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
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
3
Q
What is the mesosphere?
A
- below asthenosphere
- hot, but stronger due to pressure
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
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
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
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)
8
Q
What is the Benioff zone?
A
A narrow zone of earthquakes dipping away from the deep-sea trench
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
10
Q
How were the Himalayas formed?
A
- collision of the Eurasian + Indian plates (both continental)
- sediment accumulated on the continental shelf + slope
11
Q
What are ocean ridges?
A
- linear features found on divergent (constructive margin)
- e.g. Mid Atlantic Ridge
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
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
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
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
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
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)
18
Q
Where does chemical weathering occur?
A
- most prevalent sub-surface
- where percolating water has picked up organic acids
19
Q
Examples of chemical weathering?
A
- carbonation/solution
- hydrolysis
- hydration
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
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
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%
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
