Unit 5: Rocks and Weathering Flashcards
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Lithosphere
The rigid outer layer of the earth that includes the crust and the upper mantle
Asthenosphere
The upper layer of the earths mantle below the lithosphere in which there is relatively low resistance to plastic flow and convection is thought to occur
Igneous rocks
Forms from magma or lava solidification
Hard, no layers
Intrusive (granite): slow magma cooling
Extrusive (obsidian): rapid lava cooling
Sedimentary rocks
Forms fro sediment compaction
Crumbly, layers
Clastic (sandstone): compacted broken rocks
Chemical (limestone): compacted dissolved minerals
Organic (coal): compacted biogenic matter
Metamorphic rocks
Forms by transformation of other rocks
Relatively hard, may or may not have layers
Foliated (slate): has layers
Non-foliated (marble): no layers
Continental plates
Sial - silica and aluminium
30-40 km thick, 60-70 km under mountain chains
Comprises of numerous rocks, granite most common
Lighter, density of 2.7g/cm^3
Very old mainly over 1500 million years
Oceanic plates
Sima - silica and magnesium
6-10 km thick on average
Few types of rock, mainly basalt
Heavier, density of 3g/cm^3
Very young mainly under 200 million years
Convection current theory
States hughe convection currents occur in the earths interior. Hot magma rises through the core to the surface and spreads out at mid-ocean ridges. Cold solidified crust shrinks into earths interior because it is heavier and denser than surrounding materia. Cause as radioactive decay in the core
Dragging theory
Plates are dragged or subducted by their oldest edges which are cold and heavy. Plates are hot at the mid-ocean ridge but cold as they move away. Complete cooling takes about 1m years. As cold plates descend at the trenches pressure causes the rock to change and become heavier
Volcanic hotspots
A hotspot is a plume of lava that rises vertically through the mantle. Most are near plate margins and may have caused original rifting of the crust. They can cause movement outward flow of viscous rock from the centre may cause a drag force on the plates causing movement. A volcanic hotspot is an area of the mantle that experiences a plume of ultra hot magma causing volcanic activity on the surface. Narrow plume rises vertically from seismically slow lower pantle. Hotspot is stationary. Lithosphere moves
DIvergent (constructive) plate boudary
Plates moving apart from each other resulting in minor earthquakes and eruptions
Convergent (destructive) plate boundary
Subduction occurs resulting in large earthquakes and eruptions. Ocean trenches are also formed
Convergent (collision) plate boundary
2 or more continental plates collide with minimal subduction and creation of fold mountains. No eruptions but large earthquakes
Conservative/transform plate boundary
Plates slide past each other without subduction. Major earthquakes but no eruptions
Constructive boundary
Plates pulled apart by convection currents (sea floor spreading)
Both plates are oceanic
Mid-ocean volcano formed
Destructive boundary
Heavier oceanic plate
Oceanic plate sinks under continental plate (subduction)
Oceanic plate melts as it enters the hot mantle. Causes an increase in pressure
Volcanic eruption when too much pressure in mantle
Volcano formed above where plate melts
Lighter continental plate
Conservative boundary
Plates can be oceanic or continental
Pressure builds causing large earthquakes
Plates moving in opposite directions at slightly different speeds or angles
Alfred Wegener and Francis Bacon evidence
In 1912, Alfred Wegener had the idea of continental drift. Francis Bacon in 1620 stated how the shape of Africa was similar to that of South America. Wegener proposed continents were slowly drifting about the earth. Starting with the Carboniferous period 250 million years ago a large continent, Pangea, broke and began to drift forming continents
Harry Hess evidence
In the 20th century Harry Hess suggested convection currents would force molten magma up in the interior and crack the crust above. In the 1960’s research on rock magnetism supported this. The rocks of the mid-Atlantic ridges were magnetised in alternate directions in identical bands on both sides. This suggested that fresh magma came up through the centre and forced rocks apart. Increasing the distance from the ridge means rocks were older. Supported the idea that new rocks were being created at the centre of the ridge and older rocks are pushed apart
J Wilson evidence
In 1965 J Wilson linked continental drift and seafloor spreading into a concept of mobile belts and rigid plates, forming the basis of plate tectonics
Evidence of plate tectonics
Past and present distribution of earthquakes
Changes in earths magnetic field
Fit of continents
Glacial deposits in Brazil match those in west Africa
Fossil remains in India match those of Australia
Geological sequence of sedimentary and igneous rocks in parts of Scotland match those in Newfoundland
Ancient mountains can be traced from east Brazil to west Africa and from Scandinavia through Scotland to Newfoundland and the Appalachians
Fossil remains of na aquatic reptile, Mosasaurus (270 million years ago) are only in part of Brazil and south west Africa
Sea-floor spreading
The process by which new oceanic crust is formed at mid-ocean ridges and spreads outwards pushing older crust away from the ridge
Sea-floor spreading evidence
Wegener’s hypothesis of continental drift wasn’t widely accepted because he had no mechanism to explain how continents move. The idea was not received until new technology made ocean floor exploration possible. Harry Hess proposed seafloor spreading in which basaltic magma from the mantle rises to create new ocean floor at mid ocean ridges
What happened in 1948?
A survey of the floor of the Atlantic ocean revealed a continuous ridge running from north to south
1000 km wide
Heights of 2.5 km
Composed of volcanic rock
Similar found in Pacific ocean
Palaeomagnetism
Iron particles in lava erupted on the ocean floor are aligned with the earth’s magnetic field
As the lavas solidify these particles provide a permanent record of the earths polarity at the time of eruption
Geomagnetic polarity reversals
The earths polarity reverses at regular intervals (400000 years)
The result as a series of magnetic stripes with rocks aligned alternately towards the north and south poles
The striped pattern which is mirrored exactly on either side of a mid-ocean ridge suggest the ocean crust is slowly spreading away from the boundary and new rocks are being added equally on both sides
Age of the ocean floor
Very young places on or near ridges
Much older ages were recorded for floor rocks near continental masses (200m years)
Older crust is continuously being pushed aside by new crust
Ocean ridges
Giant submarine mountain ridges that have heights up to 3000m
Found on constructive plate boundaries where new lithosphere is created
The first ocean ridge discovered was the mid-Atlantic ridge. It was found when engineers were attempting to lay a submarine cable from north America to Europe
Subduction zone
The area where an oceanic plate meets another plate and sinks below
The oceanic plate has a similar density to the asthenosphere below and can easily be pushed down below the continental
Subduction zones dip between 30 and 60 degrees but each will get steeper with depth. The older a crust is the steeper the dip due to the increased density relative to the asthenosphere
The size of the earth is constant so the amount of land destroyed at the subduction zone is equal to the amount of land created at constructive plate boundaries
Ocean trenches
Long, narrow depressions in the ocean floor with depths of 6000-11000m
They occur as a result of subduction zones and are often found near land or island arcs
They are asymmetrical with the steeper side closest to the land mass
Benioff and Wadati zone
AN area that extends below ocean trenches to a depth of 68-km where earthquakes often occur
Was named after seismologists Hugo Benioff and Kiyoo Wadati
Known for deep-focus earthquakes
Zone of Seismicity corresponds to the down-going slab in a subduction zone
Dip can be 30-60 degrees
The Benioff zone may extend from near surface to depths of 650-700 km
Most earthquakes occur within 1000 C isotherm due to internal deformation and dehydration embrittlement of the subducting slabs
What is a hotspot?
Volcanic hotspots are created as plumes of magma rise through the earths mantle. Mantle plumes are long-lived areas of high levels of heat flow in the mantle. Plumes consist of an upwelling long thin conduit and a bulbous head with spreads at the base of the lithosphere and produces a lot of magma, due to partial melting with arises from a drop in pressure. These have their origin at the Gutenberg Discontinuity which is the boundary that divides the outer core and mantle (2900km). It is believe that heat from the core is passed to the mantle by conduction and heat portions of the lower mantle become less dense and buoyant and rise as diapirs which become mantle plumes. The magma build-up will create volcanic activity on the ocean floor as it breaks through weak areas. The volcanoes may break through the surface of the ocean waters to form islands. The hot spot stays in the same place but because the plate above moves over time it creates a chain of islands with the oldest being furthest from the hotspot. Hotspots have seismic and volcanic activity. Volcanic earthquakes are generated by magma and tectonic earthquakes occur because of structural weakness at the base of volcanoes or in the crust below
Fold mountain building
Created when 2 or more tectonic plates are pushed together. As they collide, compressing boundaries, rocks and debris are warped/folded into rocky outcrops ,hills, mountains and ranges. Created through orogeny. This event takes millions of years. Nappes are common, dramatic folded rocks or rock formations. Earths crust is warped into folded form. Often associated with continental crust. Created at convergent boundaries (collision or compression). At a compression zone tectonic activity forces crustal compression at the leading edge of the crust so most fold mountains are found on the edge or former edge of a continental boundary. Rocks on the edge are weaker and less stable so more susceptible to folding and warping. Most are made of sedimentary rock and metamorphic rock formed under high pressure and low temperatures
Types of folds
Fold mountains are defined by folds and their shape. Usually more than 1 type in a mountain. Anticlines and synclines are the most common up and down that result from compression. Anticline is an upside down U shape with the oldest rocks in the centre. A syncline has a U shape with the youngest rocks in the centre. A dome is a series of symmetrical anticlines. Oldest in the centre. A basin is a depression in the earths surface. Youngest in the centre
Monocline
A type of fold in which all layers incline or dip in the same direction
Chevron fold
A sharp, straight fold where rock layers are like zig-zags
Slump fold
A result of slope failure. This happens when sediments were soft before they became a single mass. Lithified to become a slump
Ptygmatic folds
A type of slump fold created when the folding material is more viscous than the surrounding material. Many are created as metamorphic rock melts and intrudes into another layer forming a dike
Disharmic folds
Where different rock layers have different fold shapes
Volcanic island arcs
Long, curved chain of oceanic islands associated with intense volcanic and seismic activity and orogenic processes. Most consist of 2 parallel, arcuate rows of islands. The inner row is composed of a string of explosive volcanoes and the outer row is made up of non-volcanic islands. With single arcs, many constituent islands are volcanically active. An island arc typically has a land mass or partially enclosed, shallow area on its concave side. Along the convex side there is a long, narrow deep-sea trench. Destructive earthquakes occur often. These are deep focus seismic events from 370 miles below an arc. They tend to have a foci of progressively greater depth towards the concave side
How are volcanic island arcs formed?
Formed when 2 lithospheric plates converge and one oceanic is forced into the partly molten lower mantle below the continental. An island arc is built from the surface of the overriding plate by extrusion of basalts and andesites. The basalts are from semi-molten mantle and andesites from the partial melting of the descending plate
Features of volcanic island arcs
Ahead of the subduction zone there is a low bulge on the sea floor (trench outer rise) causing by the bending of the plate as it subducts. The outer slope of the trench is generally gentle but broken by faults as the plate bends. The floor of the trench is flat and covered by sediment/ash. Te trench inner slope is steeper and contains fragments of the subducting plate. The subduction complex (or accretionary prisms) is the slice of descending slab and may form significant landforms. Most subduction zones contain an island arc parallel to a trench on the overriding plate. 150-200 km from trench
Weathering
The decomposition and disintegration of rocks in situ
Decomposition
Chemical weathering
Disintegration
Mechanical weathering that breaks the rock into smaller fragments
In situ
In the natural or original position or place
Weathering vs erosion vs deposition
Weathering is the breaking down or dissolving of rocks and minerals on earths surface
Erosion is the transportation or movement of the weathered materials
Deposition is the dropping of the weathered material
Features of weathering
Many minerals are formed under high pressure in the earths core. As they cool nearer the surface they stabilise
Can cause irreversible changes to rocks. Some can change from a solid to clastic (fragmented) state. Some can change from solid to plastic (pliable) state
Can change the volume, density, grain size, surface area, permeability, consolidation and strength of rocks
It can create new minerals and solutions
Some minerals can be very resistant to it
Minerals and salts may be removed, transported, concentrated or consolidated
It creates new landforms and features
Freeze-thaw weathering
Also called frost action is weathering due to freezing temperatures. Water gets into the cracks of rocks, freezes and expands by about 10%. This puts pressure on a rock causing it to shatter and break off
Heating/cooling processes in weathering
Repeated heating and cooling of rocks can cause them to be broken down and weathered away. Rocks expand rapidly when hot and contract rapidly when cold causing the breaking apart of layers of rock. The changes in temperature cause stress on the outer layers of the rock so they peel off (exfoliation)
Salt weatheirng
Occurs due to salt crystal growth in the cracks and pores in rocks. When saline solutions get into cracks and evaporate, it leaves the salt crystals that were in the solution. As these crystals accumulate over time, the build up of pressure expands the gap in the rocks. This causes rocks to break off or disintegrate
Pressure release weathering
Caused when rocks that are under a lot of pressure no longer have to bear a heavy load causing expansion and fracturing. When there is a removal of weight, the underlying rocks will expand when the pressure is released causing fractures to form on the rock surface
Biological weathering
The action of burrowing animals or growing roots to destabilise rocks. As the plant grows, the roots enter cracks in the rock under the soil. As the plant and roots grow, the roots cause the crack to get larger and rock breaks away
Chemical weathering
The decomposition of rock from a chemical change. This is often the result of the interaction between rocks and moisture which leads to dissolved particles and the formation of clays. More likely to take place in warm, moist vegetated areas
Oxidation
Occurs when rocks become exposed to air. Often can be seen as earth is moved, holes are dug or mass movement exposes underlying rock and soil. Iron-rich soils and rocks may appear grey or blue until exposed to air. Previously ferrous soil and rock will oxidise and change to a ferric state, turning rusty. This also occurs in iron-rich metals that have become exposed to air
Hydrolysis
Particularly significant in the decomposition of rocks to form clays. It is the process by which chemical bonds are broken and the components partner to form different elements. It is a reaction involving the breaking down of a bond in a molecule with water. Mainly occurs between a hydrogen ion in water and changes a solutions pH. On granite landscapes, hydrogen in water reacts with minerals in rock and are washed through attaching to silicic acid and potassium hydroxyl in chelation. Positive ions attach to negative ions. The product is a fine grey clay (kaolin)
Acidification
The process by which liquids become acidic and is a common way in which rocks and minerals are dissolved. The greater the concentration of acids, the greater the effect of weathering
Carbonation
The process where carbon dioxide, often from rainwater produced carbonic acid. This is a weak acid solution that reacts with calcium carbonate rocks
Acid rain
Caused by greater concentrations of carbon dioxide, sulphur dioxide and nitrogen oxide due to human activities. These gases form acids as they combine with water vapour as rain
Hydration
Related to the absorption of water by rocks. Certain rocks are particularly capable of absorbing water into pores and cracks. the rocks swell sometimes repeatedly in dry and wet conditions and can change state
