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
How does rock structure influence weathering?
- **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
How does vegetation influence weathering?
- 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
How does relief influence weathering?
- 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
What does the peltier model show?
- 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
What is mass movement?
- 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
What is rain splash?
- 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
What is sheet wash?
- 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
What are rills?
- 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
What are the main types of mass movement?
- creep/solifluction
- debris flows + lahars
- slope failure - slides + falls
34
Factors that increase shear force?
- loading of a slope - water or vegetation
- lateral pressure - water freezing in cracks, swelling due to hydration
- transient stress - earthquakes
35
Factors that decrease shear strength?
- 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
Role of water in mass movement?
- 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
What is creep/solifluction/heave?
- 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
How do slumps occur?
- occurs on weaker rocks + clay
- rotational movement along a slip plane
39
How do flows occur?
- 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
How do slides occur?
- 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
How do falls occur?
- occurs on steep slopes - greater than 45°
- rocks fall under gravity
- can be triggered by freeze-thaw, seismic waves or gravitational stress
42
Methods of control for falls?
- flattening the slope
- netting/pinning
- drainage
- reinforcement of rock walls by grouting with cement, anchor bolts
- covering of wall with steel mesh
43
Methods of control for slides + slopes
- grading or benching to flatten the slope
- drainage of surface water with ditches
- sealing surface cracks to prevent infiltration
- buttresses roots
44
What is pining?
- 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
What is netting?
- 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
Difference between pinning and netting?
- netting prevents scree or debris (small rocks) compared to pinning which prevents larger movements of material
47
What is grading?
- 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
What are the effects of mass movement flow on the slope?
- shape of slope changes
- material moved from top to bottom due to gravity
- development of scar at source
49
What are the small-scale movements of sediment on slopes?
- rainsplash
- heaves
- rill action
50
Convergent/ destructive plate boundaries (oceanic/oceanic)
- 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
Convergent/destructive plate boundaries (continental/continental)
- 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
What forms at a conservative plate boundary
- earthquakes
53
What forms at a constructive/divergent plate boundary
C/C = volcanoes, rift valleys, earthquakes
O/O = ocean ridges, earthquakes, volcanoes
54
What forms at a destructive/convergent plate boundary?
- C/C = fold mountains, earthquakes
- O/O = ocean trenches, island arcs, earthquakes, volcanoes
- C/O = fold mountains, earthquakes, volcanoes
55
What causes convection currents?
- 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
Diagram of mass movement
- mass movement can happen over a range of time scales
- also depends on moisture in the hill slope material
57
Order of internal strength
- non-cohesive materials e.g. coarse grain (sand)
- cohesive materials - clays + silts
- rock - sandstone + skate
