3.6 Weathering and mass movement at the coast

Question 1

What is weathering?

Answer 1

Weathering is the gradual breakdown of rock, in situ, at or close to the ground surface. It can be either mechanical, chemical or biological. By breaking rock down, weathering creates sediment which the sea can then use to help erode at the coast- weathering helps to increase the rate of erosion of some coasts

Question 2

What is mass movement?

Answer 2

Movement of weathered material down slope as a result of gravity

Question 3

What are the different types of mechanical weathering?

Answer 3

freeze-thaw weathering
salt-weathering
wetting and drying

Question 4

What is freeze-thaw weathering?

Answer 4

(also known as frost shattering)
occurs when water enters a crack or joint when it rains and then freezes in cold weather. When water freezes, it expands in volume by about 10%. This expansion exerts pressure on the rock, which forces the crack to widen. With repeated freezing and thawing, fragments of rock break away and collect at the bottom of the cliff as scree

Question 5

What is an example of a major event caused by freeze-thaw weathering?

Answer 5

A major rock fall at the white cliffs of Dover in February 2001 was caused mainly by freeze-thaw weathering. This followed a very wet autumn and a cold February.

Question 6

What is the main type of rock affected by freeze-thaw weathering?

Answer 6

chalk

Question 7

What is salt weathering?

Answer 7

When salt water evaporates, it leaves salt crystals behind. These can grow overtime and leave stresses in the rock, causing it to break. Salt can also corrode rock, particularly if it contains traces of iron

Question 8

What is wetting and drying?

Answer 8

Rocks rich in clay, such as shale, expand when they get wet and contract when they get dry. This can cause them to crack up and break

Question 9

What are the several ways in which biological weathering can occur?

Answer 9

-Thin plant roots start to grow into cracks in a cliff face. These cracks then widen as the roots grow thicker, which breaks up the rock
-water running through decaying vegetation becomes acidic, which leads to increased chemical weathering
-Birds and animals dig burrows into cliffs
-Marine organisms are also capable of burrowing into rocks or secreting acids (known as rock boring)

Question 10

What is the different types of chemical weathering?

Answer 10

Carbonation
Hydrolysis
Oxidation

Question 11

What are the various factors that can affect the resultant mass movement at the coast?

Answer 11

• the angle of the slope or cliff
• the rock type and its structure
• the vegetation cover
• how wet the ground is

Question 12

What are the different types of mass movement?

Answer 12

Flow: Imperceptible
soil creep
Solifluction

Flow: slow to rapid
earth flow/mudflow

Slide: slow to rapid
Rock/debris fall
rock/debris slide
slump

Question 13

What is a soil creep?

Answer 13

Slowest form of mass movement
Almost continuous process
Very slow downhill movement of individual soil particles

Question 14

What is soliflucution?

Answer 14

Averages between 5cm and 1 metre a year
Occurs mainly in tundra areas, where the ground is frozen
When the top layer of soil thaws in summer, but the layer below remains frozen as permafrost, the surface layer becomes saturated and flows over the frozen subsoil and rock.

Question 15

What are earth flows and mudflows?

Answer 15

An increase in the amount of water can reduce friction, causing earth and mud to flow over underlying bedrock

Question 16

What’s the difference between a slide and flow?

Answer 16

In a slide, the material remains intact, whereas in a flow, the material becomes jumbled up

Question 17

What is a rock fall and when do they occur?

Answer 17

Rock falls are most likely to occur when strong, jointed and steep rock faces are exposed to mechanical weathering
occur on slopes over 40 degrees
The material, once broken away from the source either bounces or falls vertically to form scree at the foot of the cliff

Question 18

What are block falls?

Answer 18

Similar to rock falls
A large block of rock falls away from the cliff as a single piece, due to the jointing of the rock

Question 19

What are rock/debris slides?

Answer 19

Rocks that are jointed, or have bedding planes roughly parallel to the slope of the cliff surface are susceptible to landslides
An increase in the amount of water can reduce friction - causing sliding
In a rock or landslide, slabs of rock can slide over underlying rocks along a slide or slip plane

Question 20

What are slumps and when do they occur?

Answer 20

Often occur in saturated conditions
there is a rotational movement
occur on moderate to steep slopes
They are common when softer materials overlie more resistant or impermeable rock
slumping causes rotational scars
Repeated slumping creates a terraced cliff profile

Question 21

What is carbonation?

Answer 21

rainwater absorbs CO2 from the air to form a weak carbonic acid. This reacts with calcium carbonate in rocks such as limestone and chalk- to form calcium bicarbonate, which is easily dissolved. The cooler the temperature of the rainwater, the more carbon dioxide - increasing the effectiveness of carbonation in winter

Question 22

What is hydrolysis?

Answer 22

The breakdown of minerals to form new clay minerals, plus materials in solution, due to the effect of water and dissolved CO2

Question 23

What is oxidation

Answer 23

The addition of oxygen to minerals, especially iron compounds, which produces iron oxides and increases volume contributing to mechanical breakdown

Question 24

What is the process of rotational slides?

Answer 24

1. The bedding plane between the impermeable clay and permeable sand dips seaward, which promotes mass movement.
2. Cracks develop in the cliff top during dry weather as soil and sediment dry out ; these later become routes that rainwater can take into the sand
3. Heavy rain saturates the permeable sands, loading the cliff material
4. Water percolates through the permeable sand, but is forced to move along the boundary of the impermeable rock below. This contributes towards a high water pressure and creates internal pressure within the cliff
5. Toe erosion by marine processes undercuts the cliff from below, adding to instability
6. Curved failure surfaces develop in the sand, and the whole cliff begins to rotate about a pivot point

Question 25

Discuss the mass movement event at Burton Bradstock, Dorset

Answer 25

on the 24th July 2012, two cliffs fell 20 minutes apart
400 tonnes of rock fell- one person was killed and another fatally injured

Question 26

What is the geology like at Burton Bradstock?

Answer 26

Composed of Bridport sand: horizontal beds of micaceous silts and fine grained sandstone which alternate at regular intervals with harder bands of sandstone

Question 27

What makes the geology at Burton Bradstock susceptible to weathering?

Answer 27

Differential weathering has worn away the soft rock
Sandstone is permeable, making it vulnerable to frost shattering and salt weathering , while calcite cement, which binds sandstone grains together is prone to carbonation and solution
Vertical joints in the sand stone also make the cliff susceptible to toppling

Question 28

What caused the event at Burton Bradstock to happen in this specific incident?

Answer 28

Wet weather in June 2012 would have saturated the normally permeable sandstone and contributed to an increase in pore water pressure. Meanwhile, a drier period immediately before the failure may have caused some shrinkage, further weakening the rock.
The presence of a notch at the base of the cliff also suggests marine erosion had attacked the cliff at some point, although the gravel beach would have offered some protection

Question 29

What happened at Porthkerry in South Wales?

Answer 29

31st October 2011
The rock fall occurred during heavy weather and was close to the time of a high spring tide
around 15,000 metre cubed of rock was involved in the fall
Rock fall was toppling type- that is a block became isolated from the cliff due to fissuring and fell onto the foreshore, breaking up in the process and forming a debris fan, extending 80m across the beach and 10m up against the cliff face

Question 30

What is the geology like at Porthkerry?

Answer 30

part of the Blue Lias formation
A thick sequence of regularly bedded limestones and mudstones
The bedding/jointing tended to produced cuboidal structures, resulting in little or no mass strength adjacent to the cliff face
seaward dip of 3-4 degrees

Question 31

What caused the mass movement at the event in Porthkerry?

Answer 31

the cliff seemed to be saturated with considerable groundwater seepage
It’s presumed the failure occurred in the mudstone, allowing the overlying strata to collapse downward onto the beach