Tectonics

Question 1

What are the Layers of the earth?

Answer 1

Inner core, Outer core, Mantle, Crust

Question 2

Characteristics of oceanic crust?

Answer 2

• Newer, most less than 200 million years old
• Denser
• Can be subducted (sink)
• Can be renewed and destroyed

Question 3

Characteristics of Continental crust?

Answer 3

• Older, most over 1500 million years old
• Less dense
• Cannot be subducted (sink)
• Cannot be renewed and destroyed

Question 4

What is Pangaea?

Answer 4

The name given to the landmass that was all of the continents joined together.

Question 5

What Evidence did Wegener give to back up his theory of Pangaea?

Answer 5

Continental fit – some continents seem to fit together

Geological evidence – rocks of the same age, type and formation are found in SE Brazil and South Africa; mountain formations are similar in eastern USA and NW Europe; glacial deposits are similar in Antarctica, South America and India

Climatological evidence – Antarctica, North America and UK all have coal deposits of similar age formed in the same tropical zones during the Carboniferous period

Biological evidence – similar fossil formations are found on either side of the Atlantic. Similar plants and animals found in coal deposits in India and Antarctica.

Question 6

Evidence for plate tectonics, Pangaea and Sea Floor Spreading?

Answer 6

Mid Atlantic Ridge – in 1948 geologists discovered a continuous ridge running along the bed of the Atlantic Ocean from North to South – like a seam between S America and Africa/N America and Europe

Palaeomagnetism – in 1950s geologists observed regular palaeomagnetic striping spreading from the ocean floor ridges. When lava erupts on the ocean floor, the iron-rich minerals in the lava align themselves with the magnetic field of the Earth. As lava cools it acts as a record of the Earth’s polarity at the time of cooling (bit like the rings around a tree trunk). Earth’s polarity reverses every 400,000 years, this is recorded in the lava either side of the ridges. This suggested to geologists that the ocean floor was spreading over time.

Subduction – surveys recorded very young rock near or around the ridges with much older rock nearer the continental masses. Older crust is continually being pushed aside by new crust. BUT the Earth’s crust is not getting bigger, therefore old crust must be consumed (subducted) elsewhere. Subduction occurs when two plates move towards each other and one plate slides under the other, moving down into the mantle (lithosphere). The more dense oceanic crust moves below the less dense, more buoyant continental crust.
Continental crust consists therefore of older, more complex rock types.

Convection currents in the mantle (asthenosphere) – zones of hotter, more liquid magma exhibit a continuous circulatory motion in the asthenosphere that causes the crustal plates to move. Pulling apart the crust at ocean ridges and rift zones, and pulling oceanic crust down at subduction zones.

Question 7

Name all of the Theorists and their ideas

Answer 7

Lyell (1830): ‘Principle of Geology’, Temple of Serapis, book is the cornerstone of modern geology

Wegener (1912): continental fit, continental drift, termination of mountains, cross-continental fossilizations

Hess (1948): paleomagnetism & sea floor sreading

Dorman & Barazangi (1969): seismic & aseismic zones, linear seismic patterns at margins

Stewart (2013): Africa: Victoria falls & basalt, Great Pyramids & numelites, Wadi al Hitan, West African craton, eclogite & subduction, Rift Valley, Red Sea inundation. Australia: glossopteris fossilizations in coal deposits similar to Antarctica; Great Artesian Basin & opal formation

Question 8

What is a plate margin?

Answer 8

Plate margins (sometimes called boundaries) are where two plates meet.

Question 9

Describe Ocean Trenches

Answer 9

• Form the deepest parts of the Earth
• Occur at destructive (subduction) margins
• As oceanic plate is subducted beneath the continental plate creates deep ocean trenches. The deepest ocean trenches are created by oceanic-oceanic subduction.
  -Mariana trench is deepest part of the world’s ocean.
  Create by the subduction of the Pacific plate beneath the Mariana plate (oceanic-oceanic)

Question 10

Describe Young fold mountains and how they are formed

Answer 10

• Formed in last 65 million years, young fold mountains are the highest areas of our planet.
• Ranges include the Himalayas, the Rockies, the Andes and the Alps.
• Older fold mountains like the Cumbrian mountains have been worn down via erosion
• Found at destructive (subduction) and destructive (collision) plate margins

How they are formed:

1. Geosynclines are huge depressions found naturally on the ocean floor
2. Rivers deposit sediment in these geosynclines
3. Sediments are compressed and turned to sedimentary rocks like limestone
4. Plates are forced together at destructive margins (can be subduction or collision)
5. Sedimentary layers are forced upwards into fold mountains

Question 11

Describe Conservative Margins?

Answer 11

Often referred to as passive or transform slip margins – occur where two plates are moving parellel to each other
E.g. San Andreas Fault in California, and Alpine Fault in New Zealand
Areas of seismic activity, but not vulcanism

Question 12

Describe Hot Spots?

Answer 12

Volcanoes can be created entirely independently of plate margins
E.g. The Hawaiian Islands where formed in the middle of the Pacific Ocean, some 3000km from the nearest plate margin
Plumes of superheated, less dense magma rise from deep within the asthenosphere up into the lithosphere and form basaltic shield volcanoes on the ocean floor.
As the shield volcano erupts, builds up over time to form an island.
As the island becomes part of the plate it moves away from the magma plume (the hot spot) and a new island can be created from out of the lithosphere.
This sequence can form a chain of islands if the plate is moving quickly enough over the hot spot.

Question 13

What are constructive plate boundaries?

Answer 13

A constructive plate boundary, sometimes called a divergent plate margin, occurs when plates move apart. Volcanoes are formed as magma wells up to fill the gap, and eventually new crust is formed. An example of a constructive plate boundary is the mid-Atlantic Ridge.

Question 14

Describe the formation of Oceanic Ridges

Answer 14

As two plates pull apart they form a weaker zone in the crust. This zone is exposed to high temperatures.
High temperatures cause this weaker crust to expand and stretch, forming a ridge.
The split in the crust provides a lower pressure zone where more liquid lava can erupt to form submarine volcanoes.
Continued eruptions may cause volcanoes to develop until they reach the surface – islands can be formed this way (e.g. Iceland was formed by the Mid Atlantic Ridge, Surtsey was created in 1963 by the same ridge.

Question 15

Describe the formation of rift valleys

Answer 15

Rift valleys occur where two continental plates move
apart. As the sides of the rift move apart, central sections drop down. This creates large parallel depressions – known as rift valleys. As this process continues the valley gets wider and wider until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. Further rifting, widening and deepening can result in the valley being inundated by the sea (as is the case where the rift valley widens into the Red Sea) E.g. Great East African Rift Valley

Question 16

Describe Extrusive landforms

Answer 16

Extrusive landforms are created by igneous activity at the surface of the earth. There is wide range of volcanic landforms that includes large depressions (calderas and Maars), conical mountains (volcanoes), and broad lava plains and plateaus.

Question 17

Describe Intrusive landforms

Answer 17

Intrusive volcanic landforms are collections of cooled magma that formed within the Earth’s crust and were later exposed by surface erosion and uplift. The collective term for these land forms is pluton.

Question 18

Describe basaltic lava

Answer 18

Basaltic (basic) lava – originates from the upward movement of mantle material. Most common around spreading ridges, hot spots and rifts. RUNNY

Question 19

Describe Andesitic lava

Answer 19

Andesitic (intermediate) lava – found at destructive plate margins where crust is being destroyed. SEMI-VISCOUS. Eruptive & volatile.

Question 20

Describe Rhyolitic lava

Answer 20

Rhyolitic (acid) lava – found at destructive and collision margins. VISCOUS. Eruptive & highly volatile

Question 21

Describe Pyroclastic debris

Answer 21

Pyroclastic debris – volcanic fragments ranging from fine ash and lapilli to large volcanic bombs. Pryoclastic eruptions occur under intense pressure – more viscous lavas (rhyolitics) block vents creating violent eruptions. Incandescent shards can be released. Ash can mix with rainfall/snowmelt to form lahars (mudflows)

Question 22

What is a fissure?

Answer 22

A fissure vent, also known as a volcanic fissure or eruption fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few meters wide and may be many kilometers long. When two plates move apart lava is released through the fissures rather than through a central vent. These volcanoes are common
in Iceland.

Question 23

Describe shield volcanoes

Answer 23

These have very wide, gently sloping sides where basic runny lava has flowed out of a central vent. e.g. Mauna Loa on Hawaii.

Question 24

Describe acid/dome volcanoes

Answer 24

These are steep, convex sided volcanoes where
the viscous lava has solidified quickly near to the crater. If the lava solidifies in the vent a spine is formed which pushed up by subsequent eruptions. e.g. Mt Pelee in Martinique.

Question 25

Describe ash/cinder cones

Answer 25

Here ash and cinders build up layers to form a symmetrical cone with slightly concave sides e.g. Paricutin in Mexico.

Question 26

Describe composite cone volcanoes

Answer 26

Composite cones: are formed from alternating gentle eruptions releasing acidic lava and followed by violent eruptions releasing ash. These materials build up a steep sided cone. Parasitic cones may develop from the main vent.

Question 27

Describe Calderas

Answer 27

The magma chamber below the volcano may be cleared out by gas build up being released in huge explosions of acidic lava and some ash. This causes the sides of the crater to be removed, widening the crater which can fill with water to form a lake or if below sea level a lagoon. These are called calderas.

Question 28

List and describe the minor extrusive landforms

Answer 28

1. Geysers are formed when water in the lower crust is heated by hot rocks making steam which with increased pressure causes the water and steam
   to explode onto the surface. e.g. Old Faithful in Yellowstone and Geysir in Iceland
2. Hot springs are formed when water seeps into underground hot volcanic rocks in a volcanic region, it can be heated to boiling point (1000C). This heated water can then flow back to the Earth’s surface continuously and quietly. It also contains minerals dissolved by the water
3. Mud pots are formed in geothermal areas where water is in short supply. The little water there is rises to the surface at a spot where the soil contains a lot of volcanic ash or clay. Grey or white boiling mud is formed as a viscous, hot slurry often squirting over the top of the mud pot.

Question 29

List and describe the intrusive landfroms

Answer 29

Intrusive landforms include batholiths, dikes, sills, plutons and laccoliths.

Batholith
A large, deep seated mass of magma that cools very slowly to form coarse grained rock. Can be several hundred of kilometres wide (e.g. Cornubian Batholith, Cornwall).

Sill
A horizontal intrusion along the bedding planes with vertical cooling cracks. Cools rapidly, contracts and cracks (e.g. Whin Sill, North Yorkshire)

Dike (or dyke)
A vertical intrusion with horizontal cooling cracks that cuts across the bedding planes. Cools rapidly and cracks (e.g. Kildonan, Arran in Scotland)

Question 30

What is plageomagnetism?

Answer 30

When lava erupts on the ocean floor, the iron-rich minerals in the lava align themselves with the magnetic field of the Earth. As lava cools it acts as a record of the Earth’s polarity at the time of cooling (bit like the rings around a tree trunk). Earth’s polarity reverses every 400,000 years, this is recorded in the lava either side of the ridges. This suggested to geologists that the ocean floor was spreading over time.