3.4 Weathering, Mass Movement and Runoff

Question 1

What is weathering?

Answer 1

The breakdown / disintegration of rock in situ at the ground surface.

Question 2

Outline weathering as a positive feedback loop.

Answer 2

If the rate of debris removal exceeds the rate of weathering and mass movement, a positive feedback may operate - as the rate of weathering and mass movement could increase

Question 3

Outline weathering as a negative feedback loop.

Answer 3

If debris removal is slow and ineffective, this will lead to a build up of an apron of debris that reduces the exposure of the cliff face as it extends up the cliff face.

Weathering and mass movement rates will decrease.

Question 4

What are the 3 different types of weathering?

Answer 4

Mechanical
Biological
Chemical

Question 5

Outline the 3 types of mechanical (physical) weathering.

Answer 5

• FREEZE THAW. Water enters a crack in the rock when it rains, and then freezes in the cold weather. When water freezes, it expands by 11%, exerting pressure on the rock, forcing the crack to widen. With repeated freezing and thawing, fragments of rock break away and collect at the bottom of the cliff as scree.
• WETTING ABD DRYING. Frequent cycles of wetting and drying are common at the coast. Rocks rich in clay (eg shale) expand when wet and contract when dry - causing the, to crack and break up.
• SALT CRYSTALLISATION. When sand water evaporates, it leaves salt crystals behind. These can grow over time and exert stresses in the rock, causing it to break up. Salt can also corrode rock, esp if it contains traces of iron.

Question 6

Define mechanical weathering.

Answer 6

The break up of rocks without any chemical changes taking place.

Question 7

Define biological weathering.

Answer 7

The breakdown of rocks by organic activity.

Question 8

Outline the 3 different ways biological weathering can break down at the coast.

Answer 8

• thin plant roots grow into small cracks in a cliff face. These crack widen as the roots grow, breaking up the rock.
• birds and animals dig burrows into a cliff.
• water running through decaying vegetation becomes acidic, which leads to increased chemical weathering.

Question 9

Define chemical weathering.

Answer 9

Chemical weathering involves a chemical reaction where salts may be dissolved, or a clay-like deposit may result which is easily eroded.

Question 10

Outline the 3 different types of chemical weathering.

Answer 10

• CARBONATION. Rainwater absorbs CO2 from the air to form a weak carbonic acid. This reacts with calcium carbonate in rocks to form calcium bicarbonate, which is easily dissolved.
  The cooler the temp of the rainwater, the more CO2 is absorbed.
• OXIDATION. The reaction of rock minerals with oxygen, eg iron, to form a rusty red powder, leaving rocks more vulnerable to weathering.
• SOLUTION. The dissolving of rock minerals eg halite.

Question 11

Define mass movement.

Answer 11

The downhill liege,not of material under the influence of gravity.

Question 12

Give an example of a slow mass movement.

Answer 12

Soil creep.

Question 13

Give an example of fast mass movement.

Answer 13

Rockfalls, landslides.

Question 14

Outline the 7 types of mass movement.

Answer 14

• soil creep
• landslide
• mudflow
• rockfall
• runoff
• slump
• solifluction

Question 15

Outline the mass movement: soil creep.

Answer 15

Soil creep is the extremely slow form of movement of individual soil partials downhill.

Particles rise towards the ground surface due to wetting of freezing and then return vertically to the surface in response to gravity as the soil dried out.

Question 16

Outline the mass movement: landslide.

Answer 16

A landslide involves a block of rock moving v rapidly downhill along a slide plane. The block of material stays largely intact.

Frequently triggered by earthquakes or heavy rainfall, when the slip surface becomes lubricated and friction is reduced.

Question 17

Outline the mass movement: mudflow.

Answer 17

A mudflow involves earth and mud flowing downhill, usually over unconsolidated or weak bedrock eg clay, often after heavy rainfall.

Water gets trapped in the rock, increasing pore water pressure, which forces rock particles apart and leads to slope failure.

Question 18

Outline the mass movement: rockfall.

Answer 18

A rockfall involves the sudden collapse of breaking away of individual rock fragments at a cliff face. Most commonly associated with steep cliffs in heavily jointed and resistant rock.

Often triggered by mechanical weathering, rocks bounce down from the slope to form scree at the foot of the slope.

Question 19

Outline the mass movement: slump.

Answer 19

Commonly occur on weak, unconsolidated clays and sands, often when permeable rock overlies impermeable rock. This causes a buildup of pore water pressure.