3.6 Weathering and mass movement at the coast Flashcards
What is weathering?
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
What is mass movement?
Movement of weathered material down slope as a result of gravity
What are the different types of mechanical weathering?
freeze-thaw weathering
salt-weathering
wetting and drying
What is freeze-thaw weathering?
(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
What is an example of a major event caused by freeze-thaw weathering?
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.
What is the main type of rock affected by freeze-thaw weathering?
chalk
What is salt weathering?
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
What is wetting and drying?
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
What are the several ways in which biological weathering can occur?
-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)
What is the different types of chemical weathering?
Carbonation
Hydrolysis
Oxidation
What are the various factors that can affect the resultant mass movement at the coast?
- the angle of the slope or cliff
- the rock type and its structure
- the vegetation cover
- how wet the ground is
What are the different types of mass movement?
Flow: Imperceptible
soil creep
Solifluction
Flow: slow to rapid
earth flow/mudflow
Slide: slow to rapid
Rock/debris fall
rock/debris slide
slump
What is a soil creep?
Slowest form of mass movement
Almost continuous process
Very slow downhill movement of individual soil particles
What is soliflucution?
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.
What are earth flows and mudflows?
An increase in the amount of water can reduce friction, causing earth and mud to flow over underlying bedrock
What’s the difference between a slide and flow?
In a slide, the material remains intact, whereas in a flow, the material becomes jumbled up
What is a rock fall and when do they occur?
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
What are block falls?
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
What are rock/debris slides?
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
What are slumps and when do they occur?
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
What is carbonation?
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
What is hydrolysis?
The breakdown of minerals to form new clay minerals, plus materials in solution, due to the effect of water and dissolved CO2
What is oxidation
The addition of oxygen to minerals, especially iron compounds, which produces iron oxides and increases volume contributing to mechanical breakdown
What is the process of rotational slides?
- The bedding plane between the impermeable clay and permeable sand dips seaward, which promotes mass movement.
- 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
- Heavy rain saturates the permeable sands, loading the cliff material
- 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
- Toe erosion by marine processes undercuts the cliff from below, adding to instability
- Curved failure surfaces develop in the sand, and the whole cliff begins to rotate about a pivot point
Discuss the mass movement event at Burton Bradstock, Dorset
on the 24th July 2012, two cliffs fell 20 minutes apart
400 tonnes of rock fell- one person was killed and another fatally injured
What is the geology like at Burton Bradstock?
Composed of Bridport sand: horizontal beds of micaceous silts and fine grained sandstone which alternate at regular intervals with harder bands of sandstone
What makes the geology at Burton Bradstock susceptible to weathering?
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
What caused the event at Burton Bradstock to happen in this specific incident?
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
What happened at Porthkerry in South Wales?
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
What is the geology like at Porthkerry?
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
What caused the mass movement at the event in Porthkerry?
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
