L8 - Coasts: weathering, mass movement

• weathering is the breakdown of rock 'in-situ', meaning where it is
• processes of weathering affect rocks exposed at the coast
• mechanical weathering is sometimes knows as physical weathering, it is the break down of rock without changing its chemical composition
• the main type of weathering affecting the coast is freeze thaw, it is particularly effective if the rock is porous (contains holes) and permeable (allows water to pass through)
• mechanical freeze thaw:

  • this happens when the temperature alternates above and below 0 C (the freezing point of water)
  • water gets into rocks that have cracks
  • when the water freezes it expands, which puts pressure on the rock
  • when the water then thaws, it contracts, which releases the pressure
  • repeated freezing and thawing widens the cracks and causes the rock to break up and become detached. this may end up at the bottom of the slope as scree
• chemical weathering is the break down of rock by changing its chemical composition and there are 2 main types: solution and carbonation
• some minerals and rocks dissolve in rainwater or seawater, this is solution
• carbonation:

  • it tends to happen in warm and wet conditions
  • rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid
  • carbonic acid reacts with rock that contains calcium carbonate, eg chalk
  • this leads to rocks dissolving
• erosion is the removal of material by the waves
• hydraulic action:

  the relentless force of destructive waves pounding the base of the cliffs. this causes repeated changes in air pressure as water is forced into and out of faults. the forward movement of water compresses air into the cracks and as the wave retreats, there is an 'explosive' effect causing the rock to break up
• abrasion:

  destructive waves have enough energy to 'throw' sand and shingle at the cliff. these scratch and scrape at the cliffs surface. this process of erosion is called abrasion. this is concentrated between high and low water marks and is very effective in high energy conditions
• attrition:

  this is the grinding down of individual particles. during transport, pebbles collide with each other. over time this wears away jagged edges to make smooth, rounded pebbles. they will also break down into smaller pieces that also become smoother over time
• the rate of hydraulic action erosion will be higher where:

  • the coastline is exposed to a large fetch
  • strong winds blow for a long time and create destructive waves (common in winter)
• the rate of abrasion erosion will be higher where:

  • an area has no beach to act as a buffer between the sea and the cliffs
  • a headland 'juts out' into the sea. waves converge on a headland (wave refraction) and gain height and therefore, erosive energy
• the rate of attrition will be higher where:

  • there are soft rocks, which will erode faster than harder rocks
  • a rock has many joints
• longshore drift:

  1. load is carried by the waves
  2. the larger and heavier the load, the greater the velocity needed to transport it
  3. the heaviest ones are rolled onto the beach through traction and small pebbles are bounced onto the beach through saltation, whereas the lightest ones are held in suspension
  4. load is transported along the beach by longshore drift. the direction it moves depends on the direction of the prevailing wind
• mass movement

  the movement of weathered particles down a slope under the influence of gravity
• weathering can occur in a number ways and results in the creation of a regolith layer (broken down pieces of rock) which is moved downslope
• weathering processes vary from slope to slope because of differing geography
• substantial accumulation of fairly coarse material at the foot of steep slopes are known as 'scree'
• rockfalls

  rockfalls occur on slopes of more than 40 degrees, they are made up of individual blocks
• rockfalls caused by

  extreme weather on cliffs, usually freeze-thaw weathering
• rockfall features

  broken fragments fall/bounce to the bottom of a slope where there will b an accumulation forming a scree slop. the cliff will be almost vertical
• example of a rockfall

  january 1999 beachy head, sussex. a single cliff retreat of 17m occurred with over 50,000 tonnes of chalk producing a scree slope. individual pieces of rock were up to 4m across
• slide/ landlside

  generic term for downhill movement that occurs on cliffs that are steep, it moves rapidly under the influence of gravity
• slide/ landlside

  many occur through lines of weakness that are weakened by water flowing through joints. these can lubricate layers and cause it to slide along a straight 'slip plane' and rock falls as a 'block' which maintains contact with the clipp
• slide example

  june 1993 holbeck hall hotel, north yorkshire, in the early hours, the hotels lawn dropped 20 metres. at 7:30 am the hotel was evacuated, and soon after the whole area of land slipped down the slope, pushing over the sea wall at the bottom of the cliff
• mudslide

  they have a high water content and very rapid movement
• mudslide causes

  heavy rain / steep slopes over 10 degrees with bare slopes
• mudslide example

  may 1998, naples, italy, days of heavy rain caused a flow that killed 350 people and buried 6 towns under 3m of debris
• rotational slump

  characterised by mass of weathered material moving along curved shear planes
• rotational slump occurs

  when mud material is saturated and gains weight, lubricating the lines
• slumps

  slumps occur where permeable materials (ie sands) overlie relatively impermeable rocks such as clay
• rotational slump features

  slumps are a tilting, slumped mass which exposes a steep, curved scar behind and pushes a thinner 'toe' out in front, usually there will be more than one which leads to a stepped profile
• example of rotational slump

  this has occured in dunwich, suffolk on a large scale. large sections of the coast have been taped off from the public as it is considered too dangerous