ROCKS AND WEATHERING

Question 1

What is the lithosphere?

Answer 1

The lithosphere is the upper surface layer and consists of the crust and upper mantle.

Question 2

What is the continental crust?

Answer 2

The continental crust is made up of less dense, granitic rock due to the presence of silica and oxygen and is 35 to 70km thick on average.

Question 3

What is the oceanic crust?

Answer 3

The oceanic crust is made up of more dense basaltic rock with iron and magnesium and is on average 6 to 10km thick.

Question 4

What is the convection current theory of the way in which tectonic plates move?

Answer 4

The convection current theory states that plates move due to convection currents in the Earth’s interior.
Intense heat from radioactive decay in the earth’s core causes molten rock in the mantle to move in a convection cell.
A convection cell is the pattern where the heated material rises near to the surface, cools, and then sinks back down into the mantle where it is again heated and the cell repeats in a cycle.
This pattern creates convection currents in the mantle and these currents are what drive the movement of tectonic plates.

Question 5

Describe divergent (constructive) plate boundaries.

Answer 5

These boundaries occur where two tectonic plates are moving away from eachother. Magma wells up and fills the gap created by the separating plates in the process of sea-floor spreading.

Question 6

What is the dragging theory of the way in which tectonic plates move?

Answer 6

The dragging theory states that the plates are dragged or subducted by their oldest edge. Plates are hot at constructive plate boundaries but they cool as they move away. As the cold plates are subducted at convergent boundaries, pressure causes the rock to change and become heavier thus pulling the plate along behind it.

Question 7

What is the process of sea-floor spreading?

Answer 7

Sea-floor spreading occurs at divergent (constructive) plate boundaries where the two plates are moving apart and magma rises to fill the gap between them. As the plates move slowly away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, forming a mid-ocean ridge.

Question 8

What are mid-ocean ridges?

Answer 8

Mid-ocean ridges are formed at divergent (constructive) plate boundaries where new lithosphere is created through sea floor spreading. They are giant submarine mountain chains with a total length of over 60 000km. They are 1000-4000km wide and have crests 2-3km high above the surrounding ocean basins which are 5km deep.

Question 9

Describe transform (conservative) plate boundaries.

Answer 9

Transform or conservative boundaries occur where two plates are sliding horizontally past each other.

Question 10

Describe convergent (destructive) plate boundaries.

Answer 10

These plate boundaries occur when an oceanic plate collides with another plate - either oceanic or continental - and subduction occurs. The oceanic plate is much denser than the other plate and so is pushed down beneath it into the upper mantle. This is where the oceanic crust is destroyed in the Benioff zone.

Question 11

Describe convergent (collision) plate boundaries.

Answer 11

This plate boundary occurs when two continental plates meet and a powerful collision occurs. Both plates have a much lower density than the mantle, which prevents subduction. The intense compression and folding and buckling results in the formation of large chains of fold mountains.

Question 12

What is subduction?

Answer 12

Subduction occurs at convergent (destructive) boundaries where the ocean plate meets either an oceanic or continental plate and is pushed beneath it into the upper mantle.

Question 13

What is biological weathering?

Answer 13

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Question 14

What does vadose mean?

Answer 14

Vadose = relating to or denoting underground water above the water table.

Question 15

What does phreatic mean?

Answer 15

Phreatic = relating to or denoting underground water in the zone of saturation (beneath the water table).

Question 16

What are speleothems?

Answer 16

Speleothems are secondary cave deposits formed by the precipitation of dissolved calcium carbonate e.g. stalagmites, stalactites.

Question 17

What is the difference between stalagmites and stalactites?

Answer 17

Stalactites develop from the top of the cave whereas stalagmites are formed on the base of the cave.

Question 18

What is the difference between stalagmites and stalactites?

Answer 18

Stalactites develop from the top of the cave whereas stalagmites are formed on the base of the cave.

Question 19

What is pore pressure?

Answer 19

Pore pressure is the pressure of water in the interstitial spaces (between particles). If this pore pressure builds up then it can force the particles apart under pressure, increasing the ability of the regolith to move.

Question 20

What are the two factors that cause slope failure?

Answer 20

1. a reduction in the internal resistance, or shear strength, of the slope.
2. an increase in shear stress, that is the forces attempting to pull a mass downslope.

Question 21

What are the two factors that cause slope failure?

Answer 21

1. a reduction in the internal resistance, or shear strength, of the slope.
2. an increase in shear stress, that is the forces attempting to pull a mass downslope.

Question 22

How can shear stress be increased?

Answer 22

1. Removal of lateral support through undercutting or slope steepening - erosion by rivers and glaciers, wave action, faulting, previous rockfalls or slides.
2. Removal of underlying support - undercutting by rivers and waves, subsurface solution.
3. Loading of slope - weight of water, vegetation, accumulation of debris.
4. Lateral pressure - water in cracks, freezing in cracks, swelling (especially through hydration of clays), pressure release.
5. Transient stresses - earthquakes, movement of trees in wind.

Question 23

How can shear strength be reduced?

Answer 23

1. Weathering effects - disintegration of granular rocks, hydration of clay minerals, dissolution of cementing minerals in rock or soil.
2. Changes in pore pressure - saturation, softening of material.
3. Changes of structure - creation of fissures in shales and clays, remoulding of sand and sensitive clays.
4. Organic effects - burrowing of animals, decay of tree roots.

Question 24

How can shear strength be reduced?

Answer 24

1. Weathering effects - disintegration of granular rocks, hydration of clay minerals, dissolution of cementing minerals in rock or soil.
2. Changes in pore pressure - saturation, softening of material.
3. Changes of structure - creation of fissures in shales and clays, remoulding of sand and sensitive clays.
4. Organic effects - burrowing of animals, decay of tree roots.

Question 25

How can downslope movement be opposed?

Answer 25

1. Friction - will vary with the weight of the particle and slope angle. Friction can be overcome on gentle slope angles if water is present e.g. solifluction can occur on slopes as gentle as 3º.
2. Cohesive forces - act to bind the particles on the slope. Clay may have high cohesion, but this may be reduced if the water content becomes so high that the clay liquefies, when it loses its cohesive strength.
3. Pivoting - occurs in the debris layers which contain material embedded in the slope.
4. Vegetation - binds the soil and thereby stabilises slopes. However vegetation may allow soil moisture to build up and make landslides more likely.

Question 26

What is surface wash?

Answer 26

This occurs when the soil’s infiltration capacity is exceeded. It is common in winter as water drains across saturated or frozen ground, following prolonged or heavy downpours or the melting of snow. It is also common in arid and semi-arid regions where particle size limits percolation.

Question 27

What is surface wash?

Answer 27

This occurs when the soil’s infiltration capacity is exceeded. It is common in winter as water drains across saturated or frozen ground, following prolonged or heavy downpours or the melting of snow. It is also common in arid and semi-arid regions where particle size limits percolation.

Question 28

What is sheetwash?

Answer 28

This is the unchannelled flow of water over a soil surface. Sheetwash usually breaks into areas of high velocity separated by areas of lower velocity on slopes. It is capable of transporting material dislodged by rainsplash.

Question 29

What is throughflow?

Answer 29

Throughflow is the movement of water down through the soil. It is channelled into natural pipes in the soil. This gives it sufficient energy to transport material, and added to its solute load, may amount to a considerable volume.

Question 30

What is heave or creep?

Answer 30

This is the slow, small-scale process which occurs mostly in winter.

Question 31

What is heave or creep?

Answer 31

This is the slow, small-scale process which occurs mostly in winter. As permafrost forms on the active layer of the soil, it compacts it down. When this permafrost thaws the soil thus expands, causing the top layer to move downwards, assisted by the percolating water from the frost. This freeze-thaw cycle creates a slow movement of material downslope. Small-scale variations in slope, compaction, cohesion and vegetation will have a significant effect on the rate of soil creep.

Question 32

What are rills?

Answer 32

Rills are small networks of shallow channels created by thin, concentrated flows of water, the result of infiltration capacity being exceeded.

Question 33

What are rills?

Answer 33

Rills are small networks of shallow channels created by thin, concentrated flows of water, the result of infiltration capacity being exceeded.

Question 34

What is rainsplash erosion?

Answer 34

On flat surfaces raindrops compact the soil and dislodge particles equally in all directions. On steep slopes the downward component is more effective than the upward motion due to gravity so particles are dislodged and fall downwards. Rainsplash erosion downslope increases with slope angle.