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Physical Georgaphy AS > Rocks + Weathering > Flashcards

Rocks + Weathering Flashcards

1
Q

What is the lithosphere?

A
  • broken up into plates
  • majority of the lithosphere is within the mantle
  • the top of the lithosphere is the crust which is the land and sea
  • brittle, cool, rigid
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2
Q

What is the Asthenosphere?

A
  • hot, weak, plastic
  • semi-molten layer constantly moves due to flows of heat (convection currents) powered by heat from the core
  • below lithosphere
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3
Q

What is the mesosphere?

A
  • below asthenosphere
  • hot, but stronger due to pressure
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4
Q

Constructive/Divergent plate boundary

A

Oceanic/oceanic
- magma rises from gap left by 2 plates separating - forming new land when it cools
- less explosive underwater volcanoes formed as magma rises = ocean ridge
- new land forming on the ocean floor by lava filling the gaps = sea floor spreading

Continental/continental
- any land in the middle of the separation is forced apart = rift valley
- volcanoes form where magma rises
- eventually the gap will most likely fill with water + separate completely from the main island

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5
Q

Convergent/destructive plate boundary (continental + oceanic)

A
  • oceanic + continental plates forced together due to convection currents
  • oceanic plate subducts since its denser
  • in the Benioff zone, crustal melting occurs + magma is forced through cracks in the continental plate to form volcanoes
  • subduction plates drags down crustal material to form an ocean trench
  • fold mountains also occur on continental
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6
Q

Conservative plate boundaries

A
  • plates slip past each other with relative horizontal movement
  • no plates are destroyed so no landforms form
  • plate movement causes a lot of pressure build up = earthquakes
  • on the continental crust fault lines can occur where the ground is cracked by the movement
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7
Q

Subduction zones + ocean trenches

A
  • subduction zones occur when an oceanic plate is pushed into the atmosphere - as it is cooler than the surrounding magma, it is denser + sinks
  • plate creation at ocean ridges is roughly equal to the plate destroyed by subduction zones
  • ocean trench is found at the subduction zone - landform created by buckling of subducting plate (also forms a bulge)
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8
Q

What is the Benioff zone?

A

A narrow zone of earthquakes dipping away from the deep-sea trench

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9
Q

How are fold mountains formed?

A
  • ocean plate meets a continental plate
  • denser oceanic plate subducts
  • movement + subduction of the oceanic plate is caused by convection currents
  • this pushes sedimentary rock on lighter less dense continental plate upwards and buckles
  • ophiolite is thrusted into fold mountains
  • e.g. Andes
    OR
  • two continental plates meet - both folded + buckled
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10
Q

How were the Himalayas formed?

A
  • collision of the Eurasian + Indian plates (both continental)
  • sediment accumulated on the continental shelf + slope
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11
Q

What are ocean ridges?

A
  • linear features found on divergent (constructive margin)
  • e.g. Mid Atlantic Ridge
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12
Q

How are volcanic island arcs formed?

A
  • caused by subduction on a convergent plate boundary - two oceanic plates (no fold mountains)
  • bulge forms due to bending of plate, trench forms on margin but can be sediment filled (trench more prominent on destructive boundaries i.e oceanic + continental)
  • volcanic island arc caused by rising magma from melting plates
  • form arcuate (fan shaped chain) of islands
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13
Q

What are the types of physical/mechanical weathering?

A
  • freeze-thaw
  • salt-crystallisation
  • heating + cooling (exfoliation)
  • pressure release/dilation
  • vegetation roots + burrowing animals
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14
Q

What is freeze-thaw?

A
  • water expands as it cools - by 10% between 6 to 0°C
  • this puts pressure on the surrounding rocks
  • most effective in areas with cycles temperature above and below 0°C
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15
Q

What is salt crystallisation?

A
  • salt crystals are left behind by evaporation of salt water
  • sodium carbonate (Na2CO3) + sodium sulphate (Na2SO4) expand by 300% as temperate rises from 26-28°C
  • eventually causing rocks to break off or disintegrate
  • occurs on coasts + desert regions
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16
Q

What is heating + cooling (exfoliation)?

A
  • occurs in desert areas where temperature can be 40°C during the day + freezing at night
  • this causes rocks to expand + contract unevenly
  • the process occurs repeatedly + the rocks separates into layers + breakaway - exfoliation
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17
Q

What is pressure release/dilation?

A
  • overlying rocks removed - exposing granite which has formed under extreme pressure
  • the granite expands slightly - 1% + can crack
  • this creates what looks like bedding planes (pseudo)
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18
Q

Where does chemical weathering occur?

A
  • most prevalent sub-surface
  • where percolating water has picked up organic acids
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19
Q

Examples of chemical weathering?

A
  • carbonation/solution
  • hydrolysis
  • hydration
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20
Q

What is carbonation?

A
  • occurs on rocks containing calcium carbonate (limestone/chalk)
  • rainfall contains carbonic acid from dissolved carbon dioxide, which reacts with calcium carbonate CO2 + H2O -> H2CO3
  • this reaction forms calcium bicarbonate which is soluble + is washed away CaCO3 + H2CO3 -> Ca(HCO3)2
  • can affect other rocks too - bauxite (aluminium) - but only if pH is below 4
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21
Q

What is hydrolysis

A
  • occurs on rocks containing orthoclase feldspar
  • feldspar reacts with rainwater to form kaolinite (clay)
  • acid + hydroxyl are removed in solution leaving kaolinite
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22
Q

What is hydration?

A
  • minerals absorb water, expand + change -> both chemical + physical
  • anhydrite absorbs water + forms gypsum and expands by 0.5%
  • CaSO4 -> CaSO42H20
  • some shales can expand by up to 1600%
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23
Q

What is weathering?

A

Weathering is the disintegration (mechanical) and cleocomposition (chemical) of rocks in situ (in one place -not erosion)

24
Q

How does rock type influence weathering?

A
  • chemical composition - e.g. limestone consists of calcium carbonate + is therefore susceptible to carbonation/solution
  • the nature of cements in sedimentary rock - e.g. iron-oxide based cements are prone to oxidation, where as quartz cements are very resistant
25
Q

How does rock structure influence weathering?

A
  • joint patterns exert a strong control on water movement -> acting as lines of weakness, creating differential resistance with the same rock type
  • grain size influences the speed at which rocks weather -> coarse-grained rocks weather quickly due to a large void space + high permeability, fine-grained rocks offer a greater surface area making them highly susceptible to weathering
  • mineral type -> rocks formed of resistant minerals such as quartz, muscovite + feldspar in granite, will resist weathering, rocks formed from weaker minerals will weather rapidly
26
Q

How does vegetation influence weathering?

A
  • vegetation weathers rocks through secretion of organic acids - chemically weathering the soil
  • the growth of roots - physically weathers the soil
  • depth of soil may have an affect on the amount of weathering that occurs -> soil may protect rocks from further breakdown or they may increase the rate of weathering due to the vegetation it supports
27
Q

How does relief influence weathering?

A
  • for weathering to continue, weathered material needs to be removed
  • if the slope is too shallower, removal might not occur
  • if the slope is too steep, water may flow over the surface
  • therefore, intermediate slope angles produce the most weathering
  • also temperate difference between north and south facing slopes could influence weathering -> only if the difference is around a critical temperature e.g. around 0°C for freeze thaw
28
Q

What does the peltier model show?

A
  • the relationship between temperature + rainfall, and the different weathering types that prevail
  • van’t Hoff’s law -> weathering increase 2-3x for every 10°C
  • with temperature around 0°C - both chemical + physical occur
  • in cold conditions + low precipitation - likely to be no chemical weathering - mechanical therefore dominant (becomes more severe as precipitation increases)
  • in high temperature + high precipitation- chemical occurs
  • but in too hot condition there is little moisture so chemical can’t occur - some exfoliation in high T°C
29
Q

What is mass movement?

A
  • the downslope movement of material under the influence of gravity
  • it occurs when the shear force applied to the slope exceeds the shear strength of the material
  • shear strength is decreased by undercutting or through a loss of cohesion due to saturation
  • shear force is increased by slope loading or transient forces such as earthquakes
  • mass movement is characterised by speed + water movement
30
Q

What is rain splash?

A
  • when the impact of individual raindrops dislodge grains
  • they start to move downslope under the influence of gravity
  • most effective on bare slopes/ agricultural land
31
Q

What is sheet wash?

A
  • involves the falling of raindrops + their merging to form a near-continuous sheet of water due to water not being able to infiltrate
  • type of overland flow
  • top layer of the slope is transported downhill
  • which moves down hill slopes, gathering momentum + representing an erosive force of high potential
  • occurs when the gradient is too low for the water to concentrate together into streams
32
Q

What are rills?

A
  • semi-permanent channels cut onto gentle sloping areas due to erosive flowing water moving downwards naturally with gravity
  • normally on agricultural land/bare land
  • if they become permanent, they are called Gullies
33
Q

What are the main types of mass movement?

A
  • creep/solifluction
  • debris flows + lahars
  • slope failure - slides + falls
34
Q

Factors that increase shear force?

A
  • loading of a slope - water or vegetation
  • lateral pressure - water freezing in cracks, swelling due to hydration
  • transient stress - earthquakes
35
Q

Factors that decrease shear strength?

A
  • reduction of lateral support/ undercutting
  • weathering effects - hydration of shale/clay or carbonation etc.
  • changes in structure- cracks/fissures in shale, remoulding of sands etc.
  • organic effects - burrowing/ roots
36
Q

Role of water in mass movement?

A
  • water reduces the cohesion of particles by saturation -> decreases frictional strength
  • percolation/sheet wash will also remove finer particles
  • also increases the weight - due to rising water tables, prolonged rainfall
37
Q

What is creep/solifluction/heave?

A
  • the slow downslope movement of unconsolidated material due to the influence of gravity - rarely more than 1-3 mm a year
  • clay rich materials are liable to plastic flow - moving due to increased weight when saturated
  • freeze thaw can lead to heave where expansion of water leads to bulging of the soil + the soil piles up at right angles to the slope, thawing drops the soil causing downslope movement - this process is called heaving
  • wetting + drying can have the same effect as freeze thaw
  • in areas of permafrost - waterlogged conditions lead to accelerated creep - solifluction -> creating small ridges on the side of hillsides called terracettes
38
Q

How do slumps occur?

A
  • occurs on weaker rocks + clay
  • rotational movement along a slip plane
39
Q

How do flows occur?

A
  • soils/clays weaken by erosion + loses cohesion due to saturation -> the cohesive bonds within the soil break, causing the soil to move downhill like a viscous liquid
  • tends to be smaller material (sand sized)
  • high water content leads to mudflows after heavy rainfall (less infiltration)
  • lower water content leads to earth flows
40
Q

How do slides occur?

A
  • fast movement of rocks which mostly occurs with weak rocks on steep slopes - often the slip plane is a bedding plane
  • occurs along faults in the rock
  • an entire mass of material moves along the slip plane but the material can break up when it reaches the bottom of the slope
41
Q

How do falls occur?

A
  • occurs on steep slopes - greater than 45°
  • rocks fall under gravity
  • can be triggered by freeze-thaw, seismic waves or gravitational stress
42
Q

Methods of control for falls?

A
  • flattening the slope
  • netting/pinning
  • drainage
  • reinforcement of rock walls by grouting with cement, anchor bolts
  • covering of wall with steel mesh
43
Q

Methods of control for slides + slopes

A
  • grading or benching to flatten the slope
  • drainage of surface water with ditches
  • sealing surface cracks to prevent infiltration
  • buttresses roots
44
Q

What is pining?

A
  • used to attach wire nets (or sometimes concrete blocks) to a rock face or slope so that the risk of rock falls is reduced or the risk of erosion is reduced
  • often used on steep faced cliffs next to roads etc.
45
Q

What is netting?

A
  • netting may help collect fragments of scree, which can be safely removed at a later date
  • this is often used in areas where tourism is important, and where the risk of rock fall is high
46
Q

Difference between pinning and netting?

A
  • netting prevents scree or debris (small rocks) compared to pinning which prevents larger movements of material
47
Q

What is grading?

A
  • pre-profiling of slopes so that they become more stable
  • afforestation is the planting of new forest in upper parts of a catchment to increase interception + reduce overland flow
  • they may take many years to be effective as the young, immature trees intercept relatively small amounts of water
48
Q

What are the effects of mass movement flow on the slope?

A
  • shape of slope changes
  • material moved from top to bottom due to gravity
  • development of scar at source
49
Q

What are the small-scale movements of sediment on slopes?

A
  • rainsplash
  • heaves
  • rill action
50
Q

Convergent/ destructive plate boundaries (oceanic/oceanic)

A
  • heavier plate subducts leaving an ocean trench - fold mountains will also occur
  • build up pressure causes underwater volcanoes bursting through ocean plate
  • lava cools + creates new land - island arcs
51
Q

Convergent/destructive plate boundaries (continental/continental)

A
  • both places are not as dense as oceanic - lots of pressure builds
  • ancient oceanic crust is subducted slightly - but no subduction of continental crust
  • pile up of continental crust on top of lithosphere due to pressure between plates = fold mountains
52
Q

What forms at a conservative plate boundary

A
  • earthquakes
53
Q

What forms at a constructive/divergent plate boundary

A

C/C = volcanoes, rift valleys, earthquakes
O/O = ocean ridges, earthquakes, volcanoes

54
Q

What forms at a destructive/convergent plate boundary?

A
  • C/C = fold mountains, earthquakes
  • O/O = ocean trenches, island arcs, earthquakes, volcanoes
  • C/O = fold mountains, earthquakes, volcanoes
55
Q

What causes convection currents?

A
  • heat from inner core connects through mantle into asthenosphere
  • hot magma rises because it becomes less dense with heat
  • magma is cooler at the top as it is further away from heat source - becomes more dense + sinks back down to the bottom
  • cooler magma is reheated + begins to rise again - creating a continuous loop
56
Q

Diagram of mass movement

A
  • mass movement can happen over a range of time scales
  • also depends on moisture in the hill slope material
57
Q

Order of internal strength

A
  • non-cohesive materials e.g. coarse grain (sand)
  • cohesive materials - clays + silts
  • rock - sandstone + skate

Physical Georgaphy AS (5 decks)

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