Geomorphology Flashcards

Question

Plate Tectonics: Headings?

Answer 1

1. Axioms 2. Plates 3. Plate Boundaries 4. Evidence 4.1. Sea Floor Spreading 4.2 Paleomagnetism 5. PT and Continental Displacement 6. PT and Creation of Oceans 7. PT and Mountain Building 8. PT and Vulcanicity 9. PT and Earthquakes 10. Criticism 11. PT vs Continental Drift

Answer 2

Lithosphere is mobile and floating above denser but partially molten asthenosphere Lithosphere is broken into amorphous pieces, larger and smaller known as major and minor plates new material is generated by sea-floor spreading at the mid-ocean ridges surface area is conserved, therefore plate material must be destroyed through another process motion of plates is accomodated only along plate boundaries. The mobile plates are interacting along dynamic linear zones and geological processes are super active along such areas

Answer 3

The lithosphere, 50-100km thick, is rigid and is broken up into seven very large continental- and ocean-sized plates, six or seven medium-sized regional plates, and several small ones. A tectonic plate may be a continental plate or an oceanic plate, depending on which of the two occupies the larger portion of the plate. The Pacific plate is largely an oceanic plate whereas the Eurasian plate is a continental plate. The lithosphere rests on and slides over an underlying partially molten (and thus weaker but generally denser) layer of plastic partially molten rock known as the asthenosphere Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of detachment. The lithosphere itself includes all the crust as well as the upper part of the mantle (i.e., the region directly beneath the Moho), which is also rigid. However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity. This process begins at about 100 km (60 miles) below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.3 grams per cubic cm. The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm

Answer 4

Major Plates: 1. The Antarctic (and the surrounding oceanic) plate: continental core; 2. The North American plate: mostly continental with oceanic slab (NW Atlantic); moving W; riding over Juan de Fuca minor plate 3. The South American plate: mostly continental with oceanic slab of south Atlantic Ocean; moving W; riding over NAZCA plate 4. The Pacific plate: It is entirely oceanic and moving in NW direction and is subducting below Eurasian and Indo-Australian Plate 5. The India-Australia-New Zealand plate: largely oceanic; moving in N-NE direction and is subducting below Eurasian Landmass 6. The Africa with the eastern Atlantic floor plate: mostly continental; moving NE and subducting below Eurasian Landmass 7. Eurasia and the adjacent oceanic plate: moving in easterly direction; mostly continental though western part is oceanic Young Fold Mountain ridges, oceanic trenches, and/or transform faults surround the major plates. Diagram: https://1drv.ms/u/s!AvN_8sA-Zf0djVm5bQI8DuwufADr?e=7Gzulg Diagram, self: https://1drv.ms/u/s!AvN_8sA-Zf0djW4HuhsDayHh5nNH?e=kWzwHR Can also refer the video in folder abt how to draw

Answer 5

1. The continents have a crust that is broadly granitic in composition and, with a density of about 2.7 grams per cubic cm , is somewhat lighter than oceanic crust, which is basaltic (i.e., richer in iron and magnesium than granite) in composition and has a density of about 2.9 to 3 grams per cubic cm 2. Continental crust is typically 40 km (25 miles) thick, while oceanic crust is much thinner, averaging about 6 km (4 miles) in thickness 3. The distribution of these crustal types broadly coincides with the division into continents and ocean basins, although continental shelves, which are submerged, are underlain by continental crust. 4. their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge. These crustal rocks both sit on top of the mantle, which is ultramafic in composition (i.e., very rich in magnesium and iron-bearing silicate minerals). The boundary between the crust (continental or oceanic) and the underlying mantle is known as the Mohorovičić discontinuity (also called Moho) The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle.

Answer 6

Minor Plates: Some important minor plates include: 1. Cocos plate: Between Central America and Pacific plate 2. Nazca plate: Between South America and Pacific plate 3. Arabian plate: Mostly the Saudi Arabian landmass 4. Philippine plate: Between the Asiatic and Pacific plate 5. Caroline plate: Between the Philippine and Indian plate (North of New Guinea) 6. Fuji plate: North-east of Australia 7. Juan De Fuca plate: South-East of North American Plate Sub plates: (In the process, of development as a plate)are the china sub plate, Somalian, Arabian, Persian Sub plate.

Answer 7

The Indian plate includes Peninsular India and the Australian continental portions. The Indian plate is one of the major plates whose northern boundary lies along the axis joining the northern flank of the peninsular plateau i.e. Rajmahal hills, Chota Nagpur, Baghelkhand, Bundelkhand, Malwa plateau and Aravalli hills. Its western boundary lies in the Arabian Sea by the Carlsberg Sea Ridge – 1752 m deep, the Indian Ocean by the Mid-Indian Sea Ridge – 2067 m deep and the southern boundary by the South-East Indian Sea Ridge in the Indian Ocean – 3017 m deep and the eastern boundary by the New Zealand Islands. And the Karmadic Ridge (located in the north-east of New Zealand) and the north-eastern border is located along the line joining New Guinea, Java, Sumatra and Andaman Islands In the east, it extends through Rakim Yoma Mountains of Myanmar towards the island arc along the Java Trench. The Western margin follows Kirthar Mountain of Pakistan. It further extends along the Makrana coast of Balochistan and joins the spreading site from the Red Sea rift southeastward along the Chagos Archipelago. The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly West to East direction and merging into the spreading site, a little south of New Zealand. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWHsxhub0jBRk8qN?e=scduNI After breaking away from gondwana, the Indian plate moved away @ 12cm/yr. As it approached the eurasian plate the sediments deposited in the Tethys sea was folded to form Himalaya mountains. The Indian plate first collided with Eurasian plate near Ldakh which slowed down its motion.At the same time its rotation direction also changed. On hitting Ldakh the Indian plate started closing like a door. Today it's flow speed has decreased to 5cm/yr. Diag: https://tse2.mm.bing.net/th?id=OIP.YY5k-O1mxl4amfTAGhBcTgHaHj&pid=Api&P=0

Answer 8

According to American and Indian scientists of NASA, asteroids are the reason for the fission of the supercontinent Gondwanaland. A paper to this effect appeared in the January 1993 issue of the 'Geology Journal' of the United States, written by John R. Marshall and Hans Agarwal. The impact of a meteorite or asteroid falling on the Earth comes from the debris of asteroids found on the Earth's crust, which is called Tillite. The Madagascar plate experienced two major rifting events during the break-up of Gondwana. First, it separated from Africa about 160 Mya (million years ago), then from the Seychelles and India 66–90 Mya According to scientists from the National Geophysics Research Institute (NGRI), Hyderabad, Madagascar was first physically associated with the Dharwad Protocontinent of India, which broke away from the Indian Plate about 140 million years ago. It reached its present position after about 60 million years after fragmentation. Until now, Madagascar has been considered a part of the continent of Africa. Before the fission, India and Madagascar were moving north at the rate of 4 cm per year. After fragmentation, the Indian plate collided with the Eurasian plate and formed the Karlsberg and Indian Ocean Ridges between the Madagascar minor plate and the Indian main plate, which are made of basalt on the composition plate edge. There are many evidences of the association of the Madagascar plate and the Indian plate to the Dharwad region in the past. 1. Correspondence in Geo-Magnetism data obtained from geo-satellites and geologic and tectonic investigations from the eastern and western sides of Madagascar. 2. Similarity in the geological structure of both. 3. Equivalence in the remains of shallow seas located near both. Prior to the study by the Department of Geophysical Survey of India, Bangalore, geologists believed that the Indian plate was stable in its place. The reason for the origin of the Himalayas is the southward flow of the Eurasian Plate or the Angarland, but the Geophysics Survey of India, Bangalore and the Chinese Geophysical Survey, Beijing have indisputably verified that the Indian plate itself moves in the north-east direction by 5.5 cm per year. rate is increasing.

Answer 9

1. The rate of elevation of the Himalayan ranges of 1.2 mm per annum is proof that the the northern edge of the plate is tetconically active and due to the drift in the north-east direction of the Indian plate, The Himalayan ranges are rising due to the pressure generated by the movement. 2. Earthquakes of varying magnitude along the Himalayan region from the Pamir knot to the Arakanyoma confirm that the subducted portion of the northern edge of the Indian plate is reaching the asthenosphere and transforming into magma, which exerts pressure on the Himalayan structure. Due to which earthquakes occur on the Himalayas with the help of Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The Garhwal earthquake is a vivid evidence of this. 3. According to the data obtained from the aeromagnetic survey, the depth of the depression in the upper and middle Gangetic plains is 8000-6000 m, the thickness of the depression becomes more near the Himalayan mountain range and decreases towards the south plateau. This indicates that due to the north-easterly flow of the Indian plate, its northern edge submerged in the Himalayas, forming a trench between the Himalayan range and the northern edge of the plateau, the depth of which was greater in the north. as a result, The sediments brought by the rivers to the depression got deposited in greater thickness in the northern edge. 4. Several landforms of the Konkan and Kannada coast indicate that they were formed as a result of uplift. This proves that the western coast of India (except for Kutch in Gujarat) has been uplifted in the geologic past. 5. Earlier India's Dharwad (Karnataka), Madagascar and Africa were interconnected and India was on the equator in the south position than the present. After the fragmentation, Madagascar and Africa were separated from India and the land west of the India land was submerged, on which the water of the Indian Ocean spread which was called the Arabian Sea. It was during this fault that the Western Ghat Mountains of India were born and the equatorial forests of the submerged landmass were buried and converted into petroleum. The petroleum of 'Bombay High' situated in the Arabian Sea 80 km west of Bombay is the result of this. 6. The main coal field of India or 'Koylanchal', located along the eastern and southern edges of the Chotanagpur plateau, is also evidence that this area was also covered with equatorial forest. During the tectonic event of the Meghalaya plateau separating it from the Chotanagpur plateau along the Dawki fault, these vegetation got buried and converted into coal. 7. The study of fossilized coconut fruit found between the intertrap beds of Mohgaon kalan of Madhya Pradesh by palaeontologists by carbon-14 dating method shows that it is about 70 million years old of Tertiary age. This proves that at that time there were equatorial forests with coconut species in Madhya Pradesh, that is, Madhya Pradesh was on the equator. It is at its present latitudinal position due to plate tectonics. 8. At the time of the eruption of the Rajmahal lava, about 100 million years ago, India was 2250 km south of the current position. In other words, it was at 20° south latitudinal position because 1° latitude = 111 km. Kanyakumari, which is today located at 8°4' north latitude, was at that time about 12° south latitude. 9. The shift in the geographic north and magnetic north of a place is called 'magnetic flux'. Due to the flow of the plate, the magnetic flux of each place of each plate keeps on changing. For example, in 1970, the magnetic field of Allahabad was 1/2° West, which was decreasing at the rate of 1' (1 minute) annually 10. The Bay of Bengal on the north-eastern edge of the Indian plate is subducted by substantial sedimentation by the Ganges-Brahmaputra river system. The proof of this is the vertical roots of Sundari tree found in the Ganges Delta at a depth of 15 m to 150 m above sea level (Fig. 3)! 11. The east coast of India is sinking down at a rate of 1.2 mm per year (Gardner GS - Physical Geogrophy page 443) due to the subsidence of the Bay of Bengal geodesic. 12. Magma is coming to the earth's surface through cracks created by the subduction of the Bay of Bengal. The volcanoes of Andaman and Nicobar Islands confirm this. 10 April 1991 The volcano erupted on Ko Baren Island (Middle Andaman) is a vivid example in this context. 13. Before the origin of Shivalik, the Indobrahma river flowed from east to west along the southern mountain foot of the Himalayas and fell into the Arabian Sea. Due to plate tectonics, due to the rise of the Delhi water divide, the course of this river was changed which started falling in the Bay of Bengal and its abandoned part is still in the form of Ghaggar river in Haryana, whose water dries up in Talwara lake of Rajasthan during rainy season. 14. A borewell carried out by the Natural Oil and Gas Commission in the Parvatpad region of Shivalik revealed a fresh water lake about 1700 feet deep, which is actually the valley of the Indobrahma river, in which sufficient water is also found in the sandy sediment. This valley was raised by the movement in the north-east direction of the Indian plate, which gave rise to the Shivalik range. 15. When the landmass that is now the Indian subcontinent slammed into Asia about 50 million years ago, the oxygen in the world’s oceans increased, altering the conditions for life, scientists say (https://www.financialexpress.com/lifestyle/science/apart-from-rise-of-himalayas-what-happened-when-indian-plate-collided-with-eurasian-plate-in-pre-historic-era/1561513/) due to plate tectonics, there will be another category in future from Terai to the south of Shivalik range. can come into existence. Rapid sedimentation is taking place in the Terai region by various rivers, where the sedimentation load is causing the downpour of the Terai region. Geologists have detected a fault at Devvand in Saharanpur district which is called Himalayan Front Fault. This is the result of the subsidence of the Terai belt.

Answer 10

Three types: 1. constructive or Divergent plate boundaries 2. Destructive or Convergent Plate boundaries 3. Conservative or Shear or Transform plate Boundaries Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWIp-GbUTUG8Dtgw?e=uisU8L

Answer 11

https://1drv.ms/u/s!AvN_8sA-Zf0djW-mixOZtckFbKjh?e=PPb6cf

Answer 12

Along Such Boundaries, high energy flow from the interior has been found which is suggestive of the rising limb of convectional currents. It is a linear mobile Zone of sea floor spreading. Divergence takes place in continental as well as oceanic plates, in oceans MOR (mid oceanic Ridge) and on continents great rift valleys are formed. As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. This molten material, known as magma, is basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins Divergence and creation of oceanic crust are accompanied by much volcanic activity and by many shallow earthquakes as the crust repeatedly rifts, heals, and rifts again. Brittle earthquake-prone rocks occur only in the shallow crust. Deep earthquakes, in contrast, occur less frequently, due to the high heat flow in the mantle rock. These regions of oceanic crust are swollen with heat and so are elevated by 2 to 3 km (1.2 to 1.9 miles) above the surrounding seafloor. Divergent movement of plates results in (i) volcanic activity of fissure flow of basaltic magma, (ii) creation of new oceanic crusts, (iii) formation of submarine mountain ridges and rises, (iv) creation of transform faults, (v) occurrence of shallow focus earthquakes, (vi) drifting of oceanic plates etc. On a global scale, these ridges form an interconnected system of undersea “mountains” that are about 65,000 km (40,000 miles) in length and are called oceanic ridges.

Answer 13

Process of sea floor spreading i.e. when divergence is under oceanic crust: 1. The convectional currents when rises, it pushes hot plume of magma from about 700 km of depth 2. this hot magma and energy is involved in a process called intra crustal thinning which is simply the gradual but regular melting of crustal material. 3. When the crust is weak enough, the lateral flow of convectional current beds crack into the oceanic slab. Eventually, the plates are horizontally displaced and sea-floor spreading begins. 4. The best-known example of divergent boundaries is the Mid-Atlantic Ridge 5. The fissure gradually widens and the magma pours out which gets deposited along trailing edges of the plate. The huge deposition creates mountainous ridges 6. It is along the trailing slabs, where the magma clings on and newer plates are created, thus it is also called the constructive boundaries. The rift valley is found in the central parts of the ridge, which signifies the tensional forces. 7. Since the magma comes from the asthenosphere it is peridotitic but when it comes on the surface due to magmatic differentiation becomes basaltic. Hence the new oceanic crust is basaltic. Process of Continental Rifting i.e. when divergence is under continental crust 1. Upwelling of magma causes the overlying lithosphere to uplift and stretch 2. If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass. 3. As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.

Answer 14

This is also known 'Subduction zone' or the 'Zone of collision' or 'The Benioff Zone' it is the Zone of maximum and most complex geological processes. Events like volcanism, Earthquake, fold mts, deep trenches, are associated with these of boundaries. Process: 1. When two plates collide, the denser plate is subducted below the lighter one because lesser the density greater the buoyancy. If the density is similar the plate with greater velocity will Be subducted. If the plates have similar velocity and density, then the older and the rigid plate subducts. 2. Because oceanic crust cools as it ages, it eventually becomes denser than the younger oceanic crust or continental crust, and so it has a tendency to subduct, or dive under, adjacent continental plates or younger sections of oceanic crust. 3. The plate subducts at an angle of 45 degrees, when it enters the asthenosphere (the margin of subducting plate) 4. At a depth of 100 km to 300 km, the partial melting begins and differentiation of magma takes place but silica has upward mobility and the differentiated magma begins to rise. This is known as volcanic outpour. 5. The rising magma creates a vent or a fissure and appears on the surface as volcanic arc. 6. As the two plates further converges and the sedimentary material (only in case of C-C or O-C collisions) and the relief features of the subducting plate is scratched and gets deposited in the trench. Later on when the compressional force is increased these materials are squeezed, compressed, and folded leading to orgenesis. The mountain thus raised is known as tectonic arc. Convergent movement of plates results in (i) occurrence of explosive type of volcanic eruptions, (ii) deep focii earthquakes, (iii) formation of folded moun- tains, island arcs and festoons, oceanic trenches etc. Convergent plate Boundaries can be of three types

Answer 15

1. Ocean-Ocean collision: (i) older oceanic crust having relatively denser material is subducted into upper mantle. (ii) Such collision and subduction occurs along east Asia. The mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate. (iii) the resultant tectonic expression of plate collision and subduction includes deformation in crustal area, vulcanism, metamorphism, formation of oceanic trenches, island arcs and festoons etc., and occurrence of earthquakes (iv) Trench indicate the zone of subduction. eg. Kuril trench, Japan trench, Philipines trench. (v) volcanic arc is represented by Kuril is., Japanese Is., Philippines (v) Back arc basin (explained in later f/c): eg. sea of Okhotsk, Japan sea, East China sea, South China sea etc. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXAYcrd6A8zITKVd?e=ZtgZVf

Answer 16

Ocean-Continent collision: (i) the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted. (ii) Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle. This explains why ocean floor rocks are generally less than 200 million years old whereas the oldest continental rocks are more than 4 billion years old. (iii) resultant tectonic expressions are deformation of crustal rocks, metamorphism, volcanic eruptions, formation of folded mountains and occurrence of deep-focus earthquakes. (iv) Collision of American and Pacific plates is a typical example of this category and formation of majestic western cordillera of N. America and Andes of S. America is significant resultant tectonic expression of such situation. (v) It may be mentioned that one of the manifestions of continent-oceanic plate collision is the exposure of deep ocean rocks through their thrusting in resultant mountain masses. This process is called obduction which is opposite to subduction (vi) further the sediments also get accumulated in trench and later folded to form tectonic arcs (folded mts). eg. O-C collision is represented by Nazca plate and S. American plate. The trench is represented by Peru-Chile trench. The tectonic arc is represented by Andes mts. The volcanic arc is represented by Ojas Del Salado volcano on Argentina-Chile border, Cotopaxi and Chimborazo volcano in Ecuador etc. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXHwXjl3Tf-EeUu3?e=RI10Xt

Answer 17

Continent-Continent Collision: (i) Two plates carrying continental crust collide when the oceanic lithosphere between them has been eliminated. (ii) responsible for creation of folded mountains and occurrences of earthquakes of varying magnitudes. (iii)The collision of Asiatic-Indian plates, and European-African plates is typical example of such situation and the formations of Alpine and Himalayan mountainous chains are major manifestions Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXKT3xMohYhPxS6a?e=Xmd64Q

Answer 18

A few possible factors: 1. First of all, volcanic and igneous rocks are found along Himalayas and Alps (refer Mid Continental belt volcanoes). These however were formed because of collision between Eurasian plate and oceanic crust below the Tethys sea at the forefront of the Indian Plate and Eurasian plate. Volcanic arcs must have existed as the oceanic crust under Tethys sea subducted under Eurasian plate, but as the Indian plate continued onward, some time later it collided with Eurasian plate and folded the sediments into fold mt that destroyed these volcanic arcs. Nevertheless, part of that volcanic arc indeed is visible now in the surface. No volcanic edifice have survived the cataclysm that give rise to the Himalayas, but the rocks that formed the arc are now known as the Dras Volcanics, a band of volcanic basalts and dacites outcrops that are part of the Indus suture zone. And that are thought to originate in the Mesozoic (from about 252 to 66 Ma ago) volcanic arc that existed in the Eurasia plate as a consequence of the subduction of the oceanic crust under the Tethys sea. 2. under C-C collision, both plates are buoyed in asthenosphere and neither subducts rather they fuse together along 'suture zone' like the one present in Himalayas. Since neither plate is subducted, no magma is formed, no upwelling and no volcanic arcs. 3. even if one of the continental plate is subducted, it does not have the moisture of the oceanic crust. Moisture of the oceanic crust is necessary as it is the release of this moisture at the depths of ~100km that decreases the melting point of the lithosphere subducted. Since continental plate lacks this moisture, its melting point does not decrease enough and it does not melt. 4. Even if some magma is formed, and it rises up, it rarely reaches the surface and cools down while subterranean forming volcanic intrinsic structures and igneous rock deposits. Remember that continental lithosphere is thicker than oceanic.

Answer 19

The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about 100 km (60 miles) thick that carries a relatively thin cap of oceanic sediments. The path of descent is defined by numerous earthquakes along a plane that is typically inclined between 30° and 60° into the mantle and is called the Wadati-Benioff zone. The factors that govern the dip of the subduction zone are not fully understood, but they probably include the age and thickness of the subducting oceanic lithosphere and the rate of plate convergence. EQs and the seismic activity extends 300 to 700 km (200 to 400 miles) below the surface, implying that the subducted crust retains some rigidity to this depth. At greater depths the subducted plate is partially recycled into the mantle. The site of subduction is marked by a deep trench, between 5 and 11 km (3 and 7 miles) deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts.

Answer 20

back-arc basin, are submarine basin that forms behind an island arc. Such basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. Examples of back-arc basins include the Sea of Japan, the Kuril Basin in the Sea of Okhotsk, the Mariana Trough in the Philippine Sea, and the South Fiji Basin. Back-arc basins are sites of significant hydrothermal activity, and the deep-sea vents that occur in these regions often harbour diverse biological communities. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression Process: A back-arc basin is formed by the process of back-arc spreading, which begins when one tectonic plate subducts under (underthrusts) another. Subduction creates a trench between the two plates and melts the mantle IN THE OVERLYING PLATE, which causes magma to rise toward the surface. Rising magma increases the pressure at the top of the overlying plate that creates rifts in the crust above and causes the volcanoes on the island arc to erupt. As additional magma breaks through the cracks in the crust, one or more spreading centres develop, which widen the seafloor and expand the section of the overlying plate behind the trench. (Spreading centres that form in back-arc basins are much shorter than those found along oceanic ridges, however.) As the basin expands, the leading edge of the overlying plate may be forced oceanward, causing the trench to “roll back” (The backward motion of the subduction zone relative to the motion of the plate which is being subducted is called trench rollback ) over the subducting plate, or it may serve as a “sea anchor” by remaining fixed in place relative to the top of the subducting plate. In the latter case, the enlargement of the basin forces the trailing part of the overlying plate to move in the opposite direction.

Answer 21

where two plates pass or slide past each other along transform faults. These are called conservative because crust is neither created nor destroyed. The significant tectonic expression of such situation is the creation of transform faults which move, on an average, parallel to the direction of plate motion. Transform faults offset mid-oceanic ridges. Besides oceanic transform faults, there are also continental transform faults e.g. San Andreas fault (California, USA), Alpine fault (Af rica) etc. It may be mentioned that San Andreas fault 'is ridge to ridge transform fault.' they don't produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 earthquake that devastated San Francisco. In these areas of contact, stress is built which causes the rocks to break or slip, suddenly lurching the plates forward and causing earthquakes. These areas of breakage or slippage are called faults. The border between the Pacific and North American Plates, a stretch of the Ring of Fire is a transform boundary. The San Andreas Fault in California is an example of a transform boundary, where the Pacific Plate moves northward past the North American Plate. It is one of the most active faults on the Ring of Fire. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWynjIkKeC1VL_rZ?e=QAWbpb

Answer 22

first propounded by professor Hary Hess of the Princeton University in the year 1960. When the data of magnetic anomalies obtained during the many surveys of MOR were displayed on a chart, there emerged well defined patterns of stripes. Based on these information Hary Hess propounded that the mid-oceanic ridges were situated on the rising thermal convection currents coming up from the mantle The seafloor is spreading in a 'conveyor belt' fashion. The oceanic crust moves in opposite directions from mid-oceanic ridges, from which molten lava oozes out. These molten lavas cool down and solidify to form new crust along the trailing ends of divergent plates (oceanic crust). Thus, there is continuous creation of new crust along the mid-oceanic ridges and the expanding crusts (plates) are destroyed along the oceanic trenches. Below the oceanic ridges the rising limb of convectional currents are found. Huge amount of magma is gradually moving up with the rising convectional current. The convectional current spreads laterally on reaching the base of the crust and the downward limb is found below the oceanic trenches. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWOOMaXgAqjr1Zoy?e=qmbJdE Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWp6mkBi28SOos_j?e=xAWZcN Along the ridges, the oceanic crust is new and along the trenches, the oceanic crust is older. Thus, the rates of spreading (drifting) of different oceans have been determined. The Atlantic and Indian Oceans are spreading (expanding) very sluggishly i.e. at the rate of 1.0 to 1.5 cm per year while the Pacific Ocean is expanding at the rate of 6.0 cm per year. (only one side is measures which means actual expansion will be double). Note However, that despite the higher rate of expansion of Pacific Ocean through its MOR, it is overall sinking because it is losing at margins at a higher rate. Evidences in Support of Sea floor spreading: 1. The existence of the alternative strips of formal and reverse polarity, which lay as mirror images across the ocean ridges was the strongest evidence in support of Sea- floor spreading. 2. In line with Sea-floor spreading there is no sediment on the mid Atlantic ridge and the thickness of the sedimentary deposits increases with distance from the ridge. 3. No where ocean crust's found older that 200 million years 4. Occurrence of earthquakes near the mid oceanic ridges. Association of active volcanic islands with Mid-Atlantic ridge.

Answer 23

Palaeomagnetism refers to the preservation of magnetic properties in the older rocks of the earth. The hot and liquid lava and magma with high ferromagnesian contents, when cooled and solidified to form igneous rocks, get magnetised, the records of which are preserved in the rocks. Such magnetism preserved (frozen) in the rocks are called remanent or palaeomagnetism. It is to be remembered that the newly formed rocks are magnetised in the direction of existing geomagnetic field, and thus the magnetic inclination/dip of newly formed rocks is the same as that of the geomagnetic field at the time of the formation of said igneous rocks. Magnetism of Earth: 1. 1600 A.D. when William Gilbert, the physician of Queen Elizabeth, postulated that the earth behaved like a giant magnet and magnetism of the earth was produced in the inner part of the earth. 2. magnetic field of the earth is like a giant bar magnet of dipoles, located in the centre (core) of the earth and is aligned approximately along the axis of rotation of the earth. 3. The origin of geomagnetic field is in no case related to mantle rather it is related to the outer core of the earth because of the fact that there is gradual westward migration of geomagnetic field at the rate of 0.18° per year which proves that the rotation of geomagnetic field is slower than the rotation of the earth. This indirectly proves that the core of the earth rotates at slower rate than the overlying mantle. 4. generation of electrical currents is possible only in metallic liquid materials and such situation is found in the outer core of the earth which functions as self exciting dy namo. Thus, the energy coming out of the core is transformed into electrical currents which in asso ciation with metallic liquid substances produce geo centric dipole magnetic field. Findings of Reconstruction of Paleomagnetism of rocks from various continents: 1. revealed the fact, 'that magnetic poles have changed their positions and there has been considerable wandering in the position of poles.' Based on the calculations derived from magnetic dip observed in remnant magnetiism of rocks from that period, the positions of poles were determined in Japan, Italy, France etc. on the basis of palaeomagnetic reconstruction of Cenozoic lavas. 2. Established the fact that Cotinents are drifting and not stationary. Polar wandering curves are prepared for dif ferent continents on the basis of data derived through palaeomagnetic reconstruction. As per rule if there has not been continental drift, then the polar wan dering curves of different continents at a certain time period (same time for all the continents) shall be the same, but if the continental drift has occurred then these polar wandering curves would be different for each continent. Findnigs were of different polar wandering curves for each continent for the same time. According to A.G. Wegener all the continents were joined together in the form of Pangaea till late Permian period. If this was so, then there should be only one palaeomagnetic pole for all the continents during Palaeozoic era. This inference became true when the palaeomagnetic polar wan dering curve was prepared for Palaeozoic Pangaea by joining all the present day continents together so as to conceive the situation in Palaeozoic era. 3. Reversal of polarity: The available data of palaeomagnetism reveals the fact that about 50 per cent of the rocks of the crust have got magnetized in opposite direction to the geomagnetic field. Most of the scien tists are of the opinion that terrestrial rocks are magnetized always in the direction of geomagnetic field, but there is reversal in the direction of geomagnetic field, Scientists have measured magnetic polarity of rocks upto 4.5 million years there are four polarity epochs wherein two epochs (e.g. Gauss and Bruhnes) are of normal polarity while two epochs (e.g. Gilbert and Matuyama) are of reverse polarity. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWS8UGWFCmCB6LEc?e=qOiC8k 4. Corroborated the Theory of Sea Floor Spreading:

Answer 24

Though the sequence of events of Continental Displacement based on evidence of Paleomagnetism and sea-floor spreading is available only for the last 200 million years but on the basis of general mechanism of plate tectonics and the evidences from the continents the sequence of earlier events may be reconstructed. Acc to Valentine and Moors (1970) and Hallam (1972), 1. About 700 million years ago all the landmasses were united together in the form of one single giant landmass known as 'Pangaea l' 2. About 600-500 million years before present first Pangaea was broken because of thermal convective currents coming from within the earth, most probably from the mantle and different landmasses drifted apart. 3. These landmasses were again united together due to plate motions in one land mass known as 'Pangaea II' about 300-200 million years before present. 4. Second Pangaea began to break during early Jurassic period and N.W. Africa broke away from N. America and drifted away. The zone of sea-floor spreading continued to extend towards north and south. The separation of South America and Africa was accom plished during middle Cretaceous period, and North America and Europe began to move away from each other Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWVGrnId8E4peVW1?e=AeOVws Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWbM0U1fFHVd1VEq?e=uDcpAS

Answer 25

A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. Rift valleys are found both on land and at the bottom of the ocean and are created by the process of seafloor spreading. Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion. The Great Rift Valley System which stretches from the Middle East in the north to Mozambique in the south is a geologically active area. It features volcanoes, hot springs, geysers, and frequent earthquakes.

Answer 26

The opening of North Atlantic was accom plished in many phases. 1. Formation of new ocean basins 700 million years ago. 2. Deposition of miogeocline and eugeocline on the margins about 500 million years ago. 3. Closing of the Atlantic Ocean and the formation of part of the Applachians due to convergence of Eurasian and Ameri can plates about 400 million years ago. 4. Atlantic closed completely and the formation of the Applachians of North America and Hercynian mountains of Europe was completed about 300 million years ago. 5. Reopening of the Atlantic along with Pangea break-up abt 180 mya. Both were initiated by the eruption of the Central Atlantic Magmatic Province (CAMP), one of the most extensive and voluminous large igneous provinces in Earth's history 6. break-up of Pangaea began in the Central Atlantic, between North America and Northwest Africa, where rift basins opened during the Late Triassic and Early Jurassic. This period also saw the first stages of the uplift of the Atlas Mountains. While Northern AO developed with development of Labrador sea, separation of Eurasia-North America and Greenland and then finally separation of Eurasia and N. America, South America developed from Cretaceous period onwards. 7. Presently AO is still expanding. However, An embryonic subduction margin is potentially developing west of Gibraltar. The Gibraltar Arc in the western Mediterranean is migrating westward into the Central Atlantic where it joins the converging African and Eurasian plates. Indian Ocean : The origin and evolution of the Indian Ocean is the most complicated of the three major oceans. Its formation is a consequence of the breakup, which began about 180 million years ago, of the southern supercontinent Gondwana (or Gondwanaland); by the movement to the northeast of the Indian subcontinent (beginning about 125 million years ago), which began colliding with Eurasia about 50 million years ago; and by the western movement of Africa and separation of Australia from Antarctica some 53 million years ago. By 36 million years ago the Indian Ocean had taken on its present configuration. Although it first opened some 140 million years ago, almost all of the Indian Ocean basin is less than 80 million years old. With expansion of AO and IO, PO ceded space.

Answer 27

1. Red Sea is an example of axial trough which is located between Africa and Arabian peninsula 2. The surveyed magnetic anomalies in this area show, that both Red Sea and Gulf of Aden are spreading at the rate of 1centimetre per year (total spreading 2 cm/year) since the past 3-4 million years. 3. The Red Sea and the Gulf of Aden are located at the junction of three plates viz. Nubian plate, Somali plate and Arabian plate 4. Nubian and Somali plates are separated by Ethiopian fault. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWfzHmKHn5INdzbd?e=sEBumw

Answer 28

Pacific Ocean is a waning ocean because it is continuously being contracted in its size because of gradual encroachment of westward moving American plates. It is believed that like mid-Atlantic ridge there might have been a mid-oceanic ridge in the Pacific Ocean but it has now been remarkably deformed due to plate move ment. The magnetic survey of the Gulf of California revealed the presence of stripped magnetic anomaly. This situation validates two facts viz. (i) East Pacific Rise (ridge) is also located in the Gulf of California and there has been continuous spreading of the gulf along the ridge since the past four million years and (ii) Baja, the Californian peninsula, was previously united with the mainland of North America but later on it broke away from the continent due to spreading of sea floor. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWlHWTP6oCvX6Uiu?e=QiVVr6

Answer 29

The geologic history of the Mediterranean Sea is complex. Underlain by oceanic crust, the sea basin was once thought to be a tectonic remnant of the ancient Tethys Ocean; it is now known to be a structurally younger basin, called the Neotethys, which was first formed by the convergence of the African and Eurasian plates during the Late Triassic and Early Jurassic. The Messinian salinity crisis started about six million years ago (mya) when the Mediterranean became landlocked, and then essentially dried up. Scientists estimate that the sea was last filled about 5.3 million years ago (mya) in less than two years by the Zanclean flood. Water poured in from the Atlantic Ocean through a newly breached gateway now called the Strait of Gibraltar at an estimated rate of about three orders of magnitude (one thousand times) larger than the current flow of the Amazon River

Answer 30

Plate Collision and subduction. Can be of three types depending on the plates i.e. 1. O-O 2 O-C 3 C-C

Answer 31

collision of two oceanic plates and subduction of the boundary of the plate of relatively denser materials results in the formation of the volcanic mountain arcs or island arcs and festoons, for example, island arcs and festoons formed by Japanese islands, Phillippines etc. around the western margin of the Pacific Ocean off the east coast of Asia. The fold mountain ranges of island arcs and festoons 'form where a section of the ocean floor is subducted in the ocean basin away from a continent i.e. where ocean floor crust is on either side of the convergent plate boundary' (M.J. Bradshaw et al. 1978). The best example of the formation of mountains due to collision of two oceanic plates is the situation of Japanese island arc. Mountains of Japan range in height from 3000 m to 4000 m AMSL. It may be pointed out that all the mountains of Japan are of volcanic origin. Though Japanese mountains exhibit a number of characteristic features of folded mountains but they can no longer by regarded as fold mountains like the Alps and the Himalayas. Honshu Island represents the most characteristic example of the situation of the convergence of two oceanic plates. Honshu is bordered by Japan Trench in the east and Japan Sea in the west. The western part of the island is more frequented by volcanic activities than the eastern part. The island is characterized by two belts of metamorphic rocks on either side. It is believed that the Japan Trench was formed due to subduction of Pacific Oceanic plate under the oceanic crust to the east of Japan. According to plate tectonic theory the subducted portion of plate after reaching a depth of 100 km or more starts melting due to high temperature prevailing in the upper mantle. The magma, thus formed, ascends and appears as volcanic eruption about 200 km away from the oceanic trench. Since Japan is very close to the Japan Trench and hence western part of Japan is more frequented by volcanic activities. This process is still continuing as the Pacific plate is being continuously subducted under the oceanic crust along the Japan Trench. The eruptions of volcano in the month of June, 1991 in Japan after a dormant period of about 200 years and the eruption of Mt.Pinatubo on June 9, 1991 in Manila, Phillippines. validate the authenticity of this theory of plate tectonics. The volcanic eruptions caused by subduction of oceanic plates under the oceanic crust off the Japanese coast resulted into continuous accumulation of volcanic rocks and consequent increase in the height of island are and thus the formation of volcanic mountains could be possible. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djixaajUNKifgy-XE?e=MWgu6N

Answer 32

The collision of continental and oceanic convergent plates results in the formation of cordillera type of folded mountains e.g. the western cordillera of North America (including the Rockies). When one continental and the other oceanic plates collide due to their convergence along subduction or Benioff zone, the oceanic plate boundary being heavier due to comparatively denser materials is subducted below the continental plate boundary. The sediments deposited on the continental margins are squeezed and folded due to compressive forces caused by the subduction of oceanic plate (see Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWIp-GbUTUG8Dtgw?e=UQUexb). The Rockies and the Andes mountains were formed due to subduction of the Pacific ocean plate under the American continental plate

Answer 33

When two convergent plates composed of continental crusts collide against each other, the continental plate having relatively denser materials is subducted under the other continental plate having comparatively lighter materials than the former. The resultant lateral compression squeezes and folds the sediments deposited on either side of the continental plate margins and the sediments of the geosynclines lying between two convergent continental plates and thus forms gigantic folded mountains e.g. the Alps and the Himalayas. Mountain chains were formed due to continued collision of continental plates and consequent orogenesis along several subduction zones for long periods of time. About 70-65 million years ago (Mesozoic era) there was an extensive geosyncline, known as Tethys geosyncline, in the place of the Himalayas. Tethys geosyncline was bordered by Asiatic plate in the north and Indian plate in the south. Tethys geosyncline began to contract in size due to movement of Indian and Asiatic plates together. About 60-30 million years ago the Indian plate came very close to Asiatic plate. The Indian plate began to actively subduct under the Asiatic plate. The convergence and collision of Asiatic and Indian plates and consequent subduction of Indian plate under the former caused lateral compression due to which the sediments of Tethys geosyncline were squeezed and folded into three parallel chains of the Himalayas about 30-20 million years ago. It has been estimated that the crust has been shortened by 500 km between Asiatic and Indian plates due to convergence of two plates and subduction of Indian plate Similarly, Alpine mountains of Europe were formed due to convergence and collision of European and Afri can plates. Since the collision of these two continen tal plates was very complex and hence the structure of the European Alpine mountains is also very complex. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0dji1Hztbpj2bUQE2O?e=q2AbcZ

Answer 34

also refer Mountain Building Theories Mountain building can occur in a number of ways at a convergent margin: 1. Mountains by Subduction: (i) Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zone, and, therefore, the rocks in this region are warm and weak. (ii) Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward. (iii) eg. Andes Mountain 2. Mountains by Terrane Accretion (i) As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin. (ii) eg. addition of these accreted terranes has added an average of 600 km (400 miles) in width along the western margin of the North American continent, and the collisions have resulted in important pulses of mountain building. (iii) During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolites, are preserved on land. A classic example is the Coast Range ophiolite of California 3. Crustal thickening during Continental Collision (i) neither continent being subducted to any appreciable extent. A complex sequence of events ensues that compels one continent to override the other. These processes result in crustal thickening and intense deformation that forces the crust skyward to form huge mountains with crustal roots that extend as deep as 80 km. (ii) As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes. Over the past 70 million years the subduction of the Neo-Tethys Sea, a wedge-shaped body of water that was located between Gondwana and Laurasia, led to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia. These collisions culminated in the formation of the Alps and the Himalayas. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djVqRpL3VM_tQ_lRB?e=faxael Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America—which includes the Rocky Mountains as well as the Cascades, the Sierra Nevada, and other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion.

Answer 35

coverd with volcanoes

Answer 36

Although most of Earth’s volcanic activity is concentrated along or adjacent to plate boundaries, there are some important exceptions in which this activity occurs within plates. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djVtURf55IqY5FXgb?e=cXzpo7 These island chains record a typical sequence of decreasing elevation along the chain, from volcanic island to fringing reef to atoll and finally to submerged seamount. Canadian geophysicist J. Tuzo Wilson and American geophysicist W. Jason Morgan explained such topographic features as the result of hotspots. Hotspots are thought to be the surface expression of giant plumes of heat, termed mantle plumes, that ascend from deep within the mantle, possibly from the core-mantle boundary, some 2,900 km (1,800 miles) below the surface. A volcano builds upon the surface of a plate directly above the plume. As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct. Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atolls, and eventually they sink below the surface of the sea to form a seamount. At the same time, a new active volcano forms directly above the mantle plume. https://1drv.ms/u/s!AvN_8sA-Zf0djVtURf55IqY5FXgb?e=iObNsO The best example of this process is preserved in the Hawaiian-Emperor seamount chain. The plume is presently situated beneath Hawaii, and a linear chain of islands, atolls, and seamounts extends 3,500 km (2,200 miles) northwest to Midway and a further 2,500 km (1,500 miles) north-northwest to the Aleutian Trench. Major hot spots include : The Iceland hotspot, under the island of Iceland in the North Atlantic. The Réunion hot spot, under the island of Réunion in the Indian Ocean. The Afar hotspot, located under northeastern Ethiopia.

Answer 37

Along the convergent boundaries, When the downward-moving slab reaches a depth of about 100 km (60 miles), it gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone (known as the mantle wedge). Melting in the mantle wedge produces magma, which is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate, known as a volcanic arc, typically a few hundred kilometres behind the oceanic trench. If both plates are oceanic, as in the western Pacific Ocean, the volcanoes form a curved line of islands, known as an island arc, that is parallel to the trench, as in the case of the Mariana Islands and the adjacent Mariana Trench. If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America. Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust. In either case, the composition of the volcanic mountains formed tends to be more silicon-rich and iron- and magnesium-poor relative to the volcanic rocks produced by ocean-ocean convergence.

Answer 38

Philippine Island Arc system is formed due to subduction of Philippine Sea plate under the Sunda Plate (major continental shelf of the Eurasian plate). The trench formed here is called Philippine Trench. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0dji7XonX2yQmQfsnu?e=cBZVQm

Answer 39

In the case of Indonesian Archipelago, the Indo-Australian plate is subducting below Sunda Plate (part of Eurasian Plate). The trench formed here is called Sunda trench (Java Trench is a major section of Sunda trench). Anak Krakatau (child of Krakatau) volcano lies close to the Java Trench. It is situated in the Sunda Strait between the Indonesians Islands of Java and Sumatra. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0dji9IlCjlgbJQJWuv?e=Bi9aeq

Answer 40

island Groups in Carribean sea: 1. The Greater Antilles is a grouping of the larger islands in the Caribbean Sea: Cuba, Hispaniola (containing Haiti and the Dominican Republic), Puerto Rico, Jamaica, and the Cayman Islands. 2. Together, the Lesser Antilles and the Greater Antilles compose the Antilles (or the Caribbean islands). 3. When combined with the Lucayan Archipelago (Bahama Archipelago), all three are known as the West Indies. 4. Lucayan Archipelago is an island group comprising the Commonwealth of The Bahamas and the British Overseas Territory of the Turks and Caicos Islands. The Caribbean Plate is a mostly oceanic tectonic plate. The northern boundary with the North American plate is a transform or strike-slip boundary (more about this in the subsequent chapters). The Caribbean Plate is moving to the east while the North American Plate is moving to the west. The Puerto Rico Trench is located at a boundary between the two plates that pass each other along a transform boundary with only a small component of subduction. The boundary between the two plates in the past has been convergent, and most of the Greater Antilles group of islands are formed due to the complex interaction between the two plates. The eastern boundary of the Caribbean Plate is a subduction zone, the Lesser Antilles subduction zone, where oceanic crust of the South American Plate is being subducted under the Caribbean Plate. This subduction zone explains the presence of active volcanoes along the Lesser Antilles. Mount Pelée is an active volcano at the northern end of Martinique Island (French overseas department) in the Lesser Antilles island arc of the Caribbean. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djjDZaCI_AixtKvQF?e=6UeEK4

Answer 41

Formation of the Isthmus of Panama involved subduction of the ancient Pacific-Farallon Plate beneath the Caribbean and South American plates, forming a volcanic arc on the edge of the Caribbean Plate. The remains of the ancient Farallon Oceanic Plate are the Juan de Fuca Plate, parts of the North American Plate and the South American Plate, the Cocos Plate and the Nazca Plate. This initial Panama Arc began to form as the Caribbean Plate moved eastward. The North and South American plates continued to move westward past the Caribbean Plate. In addition to their east-west (strike-slip) motion, the plates also acquired a north-south component of convergence, leading to the collision of the Panama Arc with South America. This collision drove uplift in both the Northern Andes and the Panama Arc, forming the Isthmus of Panama. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djjE3rrXxFeUh2XrM?e=JhXkqR

Answer 42

Japan’s volcanoes are part of three volcanic arcs. The arcs meet at a triple junction on the island of Honshu. Northern arc is formed due to the subduction of the Pacific Plate under the Eurasian Plate. The trench formed is Japan Trench. Central arc is formed due to the subduction of the Pacific Plate under the Philippine Plate (island formation is not significant along this arc). The trench formed is Izu Trench. Southern Arc is formed due to the subduction of the Philippine Plate under the Eurasian Plate. The trench formed is Ryukyu Trench. Japanese island arc was very close to the mainland. The force exerted by the Pacific plate and the Philippine plate tilted the arc towards its east giving rise to the Sea of Japan. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djjLyPG9sgmSSYUwB?e=9eS7T5

Answer 43

The Mariana Trench or Marianas Trench, the deepest trench, is located in the western Pacific Ocean. The Mariana Trench is formed due to the subduction of the Pacific Plate below the Mariana Plate. The maximum known depth is between 10,994 & 11,034 metres in its floor known as the Challenger Deep. The Mariana trench is not the part of the seafloor closest to the centre of the Earth. This is because the Earth is not a perfect sphere (its Geoid); its radius is about 25 kilometres smaller at the poles than at the equator. As a result, parts of the Arctic Ocean seabed are at least 13 kilometres closer to the Earth’s centre than the Challenger Deep seafloor.

Answer 44

Basaltic magma flows out along the divergent edge (fissure type or shield type volcano). Basaltic magma has less silica, and hence it is less viscous. It flows over a long-distance causing seafloor spreading but not volcanic islands. On the other hand, along the convergent boundary, andesitic or acidic magma flows out. Andesitic or acidic magma has more silica content, and hence it has higher viscosity. It doesn’t move quickly and also solidifies faster. This helps in building a layer over layer on a confined region giving rise to a volcanic mountain.

Answer 45

covered along with Earthquakes

Answer 46

1. sea Spreading 2. Paleomagnetism 3.Satellite laser Ranging (SLR) has confirmed the actual Mobility of plates as well as different rates of movement of each plate has been calculated. Now it is beyond doubt that plates are in motion. 4. JOIDES (Joint Oceanographic Institute for deep earth sampling) expedition has once again confirmed (a) That the oceanic crust is newer along the ridges and older along the trenches. (b) There is high energy flow in the central rift valley of the ridges.

Answer 47

The objections voiced against the PTT are basically not against the theory but they arise from our inadequate understanding of the mantle and more especially the processes that operate in the earth's interior. 1. The driving Mechanism held responsible for plate motion itself is questionable. The existence of horizontal seismic separation boundaries (discontinuities) in the mantle, is the evidence of changes in the physical composition of the different layers in Mantle and, consequently of the absence of radial convection currents which are widely considered as the main driving force for Plate Motion. Secondly, the other most important Mechanism supposed for plate, motion i.e. the Slab pull and the ridge push are unable to explain the subduction of oceanig ridge when it reaches the subduction zone due to plate motion. Presently it is increasingly being assumed that the driving force for displacement of lithospheric plates is not the thermal but the thermo gravitational convection." 2. The number of plates are highly variable exact numbers of plates is still not clear (According to NASA studies there are about 100 plates in the world) 3. There are evidences to show that one plate which is being considered as a unit, is having movement in two different direction eg. Indian plate, Morocco Plate etc. 4. The length of diverging boundaries is greater than converging boundaries it simply means that there is more creation than destruction, which is against the basic postulate of plate tectonics. 5. Plate tectonics is unable to explain why converging boundaries are found along the Pacific Rim while diverging boundaries are important in other oceans. 6. Shallow focus seismicity has not been found along all the ridges i.e. oceanic ridges of Indian Ocean 8. Rise of magma and high energy flow is not found along all the diverging boundaries i.e. oceanic ridges of Indian Ocean. 9. The Benioff zone is not present equally in all the probable plates i.e. intermediate and deep focus earthquakes are absent in North America. (Also absent in Himalayan range) 10. P.T.T. has failed to provide a satisfactory explanation for mountain building. In fact certain Mt. ranges of Hercynian and Caledonian age i.e. eastern highlands of Australia, Drakensburg of S. Africa and Brazilian highlands Cannot be explained by P.T.T. 11. The existence of Pre Cambrian and Cambrian rocks near the crest of the Mid Atlantic ridge contradicts the theory of Plate tectonics. According to P.T.T the rocks of the sea floor cannot be more than 5or 10 million year old. 12. Recently the theory of "Expanding Earth" has been revived. In 1933 K.E,,Holme had talked about the expanding earth and increase of the crustal material but PTT has the basic assumption, that the earth curst is constant. Inspite of these limitations, there is no denying the fact that plate tectonic has provided single unifying theory for understanding the variety of Earth Phenomenon which were previously neither understood in themselves nor in wider context. It is the only theory providing scientific explanation regarding the origin of crust, it provides scientific explanation to the origin of fold mountain and origin and distribution of Earth quakes and volcanoes. This is the only theory explaining the origin and location of guyot and seamount, more so the Plate tectonic has validated the contents of continental drift and sea floor spreading.

Answer 48

1. CD Theory talks about the movement in the continents while PTT talks about movement in the lithosphere. 2. CD Theory- SIAL is floating on SIMA PT Theory-Lithosphere is floating on asthenosphere. 3. The forces as suggested by CD were the tidal forces and gravitational forces, while PT talks about the convection current. 4. In CD the movement is either northward or westward, but in PT, the movement is in all directions. 5. CD talks about the Sea floor as static which is allowing the continent to move like an ice breaker but in PT the Sea floor spreading is taking place. The Sea floor is not static in PT. 6. CD doesn't talk about boundaries, but for PT, plate Boundaries are the central theme of the theory. 7. Both theories have discussed Mt. building but the suggested mechanism by CD is untenable while PT explains every step of Mt. building. 8. The Ridge formation in CD was due to the trailing pieces of the continent which could not keep place with the moving continents. But in PT ridges are formed due to rise and deposition of Asthenoshpheric magma. 9. CD doesn't talk about any destruction or creation, and it has considered continental masses to be fixed, while in PT the Continental masses are being created at the cost of oceanic crust. 10. PT can explain volcanism but CD has no relation with volcanism on the earth surface. Similarities between PT and CD 1. Both the theories talk about horizontal movement and the crust being mobile. 2. Both the theories have accepted that Mt. Building and seismicity are produced along the edges and not the central part.

Answer 49

vulcanicity includes all those processes and mechanisms which are related to the origin of magmas, gases and vapour, their ascent and appearance on the earth's surface in various forms. vulcanicity has two components which operate below the crustal surface and above the crust. The endogenetic mechanism of vulcanicity includes the creation of hot and liquid magmas and gases in the mantle and the crust, their expansion and upward ascent, their intrusion, cooling and solidification in various forms below crustal surface (e.g. batholiths, laccoliths, sills, dykes, lopoliths, phacoliths etc.) while the exogenous mechanism includes the process of appearance of lava, volcanic dusts and ashes, fragmental material, mud, smoke etc. in different forms e.g. fissure flow or lava flood (fissure or quiet type of volcanic eruption), violent explosion (central type of volcanic erup tion), hot springs, geysers, fumaroles, solfatara, mud volcanoes etc.

Answer 50

1. Volcanic cone/mt.: accumulated volcanic material ; in explosive volcanoes, in the form of cone 2. vent: circular-ish opening in hte summital part of volcanic mt 3. volcanic pipe 4. volcanic crater: enlarged from of volcanic vent; AKA Caldera 5. volcanic material: ejected material

Answer 51

1. Central Eruption type: -> explosive type: volcanic material ejected violetly -> central pipe and small opening -> volcanic cones are formed due to accumulation of volcanic material -> sub divided into (i) Hawaiian: quiet eruption; due to less viscous lava and non-violent gases; Pele's Hair; eg. Kilavea volcano erupted for 7 days in 1959 (ii) Strombolian: moderate intensity; besides lava, pumice, scoria, bombs etc also ejected; upto great height in sky; rhythmic or continuous eruptions though sometimes interrupted by long intervals; eg. Stromboli volcano or Lipari Is. in Mediterranean sea (iii) Vulcanian: great force and intensity; very viscous lava that solidifies quickly and harden betn two eruptions, thereby plugging the vent; the crust is shattered in next eruption and appear in sky as ash-laden volcanic clouds assuming a cauliflower shape; eg. Vulcanian of Lipari Is. (iv) Peleean: most violent type; most viscous lava; obstructive domes of lava are formed over the vent; each successive eruption more violent; eg. most disastrous eruption of Mt Pelee of Martinique island in Carribean sea, destroyed whole town of St. Pierre in 1902; result in 'nuee ardente' i.e. glowing cloud, spread laterally and cause avalanches on hillslopes; eg. Krakatoa volcano (in Sunda strait) eruption in 1883 (v) Visuvius : similar to Vulcanian and Stromboli type but extremely violent due to enormous amt of gsaes; caulifower clouds; eg. Plinian eruption in 79AD 2. Fissure eruption type: along a long fracture, fault and fissure; slow upwelling of magma; spread over ground surface; eg. Laki fissure eruption in 1783 in Iceland Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXOYDKv6EAEtk7an?e=9IuhCj

Answer 52

Active Volcanoes: 1. Active volcanoes are those which constantly eject volcanic lavas, gases, ashes and fragmental materials. 2. It is estimated that there are about more than 500 volcanoes in the world. 3. Etna and Stromboli of the Mediterranean Sea are the most significant examples of this category. Stromboli Volcano is known as Light House of the Mediterra nean because of continuous emission of burning and luminous incandescent gases, 4. Most of the active volcanoes are found along the mid-oceanic ridges and the eastern and western margins of the Pacific Ocean (convergent plate margins) Doemant Volcanoes: 1. those which be come quiet after their eruptions for some time and there are no indications for future eruption but sud denly they erupt very violently and cause enormous damage 2. Visuvious volcano is the best example of dormant volcano which erupted first in 79 A.D., then it kept quiet upto 1631 A.D. when it suddenly exploded with great force. Last eruption occured in 1929. Extinct Volcanoes: 1. when there are no indications of future eruption. 2. The crater is filled up with water and lakes are formed. 3. It may be pointed out that no volcano can be declared permanently dead as no one knows, what is happening below the ground surface.

Answer 53

Vapour and Gases- -> Steam and vapour constitute 60 to 90 per cent of the total gases discharged during a volcanic eruption. -> Steam and vapour include (i) phreatic vapour and (ii) magmatic vapour whereas -> volcanic gases include carbon dioxide, nitrogen oxides, sulphur dioxide, hydrogen, carbon monoxide etc. -> Besides, certain compounds are also ejected with the volcanic gases e.g. sulphurated hydrogen, hydrochloric acid, volatile chlorides of iron, potassium and other metallic matter. Magma and Lava- -> Generally, molten rock materials are called magmas below the earth's surface while they are called lavas when they come at the earth's face. -> -> Lavas and magmas are divided on the basis of silica percentage into two groups e.g. (i) acidic magma (higher percentage of silica and (ii) basic lava (low percentage of silica). -> Lavas and magmas are also classified on the basis of light and dark coloured minerals into (i) felsic lava and (ii) mafic lava or Basaltic : characterized by maximum fluidity; maximum flow speed (from a few kilometres to 100 kilometres per hour, average flow speed being 45 to 65 km per hour) due to high fluidity and low viscosity. Basaltic lava is the hottest lava (1,000 to 1.200°C). Lava flow is divided into two types on the basis of Hawaiin language i.e. (1) pahoehoe lava: has high fluidity and spreads like thin sheets. This is also known as ropy lava. Pahoehoe lava, when solidified in the form of sacks or pillows, is called pillow lava. (ii) aa aa lava flow or block lava flow: more viscous. Fragmental or Pyroclastic material: materials thrown during explosive type of eruption. Grouped into three categories. (i) Essential materials include consolidated forms of live lavas. These are also known as tephra which means ash. Essential material are unconsolidated and their size is upto 2 mm. (ii) Accessory materials are formed of dead lavas, (iii) Accidental materials include fragmental materials of crustal rocks. On the basis of size pyroclastic materials are grouped into (i) volcanic dust (finest particles), (ii) volcanic ash (2 mm in size), (iii) lapilli (of the size of peas) and (iv) volcanic bombs (6 cm or more in size), which are of different shapes viz. ellipsoidal, discoidal, cuboidal, and irregularly rounded. The dimension of average volcanic bombs ranges from the size of a base ball or basket ball to giant size. Some times the volcanic bombs weigh 100 tonnes in weight and are thrown upto a distance of 10 km.

Answer 54

Distributional Pattern of Volcanoes: 1. Found in well defined zones 2. associated with weaker zones of earth represented by folded mts and fault zones 3. associated with meeting zones of continents and oceans. Occurence of more volcanic eruptions along coastal margins and during wet season suggest close relationship betn volcanic eruptions an water. 4. Plate Margins: most of the world's active volcanoes are associ ated with the plate boundaries. About 15 per cent of the world's active volcanoes are found along the divergent plate margins whereas 80 per cent volcanoes are associated with the convergent plate boundaries . Besides, some volcanoes are also found in intraplate regions e.g. volcanoes of the Hawaii Is land, fault zones of East Africa etc. 5. Like earthquakes, there are also three major belts or zones of volcanoes in the world viz. (i) circum-Pacific belt, (ii) mid-continental belt and (iii) mid-oceanic ridge belt Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXXtASRGFbRmpiys?e=LhFI45

Answer 55

1. This volcanic belt is also called as the Fire Girdle of the Pacific or the Fire Ring of the Pacific. 2. Along convergent oceanic plate margins 3. includes the volcanoes of the (i) eastern and western coastal areas of the Pacific Ocean, of island arcs and festoons off the east coast of Asia and (ii) of the volcanic islands scattered over the Pacific Ocean. 4. This belt begins from Erebus Mountain of Antarctica and runs northward through Andes and Rockies mountains of South and North Americas to reach Alaska from where this belt turns towards eastern Asiatic coast to include the volcanoes of island arcs and festoons (e.g. Sakhalin, Kamchatka, Japan, Phillippines etc.). The belt ultimately merges with the mid-continental belt in the East Indies. 5. Most of high volcanic cones and volcanic mountains are found in this belt. 6. Most of the volcanoes are found in chains e.g. the volcanoes of the Aleutian Island, Hawaii Island, Japan etc. About 22 volcanic mountains are found in group in Ecuador wherein the height of 15 volcanic mountains is more than 4560 m AMSL. Cotopaxi is the highest volcanic mountain of the world (height being 19,613 feet). The other significant volcanoes are Fuziyama (Japan), Shasta, Rainier and Hood (western cordilliera of North America), a valley of ten thousand smokes (Alaska), Mt. St. Helens (Washington, USA), Kilavea (Hawaiiland), Mt. Taal, Pinatubo and Mayon of Phillippines etc. 7. Here volcanic eruptions are primarily caused due to collision of American and Pacific plates and due to subduction of Pacific Plate below Asiatic plate. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXQP4KP4Nm4OTiUG?e=HF1qbR Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXZ9Hif9yNXMW8uE?e=HNXtQL

Answer 56

1. convergent continental plate mergins'. 2. This belt includes the volcanoes of Alpine mountain chains and the Mediterranean Sea and the volcanoes of fault zone of eastern Africa. 3. Here, the volcanic eruptions are caused due to convergence and collision of Eurasian plates and African and Indian plates. 4. The famous volcanoes of the Mediterranean Sea such as Stromboli, Visuvious, Etna etc. and the volcanoes of Aegean Sea are included in this belt. The important volcanoes of the fault zone of eastern Africa are Kilimanjaro, Meru, Elgon, Birunga, Rungwe etc. 5. It may be pointed out that this belt does not have the continuity of volcanic eruptions as several gaps (volcanic - free zones) are found along the Alps and the Himalayas because of compact and thick crust formed due to intense folding activity.

Answer 57

1. includes the volcanoes mainly along the mid-Atlantic ridge which represents the splitting zone of plates. Thus, volcanoes mainly of fissure erutpion type 2. The most active volcanic area is Iceland which is located on the mid-Atlantic ridge. This belt begins from Hekla volcanic mountain of Iceland where several fissure eruption type of volcanoes are found. It may be pointed out that since Iceland is located on the mid-Atlantic ridge representing the splitting zone of American plate moving westward and Eurasian plate moving eastward, and hence here is constant upwelling of magmas along the mid-oceanic ridge and wher ever the crust becomes thin and weak, fissure flow of lava occurs because of fracture created due to divergence of plates. The Laki fissure eruption of 1783 A.D. was so quick and enormous that huge volume of lavas measuring about 15 cubic kilometres was poured out from 28-km long fissure. Recently, Hekla and Helgafell volcanoes erupted in the year 1974 and 1973 respectively. 3. Other more active volcanic areas are Lesser Antilles, Southern Antilles, Azores, St. Helena etc. The dreadful and disastrous eruption of Mount Pelee occurred on May 8, 1902 in the town of St. Pierre on the Martinique Island of West Indies in the Caribbean Sea. All the 28,000 inhabitants, except two persons, were killed by the killer volcanic eruption.

Answer 58

Besides the aforesaid well defined three zones of volcanoes, scattered volcanoes are also found in the inner parts of the continents. Such distributional patterns of volcanoes are called as intraplate volcanoes The mechanism of their eruption is not yet precisely known. eg. within the Pacific Ring of Fire, one branch of volcanoes runs from Hawaii to Kamchatka. Vulcanicity also becomes active in the inner parts of continental plates. Massive fissure eruption occurred in the north western parts of North America during Miocene period when 1,00,000 cubic kilometres of basaltic lavas were spread over an area of 1,30,000 km² to form Columbian plateau. Similarly, great fissure flows of lavas covered more than 5,00,000 km² areas of Peninsular India. Parana of Barazil and Paraguay were formed due to spread of lavas over an area of 7,50,000 km². Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXZ9Hif9yNXMW8uE?e=HNXtQL

Answer 59

Theory of plate tectonics now very well explains the mechanism of vulcanism and volcanic eruptions. In fact, volcanic eruptions are very closely associated with plate boundaries. Divergent Plate Boundaries: Most of the active fissure volcanoes are found along the mid-oceanic ridges ...constant upwelling of lavas... These lavas are cooled and solidified and are added to the trailing ends of divergent plate boundaries and thus there is constant creation of new basaltic crust. eg. The volcanic eruptions of Iceland and the islands located along the mid-Atlantic ridge are caused because of sea-floor spreading and divergence of plates. It is obvious that divergent or constructive plate boundaries are always associated with quiet type of fissure flows of lavas because the pressure release of superincumbent load due to divergence of plates and formation of fissures and faults is a slow and gradual process. supply of lava comes from the upper mantle just below the ridge because of differential melting of the rocks into tholeiitic basalts. supply of lavas decreases with increas ing distance from the mid-oceanic ridges. It has been found that the islands nearer to the mid-Atlantic Ridge have younger lavas whereas the islands away from the ridge have older lavas. For example, the lavas of Azores islands situated on either side of the mid-Atlantic Ridge are 4-million year old whereas the lavas of Cape Verde Island, located far away from the said ridge, are 120 million year old. Convergent Plate Boundaries -> associated with explosive type of volcanic eruptions. -> magma is forced to ascend by the enormous volume of accumulated explosive gases and thus magma appears as violent volcanic eruption on the earth's surface. -> represent the volcanoes of the circum-Pacific belt and the mid-continental belt. The volcanoes of the island arcs and festoons (off the east coast of Asia) are caused due to subduction of oceanic crust (plate) say Pacific plate below the continental plate, say Asiatic plate near Japan Trench.

Answer 60

1. Extrusive Volcanic topography (i) From Explosive type volcanoes: (a) volcanic cones (b) craters and calderas (ii) From Fissure Eruptions (a) Lava plateaus and domes (b) Lava Plains 2. Intrusive Volcanic topography (i) intrusive lava domes (ii) batholiths (iii) laccoliths (iv) phacoliths (v) lopoliths (vi) sills (vii) dikes (viii)plugs and stocks

Answer 61

1. Cinder or Ash cones 2. Composite Cones 3. Parasitic cones 4. Basic lava cones 5. Acidic lava cones 6. Lava Domes 7. Lava plugs

Answer 62

1. usually of low height and are formed of volcanic dusts and ashes and pyroclastic matter (fragmental materials). 2. formation initiated due to accumulation of finer particles around volcanic vent in the form of tiny mound, say 'ant mount' which varies in height from a few centimetres to a few metres in the beginning. 3. The size of the cone gradually in creases due to continuous accumulation of volcanic materials minus lavas. Some times, the rate of growth of the cone is so high that it gains height of 100 m or more within a week. 4. The slopes of cinder cones range between 30° and 45°. 5. Larger particles are arranged near the craters and rest at the angle between 40° and 45° and the finer particles are deposited at the outer margins of the cones. 6. Since such cones are formed of unconsolidated larger particles and are seldom compacted by lavas and hence they are permeable to water. 7. Such cones are on an average less suscepitble to erosion and hence they maintain their original forms for hundreds of years provided that they are not destroyed by ensuing violent explosion. 8. The volcanic cones of Mt. Jorullo of Mexico is an example

Answer 63

highest of all volcanic cones. These are formed due to accumulation of different layers of various volcanic materials and hence these are also called as strato-cones . In fact, these cones are formed due to deposition of alternate layers of lava and fragmental (phyroclastic) materials wherein lava acts as cementing materials for the compaction of fragmental materials. The cone becomes comparatively resistant to erosion if it is coated by thick layer of lava. On the other hand, if the outer layer is composed of fragmental materials, the composite cone is subjected to severe erosion. Most of the highest symmetrical and extensive volcanic cones of the world come under this category e.g. Mt. Shasta, Mt. Ranier, Mt. Hood (USA), Mt. Mayon of Phillippines, Mt. Fuziyama of Japan, Mt. Cotopaxi of Ecuador

Answer 64

Several branches of pipes come out from the main central pipe of the volcano when the volcanic cones are enormously enlarged. Lavas and other volcanic materials come out from these minor pipes and these materials are deposited around newly formed vents located on the outer surface of the main cone and thus several smaller cones are formed on major cone These cones are called parasite cones because the supply of lava for these cones comes from the main pipe. These cones are also known as adventive or lateral cones. Shastina cone is a parasite cone of Mt. Shasta of the USA.

Answer 65

formed of light and less viscous lava with less quantity of silica. In fact, when the lava coming out of fissuse flow is deficient in silica and is characterized by high degree of fluidity, it cools and solidifies after spreading over larger area. Thus, a long cone with significantly low height is formed. Such cones are also called as shield cones because of their shapes resembling a shield. Since these cones are composed of basaltic lavas, they are also called as basic lava cones. These are also known as Hawana type of cones

Answer 66

formed where the lavas coming out of volcanic eruptions are highly viscous and rich in silica content. In fact, such viscous lavas have very low mobility and hence they are immediately cooled and solidified after their appearance on the earth's surface. Thus, high cones of steep slopes are formed. Such cones are very often known as Strombolian type of cones

Answer 67

they are similar to shield cones in one way or the other. Lava domes differ from shield cones as regards their size. Actually, lava domes are larger and more extensive in size than the shield cones. These are formed due to accumulation of solidified lavas around the volcanic vents. Based on the mode of origin and the place of formation lava domes are divided into 3 categories e.g. (A) plug dome (formed of lavas due to filling of volcanic vents), (B) endogenous dome (formed of silica rich viscous lavas) and (c) exogenous dome (formed of silica-deficient lava with high degree of fluidity).

Answer 68

formed due to plugging of volcanic pipes and vents when volcanoes become extinct. These vertical columns of solidified lavas appear on the earth's surface when the volcanic cones are eroded away. The lava-filled volcanic pipe is called as volcanic neck . Generally, volcanic necks are cylindrical shaped and measure 50 to 60 m in height (above the ground surface) and 300 to 600 m in diameter. Some times diatreme term is used to indicate volcanic neck 'Shiprock' which towers 515 metres (1700 feet) over the surrounding, flat lying sedimentary rocks of New Mexico, is an excellent example of a diatreme exposed by the erosion of its enclosing sedimentary rocks

Answer 69

depression formed at the mouth of a volcanic vent is called a crater or a volcanic mouth, which is usually funnel shaped. The slope of the crater depends upon the volcanic cone in which crater is formed. Normally, a crater formed in a cinder cone slopes at the angle between 250 and 30°. The size of a crater increases with increase and expansion of its cone. An average crater measures 300 m in diameter and 300 m in depth but there is wide range of variations in craters from the standpoint of their size e.g. craters range from small craterlets having a diameter of a few hundred metres to large craters having the diameter of a few kilometres. The crater of extinct Aniakchak volcano of Alaska has a diameter of 9.6 km (6 miles) and the side walls are 364 m to 912 m (1200 to 3000 feet) high. If the Crater Lake of the state of Oregon (USA) is accepted as a crater, it becomes one of the most extensive craters of the world, though many scien tists consider it as an example of a caldera. A crater may be differentiated from a caldera on the basis of size and mode of formation. When a crater is filled with water, it becomes a crater lake. When the crater of volcano becomes very extensive and if there are few eruptions of very small intensity after long time, several smaller cones are formed within the extensive older crater and thus several small-sized craters are formed at the mouth of each volcanic vent inside the extensive crater. Such craters or craterlets are called 'nested craters' or 'craters within the crater' or 'grouped craters'. Such craters are formed only when the next eruption is smaller in intensity than the previous one. Three smaller craters are found within the extensive crater of Mt. Taal of Phillippines. Similarly, three and two craters are found within the craters of Visuvius and Etna volcanoes. The craters formed at the mouth of volcanic vents of parasite cones developed over an extensive volcanic cone is called adventive crater.

Answer 70

Generally, enlarged form of a crater is called caldera. There are two parallel concepts for the origin of calderas. (i) According to the first group of scientists a caldera is an enlarged form of a crater and it is surrounded by steep walls from all sides. The caldera is formed due to subsidence of a crater. This concept has been propounded by the U.S. Geological Survey. It is believed according to this concept that Aso crater of Japan and Crater Lake of the USA are the result of subsidence. (ii) The second group of scientists has opined that the calderas are formed due to violent and explosive eruptions of volcanoes. Daly, the leading advocate of "eruption hypothesis' of the origin of calderas, believes that the topographic features formed by subsidence are 'volcanic sinks.' According to the advocates of this hypothesis if calderas are formed due to subsidence there should not be any deposit of pyroclastic materials and volcanic ashes related to a particular volcanic cone near the caldera but evidences have revealed that the remains of volcanic materials related to a particular cone are found not only near the concerned caldera but are also found several kilometres away from the caldera. For example, volcanic materials have been found at the distance of 128 km from the caldera of Crater Lake (USA). The significant calderas of the world are Lake Toba of Sumatra, Crater Lake (10 km x 10 km) in USA, Krakatoa (7 km x 6 km) in Indonesia, Kilauea (5 km x 3 km) in Hawaii etc. Smaller calderas housed in a big caldera are called nested calderas or grouped calderas

Answer 71

formed due to cooling and solidification of rising magma during volcanic activity in the cracks, pores, crevices, and hollow places just beneath the earth's surface the resultant rocks are called as hypabyssal igneous rocks (as opposed to Plutonic Igneous rocks that are formed deep inside the earth). The magmas are solidified in different forms depending upon the hollow places such as batholiths, loccoliths, phacoliths, lopoliths, sills, dikes etc. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXjQh1gtTnrE13jP?e=Gpo02v Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXmvKnkg9Hvhri5n?e=pzDacl

Answer 72

Batholiths are long, irregular and undulating forms of solidified intruded magmas. They are usually dome-shaped and their side walls are very steep, almost vertical. The upper portion of batholiths are seen when the superincumbent cover is removed due to continued denudation but their bases are never seen because they are buried deep within the earth. When exposed to the surface they are subjected to intense weathering and erosion and hence their surfaces become highly irregular and corrugated. Numerous batholithic domes were intruded in the the Dharwarian sedimentaries in many parts of the peninsular India during pre-Cambrian period. Many of such batholithic domes have now been exposed well above the surface in many parts of the Chotanagpur plateau of India mainly Ranchi plateau where such batholithic domes are called as Ranchi Batholiths. Murha pahar near Pithauriya village, to the north-west of Ranchi city, is a typical example of exposed Ranchi batholithic domes. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXe0gituvJwoj3TN?e=Jdu8Gn

Answer 73

formed due to injection (intrusion) of magmas along the bedding planes of horizontally bedded sedimentary rocks. Laccoliths are of mushroom shape having convex summital form. The ascending gases during a volcanic eruption force the upper starta of the flat layered sedimentary rocks to arch up in the form of a convex arch or a dome. Consequently, the gap between the arched up or domed upper starta and the horizontal lower starta is injected with magma and other volcanic materials

Answer 74

formed due to injection of magma along the anticlines and synclines in the regions of folded mountains

Answer 75

When magma is injected and solidifed in a concave shallow basin whose central part is sagged downward, the resultant form of solidified magma is called a lopolith. The rocks of lopoliths are generally coarse-grained be cause of slow process of cooling of magmas.

Answer 76

sills are usually parallel to the bedding planes of sedimentary rocks. In fact, sills are formed due to injection and solidification of magmas between the bedding planes of sendimentary rocks. Thick beds of magmas are called sills whereas thin beds of magma are termed as 'sheets'. The thickness of sills ranges between a few centimetres to several metres. When sills are tilted together with the sendimentary beds due to earth movements and are exposed to exogenous denudational processes, they form significant landforms like cuesta, hogbacks and ridges

Answer 77

represent wall-like formation of solidified magmas. These are mostly perpendicular to the beds of sedimentary rocks. The thickness of dykes ranges from a few centimetres to several hundred metres but the length extends from a few metres to several kilometres. A well defined dyke is observable across the palaeochannel and valley of the Narmada river near Dhunwadhar Falls (Bheraghat) near Jabalpur city. The relative resistance of dykes in comparison to the surrounding country-rocks gives birth to a few interesting landforms e.g. (i) If the rocks of dykes are weaker and less resistant than the country rocks, the upper portion of dykes is more croded than the country rocks, with the result a depression is formed, which, when filled up with water, is called a 'dyke lake' (ii) If the rocks of dykes are more resistant than the country-rocks, upstanding ridges and hills are formed because of more erosion of the country rocks and (iii) If the rocks of dykes and country-rocks are of uniform resistance, both are uniformly dissected and hence no significant landform is developed but the height is gradually reduced

Answer 78

Geyser, in fact, is a special type of hot spring which spouts hot water and vapour from time to time. The word geyser has been derived from an Icelandic word 'geysir' which means gusher or spouter. This word was used to indicate the spouting water of a hot spring of Iceland known as Great Geyser or Gesir. The difference between hot springs and geyser lies in the fact that there is continuous spouting of hot water from the former while there is intermittent spouting of water from the latter. A geyser spouts water from a small and narrow vent which is connected by a circuitous pipe with the underground aquifers. This pipe is called as geyser pipe or geyser tube. The length of geyser tube ranges between 30 to 100 m at different places. The temperature of water coming out of a geyser ranges between 75° to 90°C. Geysers are classified into two types viz. (1) pool type of geyser : When a geyser spouts water through an open and relatively large pool, it is called pool type of geyser. Such geysers spout larger volume of water and vapour through long geyser tubes. No deposits are possible around the geyser pools. (ii) nozzle type of geyser: Nozzle type of geysers spout water and vapour through a very small and constricted vent. Emitted materials are deposited around the geyser vents and thus geyser cones are formed. geysers are further divided into (i) geysers of equal intervals between two successive period of spouting (wherein interval period between two successive active periods of spouting is certain and fixed, such geysers are, thus, considered to be reliable as regards the periods of interval and spouting, example, Old Faithful Geyser of the Yellow Stone National Park, USA), (ii) variable geysers (wherein the interval period be tween two successive periods of spouting is not certain), (iii) long-period geysers (wherein the active period of spouting is longest of all the geysers, ranging between a few minutes to one hour, example, Grand Geyser of Iceland spouts water for 30 minutes in continuation before the next interval period starts) and (iv) feeble geyser (wherein the active period of water spouting is very short). Continuously active geysers are, in fact, hot springs which spout water without any interval. The Excelsior Geyser of the Yellow Stone National Park of the USA is the example of this category. There is no certain observable distributional of geysers over the globe as they are found pattern in almost all the continents and in almost all the climatic zones. The geysers of the USA, Iceland and New Zealand are most widely studied geysers. -> Geysers are found in groups in the Yellow Stone National Park (USA). About one hundred geysers have been named and another hundred geysers are known to the scientists. There are four major basins of geysers viz. (i) Norris Basin, (ii) Upper Lake Basin, (iii) Lower Lake Basin and (iv) Heart Lake Basin. -> The major geyser of New Zealand is located in the western region of the northern Island which is also dominated by volcanic activities. -> The geysers and hot springs are spread over an area of 1786 km² (5000 square miles) in Iceland. The most significant geyser of Iceland is Grand Geyser.

Answer 79

Fumarole means such a vent through which there is emission of gases and water vapour. fumaroles are directly linked with volcanic activities. Emission of gases and vapour begins after the emission of volcanic materials is terminated in an active volcano. Some times the emission of gases and vapour is continuous but in majority of the cases emission occurs after intervals. It is believed that gases and vapour are generated due to cooling and contraction of magma after the termination of the eruption of a volcano. These gases and vapour appear at the earth's surface through a narrow and constricted pipe (tube). It may be pointed out that fumaroles are the last signs of the activeness of a volcano. Numerous fumaroles are found in groups near Katmai volcano of Alaska (USA). Here fumaroles are found in groups in extensive valley zone, which is called a valley of ten thousand smokes' which means fumaroles appear from 10,000 vents the diameter of which is around 3 metres. Here fumaroles are found along a linear fracture. Elsewhere, fumaroles are found along the volcanic craters. The temperature of vapour emitted from fumaroles is around 645°C. It may be mentioned that vapour constitutes 98.4 to 98.99 per cent of the total gases emitted from fumaroles. Other gases include carbon dioxide, hydrochloric acid, hydrogen sulphide, nitrogen, some oxygen and ammonia. Some minerals are also emitted with gases and vapour from fumaroles. Sulphur is the most important mineral. Fumaroles dominated by sulphur are called solfatara or sulphur fumaroles.

Answer 80

1. Artificial sources 2. Evidences from theories of origin of earth 3. natural sources- volcanic eruption, earthquakes and seismology 4. Direct sources; deep earth mining and drilling. But as mining and drilling are not practically possible beyond a certain depth, they don’t reveal much information about the earth’s interior. Mponeng gold mine (deepest mine in the world) and TauTona gold mine (second deepest mine in the world) in South Africa are deepest mines reaching to a depth of only 3.9 km. And the deepest drilling is only about 12 km deep hole bored by the Soviet Union in the 1970s over the Kola Peninsula.

Answer 81

Numerous inferences can be drawn about the constitution of the interior of the earth on the basis of density of rocks, pressure of superincumbent load (weight of overlying rocks) and increasing trend of temperature with increasing depth inside the earth.

Answer 82

Density of earth as whole Cavendish attempted to calculate the average density of the earth in 1798 on the basis of the Newton's gravitational law. According to him the average density of the earth is 5.48. Poynting calculated the average density of the earth as 5.49 g cm³ in the year 1878. Since 1950 several attempts are being made to calculate the density of the earth on the basis of satellites. outer thinner part of the earth is composed of sedimentary rocks the thickness of which ranges between half a mile to one mile (0.8 km to 1.6 km). Just below this sedimentary layer there is the second layer of crystalline rocks, the density of which ranges between 3.0 and 3.5 at different places. The average density of the whole earth is about 5.5. Thus, it appears that the density of the core of the earth will be, without doubt, more than 5.5. Generally, the density of the core of the earth is around 11.0. Thus, it is proved that (1) the density of the core of the earth is highest of all parts of the earth. The satellite studies have revealed the following results about the density of the various parts of the earth-average density of the earth = 5.517 g cm³, average density of the earth's surface = 2.6 to 3.3 g cm³ and average density of the core = 11 g cm³

Answer 83

while explaining the reason for very high density of the core, previously it was believed that very high density of the core was because of heavy pressure of overlaying rocks. Since the weight and pressure of rocks increase with increasing depth and hence the density of rocks also increases with increasing depth. Thus, it is proved that (2) very high density of the core of the earth is due to very high pressure prevailing there because of superincumbent load. This infer ence is proved wrong on the ground that there is a critical limit in each rock beyond which the density of that rock cannot be increased inspite of increasing pressure therein. It may be, thus, forwarded that (3) very high density of the core of the earth is not because of very high pressure prevailing there. The other possible explanation was (4) the core must be composed of intrinsically heavy metallic materials of high density. The experiments have revealed that the core of the earth is made of the mixture of iron and nickel. This inference is also validated on the basis of geocentric magnetic field. The metallic core is surrounded by a zone of such rock materials, the upper part of which is composed of crystalline rocks.

Answer 84

It is evident from the findings of bore holes and deep mining that temperature increases from the surface of the earth downward at the rate of 2" to 3°C for 100 metres. It may be pointed out that it becomes very difficult to find out the rate of increase of temperature beyond the depth of 8 km. The rate of increase of temperature in the continental crust has been calculated based on geothermal graphs and the following generalization has been made. In the tectonically active areas (like the Basin and Range Province of the USA) temperature remains 1000°C at the depth of 43 km from the surface of the earth while the temperature remains only 500°C at the depth of 40 km from the surface in tectonically stable areas. It is evident that high temperature of 1000°C at the depth of 43 km in the tectonically active areas is nearer to the initial melting point of the rocks of lower crust and mantle mainly basalt and peridotite. temperature of the upper part of the magma slab representing the upper portion of the oceanic crust has been estimated to be 0°C where as the temperature of the lower part of the magma slab which comes in contact with the asthenosphere re mains 1200°C which is quite nearer to the melting point. If we believe the rate of general increase of temperature with increasing depth the temperature should be around 25,000°C at the depth of 2,900 km but under such circumstances most part of the earth would have melted but this has not so happened. It is evident from this discussion that most parts of the radioactive minerals are concentrated in the upper most layer of the earth. This fact explains the situa tion of high temperature in the continental crust as described above because disintegration and decay of radioactive minerals generate more heat in the crustal areas. It, thus, appears that the rate of increase of temperature downwards decreases with increasing depth. The following facts may be presented about the thermal condition of the interior of the earth. (i) The asthenosphere is partially molten. The temperature is around 1100°C at the depth of 100 km which is nearer to initial melting point. (ii) The temperature at the depths of 400 km and 700 km (from the earth's surface) has been estimated to be 1,500°C and 1,900°C respectively. (iii) The temperature at the junction of mantle and outer molten core standing at the depth of 2,900 km is about 3700°C. (iv) The temperature at the junction of outer molten core and inner solid core standing at the depth of 5,100 km is 4,300°C.

Answer 85

The earth's surface receives heat from two sources e.g. from the sun and from its interior part itself. The heat received from these two sources is ultimately sent into the space. Solar heat drives the atmospheric and hydrological processes and gener ates denudational processes whereas the internal heat of the earth performs constructive works e.g. formation of mountains, plateaux, faults etc., vul canicity, seismic events and other tectonic events. 'In a real sense, the earth's internal heat engine builds mountains and its external heat engine, the sun, destroys them' heat in the interior of the earth is generated through the disintegration of radioactive minerals and conversion of gravity force into thermal energy. On an average, there is gradual flow of heat from the inner part of the earth to its outer part. It may be pointed out that the heat energy in the solids is in the form of vibrations of atoms. It is to be remembered that the rocks are poor conductor of heat. The transfer of heat from only 10-m thick rock layer takes 3 years. The 100-m thick lava flow takes 300 years to cool down and solidify. The transfer of heat from the lower part to the upper part of a 400-km thick layer of rocks would take a long period of 5 billion years. If we take conduction as the only mechanism of the cooling of the earth, the heat from the depth of 400 km would have not reached the earth's surface till new. The transfer of heat from the interior of the earth towards its outer part may also not be effectively performed by radiation because most of the minerals of the interior of the earth are opaque. Such materials cannot effectively transfer or lose heat through radiation. The third alternative possibility for the transfer of heat may be the process of convection but convective mechanism is more effective in liquid materials.

Answer 86

It is believed that about 4.7 billion years ago the initial temperature of the earth generated by planetary accretion and adiabatic compression would have been around 1000°C. Later on the heat of the interior of the earth would have gradually but substantially increased due to heat supplied by the disintegration of radioactive minerals. About 4.0 to 4.5 billion years ago the core and mantle would have been separated and their boundary would have evolved when the temperature would have increased to reach the melting point of iron. Thus, due to foundering of molten iron into core the gravity force equivalent to 2x 10^37 erg (one calorie = 4.9 x 10' erg) in the form of heat energy might have been released. Large-scale melting and rearrangement of material inside the earth consequent upon high thermal energy, as stated above, probably became responsible for the formation of different zones of the earth e.g. crust, mantle and core.

Answer 87

Some scientists believe on the basis of up welling and spread of hot and liquid lava on the earth's surface during volcanic eruption that there is at least such a layer below the earth's surface which is in liquid state. Such molten layer has been termed as 'magma chamber'. Thus, some part of the earth should be in liquid state. this inference is refuted if one considers the increas ing pressure with increasing depth inside the earth. Thus, the inner part of the earth may not be in molten state inspite of very high temperature prevailing therein because the enor mous weight and pressure of the overlying materials (superincumbent load) increases the melting point of the rocks. It, thus, appears that the core of the earth should be in solid state. Then, where hot and liquid lavas come from during volcanic eruption? It may be pointed out that when the pressure of superincumbent load is released due to fracturing and faulting in the crustal surface, the melting point of underlying rocks is reduced (low ered) and thus the rocks are instantaneously melted because required degree of high temperature is al ready present there. It, thus, appears that no authen ticated knowledge about the composition of the earth's interior is obtained from evidences of volcanic activities

Answer 88

Seismology is the science which studies various aspects of seismic waves generated during the occurrence of earthquakes. Seismic waves are recorded with the help of an instrument known as seismograph. It may be pointed out that seismology is the only source which provides us authenticated information about the composition of the earth's interior. The place of the occurrence of an earth quake is called 'focus' and the place which experi ences the seismic event first is called 'epicentre', which is located on the earth's surface and is always perpendicular to the 'focus'. On the other hand, the focus or the place of the origin of an earthquake is always inside the earth. The deepest focus has been measured at the depth of 700 km from the earth's surface. The different types of tremors and waves generated during the occurrence of an earthquake are called 'seismic waves' which are generally divided in 3 broad categories e.g. primary waves, secondary waves and surface waves. The nature and properties of the composition of the interior of the earth may be successfully obtained on the basis of the study of various aspects of seismic waves mainly the velocity and travel paths of these waves

Answer 89

(i) Primary waves-also called as longitudinal or compressional waves or simply 'P' waves. are analogous to sound waves wherein particles move both to and fro from the line of the propagation of the ray. P waves travel with fastest speed through solid materials. Though these also pass through liquid materials but their speed is slowed down. (ii) Secondary waves-are also called as transverse or distortional or simply S waves. These are analogous to water ripples or light waves wherein the particles move at right angles to the rays. S waves cannot pass through liquid materials. (iii) Surface waves-are also called as long period waves or simply L waves. These waves generally affect only the surface of the earth and die out at smaller depth. These waves cover longest distances of all the seismic waves. Though their speed is slower than P and S waves but these are most violent and destructive.

Answer 90

If the earth would have been composed of homog enous solid materials the seismic waves should have reached the core of the earth in a straight path but this is not the case in reality. In fact, the recorded seismic waves denote the fact that these waves seldom fol low straight paths rather they adopt curved and refracted paths. Thus, it becomes obvious that the earth is not composed of homogenous materials The seismic waves are refracted at the places of density changes. A regular change of density inside the earth causes a curved path to be followed by the seismic waves. Thus, the seismic waves become concave towards the earth's surface S waves cannot pass through liquid. After indepth study of seismic waves Oldhum demonstrated in the year 1909 that S waves disap pear at the angular distance of 120° from the epicen tre and P waves are weakened. It is evident from fig. 5.2 that S waves are totally absent in the core of the earth. It appears from this observation that there is a core in liquid state which is located at the depth of more than 2900 km from the earth's surface and surrounds the nucleus of the earth. Based on this finding the scientists have estimated that the iron and nickel of the core of the earth may be in liquid state Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djgFiITtHbxVOcO8E?e=fu4Asf Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djgOws6Gj2K1R-KlW?e=RuabVM Not only this, if we study the nature, charac teristics and velocity of seismic waves, we may find the presence of several density zones inside the earth. On the basis of velocity seismic waves are divided in three sets of waves e.g. (i) first set of P-S waves of maximum velocity, (ii) second set of Pg-Sg waves of minimum velocity and (iii) third set of P'-S' waves of medium velocity falling between the first and the second sets of waves. Thus, on the basis of changes of velocity of seismic waves it is proved that there are major changes in the velocity of waves at three places inside the earth and hence it can be safely inferred that there are three distinct zones or layers of varying densities inside the earth below the outer thin layer of sedimentary rocks. Upper Layer-Jeffreys discovered a different set of seismic waves termed as Pg-Sg waves on the basis of the record of the earthquake of the Kulpa valley in Croatia in the year 1909. On an average Pg and Sg waves travel at the rate of 5.4 km and 3.3 km per second respectively in the upper part of the earth. The density of the rocks through which these waves travel is about 2.7. It is proved on this basis that the upper layer is composed of granitic rocks. On the basis of the change in the velocity of seismic waves crust is further divided into (i) upper crust and (ii) lower crust because the velocity of P waves suddenly increases in the lower crust. Intermediate Layer-Conard identified another set of seismic waves termed as P'-S' waves on the basis of the study of Tauern earthquake of 1923. The velocities of these waves are intermediate between P-S and Pg-Sg sets of waves. P' and S waves travel at the rate of 6-7 km and 3-4 km per second respectively in the middle zone of the earth. It has been inferred on the basis of intermediate velocity of these waves that there is an intermediate layer with average density of 3 inside the earth. There is difference of opinion about the nature and type of the rocks of this intermediate layer. Accord ing to Daly and Jeffreys the intermediate layer con sists of glassy basalt whereas Wegener and Holmes have identified amphibolite as constituent rock of this layer. But most of the scientists are of the view that the intermediate layer is composed of basalt. Lower Layer-P and S waves penetrate upto greatest depth inside the earth. The velocity of P and S waves is 7.8 km and 4.5 km per second espectively. The highest velocity of seismic waves in the innermost part of the earth indicates an inner or lower layer of heavier materials, most probably peridotite or dunite. It is also possible that materials may be in non-crystalline, glassy state. The depth of this layer is estimated to be about 2900 km from the earth's surface.

Answer 91

Theories about state of core of earth/ original state of earth at the time of its formation 1. Planetisimal Hypothesis 2. Tidal hypothesis 3. Nabular Hypothesis Theories about Layers of earth 1. E. Suess 2. Daly 3. Arthur holmes Current Theory

Answer 92

1. Planetisimal Hypothesis: earth was originated due to accretion and aggregation of solid dust particles known as 'planetesimals'. Based on this corollary the core of the earth should be in solid state. 2. Tidal Hypothesis: core of the earth should be in liquid state because the earth has been taken to have been formed, according to this hypothesis, from the tidal materials ejected from the primitive sun. 3. Nabular hypothesis: core of the earth should be in gaseous state.

Answer 93

The crust is covered by a thin layer of sedimentary rocks of very low density. This layer is composed of crystalline rocks, mostly silicate matter. The dominant minerals are felspar and mica. The upper part of this layer is composed of light silicate matter while heavy silicate matter dominates in the lower part. Suess has identified three zones of different matter below the outer thin sedimentary cover. (i) Sial layer located just below the outer sedimentary cover is composed of granites. This layer is dominated by silica and aluminium (SIAL=SI+AL). The average density of this layer is 2.9 whereas its thickness ranges between 50 to 300 km. This layer is dominated by acid materials and silicates of potassium, sodium and aluminium are abundantly found. Continents have been formed by sialic layer. (ii) Sima is located just below the sialic layer. This layer is composed of basalt and is the source of magma and lava during volcanic eruptions. Silica (Si-Silica+ma-magnesium) and magnesium are the dominant constituents. Average density ranges be tween 2.9 to 4.7 whereas the thickness varies from 1,000 km to 2,000 km. There is abundance of basic matter. The silicates of magnesium, calcium and iron are most abundantly found. (iii) Nife is located just below 'sima' layer. This layer is composed of nickel (NI) and ferrium (Fe). It is, thus, apparent that this layer is made of heavy metals which are responsible for very high density (11) of this layer. The diameter of this zone is 6880 km. The presence of iron (ferrium) indicates the magnetic property of the earth's interior. This property also indicates the rigidity of the earth Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djgJt8mRZ_QMtJRI_?e=mNraCO

Answer 94

Daly has recognized three layers of different density in the earth. (i) Outer zone is compossed of silicates. Average density is 3.0 and the thickness is 1,600 km. (ii) Intermediate layer is composed of the mixture of iron and silicates. Average density is from 4.5 to 9 and the thickness is 1,280 km. (iii) Central zone is made of iron and is in solid state. Average density and diameter are 11.6 and 7,040 km respectively.

Answer 95

Arthur Holmes has recognized two major layers in the earth. The upper layer is termed as crust which is composed of whole of Suess' sialic layer and upper portion of 'sima'. The lower layer has been named by Holmes as a substratum which represents lower portion of Suess' sima. Holmes has determined the thickness of sial below the continental surface on the basis of different sources and evidences as given below. (i) On the basis of thermal conditions - 20 km or less. (ii) On the basis of surface seismic waves (L waves) - 15 km or more. (iii) On the basis of longitudinal (P waves) waves- 20-30 km. (iv) On the basis of subsidence of the deepest geosynclines - 20 km or more.

Answer 96

The interior of the earth is made up of several concentric layers of which the crust, the mantle, the outer core and the inner core are significant because of their unique physical and chemical properties. The crust is a silicate solid, the mantle is a viscous molten rock, the outer core is a viscous liquid, and the inner core is a dense solid. Mechanically, the earth’s layers can be divided into lithosphere, asthenosphere, mesospheric mantle (part of the Earth’s mantle below the lithosphere and the asthenosphere), outer core, and inner core. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.

Answer 97

The crust is the outermost layer of the earth making up 0.5-1.0 per cent of the earth’s volume and less than 1 per cent of Earth’s mass. Density increases with depth, and the average density is about 2.7 g/cm3 (average density of the earth is 5.51 g/cm³). The average density of the outer and lower crust is 2.8 and 3.0 respectively. evidences of seismology have revealed almost identical structure and composition of the two sub zones, upper and lower, of the crust. The difference of density between the upper (2.8) and lower crust (3.0) is because of the pressure of supperincumbent load. The thickness of the crust varies in the range of range of 5-30 km in case of the oceanic crust and as 50-70 km in case of the continental crust. The continental crust can be thicker than 70 km in the areas of major mountain systems. It is as much as 70-100 km thick in the Himalayan region. The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C to 400 °C at the boundary with the underlying mantle. The temperature increases by as much as 30 °C for every kilometre in the upper part of the crust. The outer covering of the crust is of sedimentary material and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature. The lower layer of the crust consists of basaltic and ultra-basic rocks. The continents are composed of lighter silicates — silica + aluminium (also called sial) while the oceans have the heavier silicates — silica + magnesium (also called sima) [Suess,1831–1914 ― this classification is now obsolete (out of date)]. The continental crust is composed of lighter (felsic) sodium potassium aluminium silicate rocks, like granite. In geology, felsic refers to igneous rocks The oceanic crust, on the other hand, is composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt. On the basic of seismic studies it has been inferred that a line of discontinuity exists betn continental crust and oceanic crust, called Conrad discontinuity. Most abundant elements of earth's crust: O (46.6%) > Si (27.7%) > Al (8%) > Fe

Answer 98

There is sudden increase in the velocity of seismic waves at the base of lower crust as the velocity of seismic waves is about 6.9 km per second at the base of lower crust but it suddenly becomes 7.9 to 8.1 km per second. This trend of seismic waves denotes discontinuity between the boundaries of lower crust and upper mantle. This discontinuity was discovered by A. Mohorovicic in the year 1909 and thus it is called as 'Mohorovicic discontinuity' or simply 'Moho discontinuity'. Mohorovicic (Moho) discontinuity forms the boundary between the crust and the asthenosphere (upper reaches of the mantle) where there is a discontinuity in the seismic velocity. It occurs at an average depth of about 8 kilometres beneath the ocean basins and 30 kilometres beneath continental surfaces. The cause of the Moho is thought to be a change in rock composition from rocks containing feldspar (above) to rocks that contain no feldspars (below).

Answer 99

The lithosphere is the rigid outer part of the earth with thickness varying between 10-200 km. It is includes the crust and the upper part of the mantle (upto depth of ~100 km). The lithosphere is broken into tectonic plates (lithospheric plates), and the movement of these tectonic plates cause large-scale changes in the earth’s geological structure (folding, faulting). The source of heat that drives plate tectonics is the primordial heat left over from the planet’s formation as well as the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle.

Answer 100

The mantle having mean density of 4.6 g cm³ extends to a depth of 2900 km inside the earth. Upper mantle with density of 4.5 while lower mantle with density 6.5 It may be mentioned that the thickness of the mantle is less than half of the radius of the earth (6371 km) but it contains 83 per cent of the total volume and 68 per cent of the total mass of the earth. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djhPmhWLrIXnpzUpu?e=VYaf71 the mantle is divided on the basis of the information received from the discovery of the International Union of Geodesy and Geophysics into 3 sub-zones e.g. (i) first zone extending from Moho discontinuity (7-35 km) to 400 km depth: The velocity of seismic waves relatively slows down in the upermost zone of the upper mantle for a depth of 100 to 200 km (7.8 km per second). This zone is called the zone of low velocity. (ii) second zone extending from 400 km to 660 km depth called Mantle transition zone and These two make up upper mantle. Upper Mantle (upto 670 km) made up of Peridotite and Gabbro rocks (iii) third zone extending from 660 km to 2900 km depth: lower mantle; rich in olivine Mechanically, mantle can be divided into upper mantle (part of lithosphere) and lower mantle divided by Asthenosphere Mantle is believed to have been formed largely of silicate minerals rich in iron and magnesium. Regarding its constituent elements, the mantle is made up of 45% oxygen, 21% silicon, and 23% magnesium (OSM). In the mantle, temperatures range from approximately 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core-mantle boundary. It is predominantly solid but in geological time it behaves as a viscous fluid. Because of the temperature difference, there is a convective material circulation in the mantle (although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile). Convection of the mantle is expressed at the surface through the motions of tectonic plates.

Answer 101

extends from the lower boundary of the mantle at the depth of 2900 km to the centre of the earth (upto 6371 km). The mantle-core boundary is determined by the 'Weichert-Gutenberg Discontinuity' at the depth of 2900 km. There is pronounced change of density form 5.5 g cm³ to 10.0 g cm³ along the Gutenberg Discontinuity. This sudden change in density is indicated by sudden increase in the velocity of P waves (13.6 km per second) along the mantle-core boundary or Gutenberg Discontinuity. The density further increases from 12.3 to 13.3 and 13.6 with increasing depth of the core. It, thus, appears that the density of the core is more than twice the density of the mantle but the volume and mass of the core are 16 per cent and 32 per cent of the total volume and mass of the earth respectively. The core is further divided into two sub-zones e.g, outer core and inner core, the dividing line being at the depth of 5150 km, called Lehman discontinuity It is generally believed that the core is composed of iron and nickel but according to the second view point the core may be formed of silicates. It is also believed that after disintegration on high pressure the electronic structures have changed into heavy metallic materials, thus the density of the core has increased. According to the third view point initially the core was composed of hydrogen but later on hydrogen was transformed into metallic materials due to excessive pressure (over 3 million atmosphere). This possibility is questioned on the ground that though the transformation of silicate or hydrogen due to very high pressure in the core may be believed tentatively but this process cannot increase the density of the core as high as it is at present. For example, the planet Mercury is smallest of all the planets of our solar system but its density is highest of all the planets. It may be argued that least compression and pressure cannot generate highest density in the core of Mercury. Most of the present day geophysicists and geochemists believe that the core is made of metallic materials mainly iron and nickel.

Answer 102

S waves disappear in this outer core. This means that the outer core should be in molten state. The outer core is composed of iron mixed with nickel (nife) and trace amounts of lighter elements. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core. The density of the outer core ranges from 9.9 g/cm3 to 12.2 g/cm3. The temperature of the outer core ranges from 4400 °C in the outer regions to 6000 °C near the inner core. Heat sources include energy released by the compression of the core, energy released at the inner core boundary as it grows (latent heat of crystallisation), and radioactivity of potassium, uranium and thorium. Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth’s magnetic field.

Answer 103

The inner core extends from the depth of 5150 km to the centre of the earth (6371 km). This lowermost zone of the interior of the earth is in solid state, the density of which is 13.3 to 13.6. inner core is generally believed to be composed primarily of iron (80%) and some nickel (nife) Seismic evidence of solid inner core: 1) based on arrival time of P waves an estimated speed: P waves travel through this zone with the speed of 11.23 km per second. 2) shear waves: detection of a wave called PKJKP that is formed as following: when P-wave meets the edge of inner core, some of it is transmitted into the inner core as an S-wave, which can travel through only if inner core is solid. When this S-wave re-transmitted into outer core as a P-wave and then returned to the surface, it is called PKJKP wave. Earth’s inner core rotates slightly faster relative to the rotation of the surface. At 6000°C, this iron core is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.

Answer 104

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Answer 105

Surge tectonics refers to the genesis of global surge waves caused by changes (weakening) in the regional gravity fields due to upward movement of deformable magma in the surge channels in the lithosphere (crust, 100-200 km thick upper layer of the earth) lying above mantle. Recently, surge tectonics involving tectonic activity within the crust has been related to climatic phenomena including El Nino. Infact, there is paradigm shift from traditional modelling of climate change based on 'ocean-atmosphere interactions' to 'earth dynamics (surge tectonics)-ocean-atmosphere interactions'. The asthenosphere, the lower part of the lithosphere (crust) is in partially molten condition wherein molten (fluid) magma is in motion. The lithosphere (crust) above hard mantle is characterized by a network of deformable magma channels which have been termed as surge channels. These surge channels are, in fact, conduits through which fluid magma moves upward from asthenosphere to upper part of the lithosphere. When the asthenosphere becomes too weak to support the lithosphere dynamically, the latter collapses into the former. The surge channel system, fluid magma and the collapse of lithosphere into dynamically weakened asthenosphere, are parts of 'GLOBAL GIANT HYDRAULIC PRESS SYSTEM'. Strictly speaking, surge tectonics means upward motion of fluid magma in surge channels (magma conduits), rise in temperature of regional oceanic water and consequent decrease in pressure and shift in regional gravity field of oceanic crust. The motions in the surge channels are caused by earth's rotation. Magma, while rising through the surge channels, undergoes its transformation (deformation) i.e. it becomes lighter (decrease in density) and less compact and hence expands. This consequent expansion in magma reduces gravitational attraction in the surge channels and weakens the regional gravity fields. The increase in seismic activity along East Pacific Rise (ridge), increase in sea level in the Pacific Ocean due to shift in regional gravity field and increase in temperature of ocean waters surrounding Indonesian archipilago etc. due to surge tectonics have been associated with El Nino phenomenon. Thus, it is concluded that El Nino phenomenon is related to surge tectonics and in turn the former affects weather and climate. This has been termed as a Gravitationally Earth Teleconnected Global Oscillation System (GETGOS) which controls climatic fluctuation

Answer 106

Dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field and sustains it over astronomical time scales (millions of years). Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect (caused due to the rotation of the earth), gives rise to self-sustaining (geodynamo) Earth’s magnetic field. Mechanism ->Earth’s magnetic field is generated in the earth’s outer core. -> Lower pressure than the inner core means the metal in the outer core is fluid. -> The temperature of the outer core ranges from 4400 °C in the outer regions to 6000 °C near the inner core. -> The differences in temperature, pressure and composition within the outer core cause convection currents in the molten iron of the outer core as cool, dense matter sinks while warm, less dense matter rises. -> This flow of liquid iron generates electric currents, which in turn produce magnetic fields. -> Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. -> This self-sustaining loop is known as the geodynamo. -> The spiral movement of the charged particles caused by the Coriolis force means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field of the planet. Diagram: https://1drv.ms/u/s!AvN_8sA-Zf0djhTcYLpu34S2R6PR?e=wA8RUw

Answer 107

The magnetosphere is the region above the ionosphere that is defined by the extent of the Earth’s magnetic field in space. It extends several tens of thousands of kilometres into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation. Many cosmic rays are kept out of the Solar system by the Sun’s magnetosphere called heliosphere. Due to the onslaught of solar winds, earth's magnetic field is distorted and magnetic fields on the sun facing side are pushed inward while the magnetic fields on the other side are pushed into a tail like shape. The former is called bow shock and the latter is called magnetotail. Earth's magnetosphere protects earth's atmosphere because in the absence of it, the solar charged high velocity particles would blast away the atm around earth, as is suspected to happen to Mars. As Mars' magnetic field died down, the incoming solar radiation blew away its atmosphere. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djhhN_loYDi3uvrDS Components of Magnetosphere: Bow shock, Magnetopause (shown in diag )

Answer 108

As solar winds clash with earth's magnetosphere, its shape changes depending on intensity of the winds. If the solar winds are weak, the magnetosphere expands and if they are strong, the magnetosphere compresses. During the changing of the shape of earth's magnetosphere, its magnetic lines break and reconnect according to new shape. This is called magnetic reconnection. Some solar charged particles enter earth's magnetosphere in this process and travel along the magnetic lines to reach the poles and collide with the ionosphere particles above the poles. The increased energy of ionosphere particles is released in form of colors. This results in Aurora Borealis and Aurora Australis

Answer 109

A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others may be temporarily created. By trapping the solar wind, the magnetic field deflects those energetic particles and protects the atmosphere from destruction The belts endanger satellites, which must have their sensitive components protected with adequate shielding if they spend significant time near that zone. Spacecraft travelling beyond low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and solar particle events. Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djhauTXS8yojDRuBz?e=3dFBVQ Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djhW9C7DwHbGjodC8?e=yqaifj

Answer 110

The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity. If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere and more of it gets in. Periods of intense activity, called geomagnetic storms, can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System. It takes just two days to reach the Earth. At the Earth’s surface, a magnetic storm is seen as a rapid drop in the Earth’s magnetic field strength. Ring Current: Ring current is the name given to the large electric current that circles the Earth above its equator during magnetic storms. Effects The ionosphere gets heated and distorted, which means that long-range radio communication that is dependent upon sub-ionospheric reflection can be difficult. Ionospheric expansion can increase satellite drag, and it may become difficult to control their orbits. Geomagnetic storms disrupt satellite communication systems like GPS. Astronauts and high-altitude pilots would face high radiation levels. Electric power grids would see a high increase in voltage that would cause blackouts. Geomagnetic storms disrupt satellite communication systems like GPS.

Answer 111

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Geomorphology Flashcards

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