Weathering, Mass Movement Coastal Processes (3) Sub-Aerial Proceses Weathering, Mass Movement

Sub-Aerial Processes Weathering Mass Movement Erosion can best be seen on cliff coastlines. The processes responsible for cliff erosion can be classified into two types: Active on the cliff face – CLIFF FACE or SUB-AERIAL PROCESSES (Weathering and Mass Movement) Active at the base of the cliff – CLIFF FOOT or MARINE PROCESSES (Hydraulic Action, Cavitation, Abrasion/Corrasion, Attrition, Solution/Corrosion) 4/5/2019

Cliff Face & Cliff Foot Processes Active on the cliff face – CLIFF FACE or SUB-AERIAL PROCESSES (Weathering and Mass Movement) Active at the base of the cliff – CLIFF FOOT or MARINE PROCESSES (Hydraulic Action, Cavitation, Abrasion/Corrasion, Attrition, Solution/Corrosion) 4/5/2019

Sub-Aerial Processes: Types of Weathering There are many examples of weathering (the full list should have been covered at AS!!) Freeze-Thaw Weathering (Mechanical/Physical Weathering) The process whereby water, that has seeped into cracks or joints in the cliff face, freezes and expands (by 9%), leading to degradation by crumbling or splitting (frost shattering) of the cliff face rocks Salt Weathering (Mechanical/Physical Weathering) The process by which salt water in the spray zone leads to salt crystals forming (salt crystallisation) after water evaporates inside cracks or pores in the cliff face near the base. The salt crystals, composed of sodium and magnesium compounds, expand in cracks and joints, weakening the rock structures which crumble easily Exfoliation (Mechanical/Physical Weathering) Desert coastlines experience diurnal temperature extremes that can cause the expansion and contraction of the surface layers of rock on the cliff face. Over time, the rock is weakened and surface layers peel off 4/5/2019

Sub-Aerial Processes: Types of Weathering (continued) Biological Weathering Plant roots can widen cracks, animals may burrow in cliff faces and move rock fragments in the process, while molluscs, sponges and sea urchins can break down rock minerals in the tidal zone by altering the rock’s chemical composition Carbonation/Solution (Chemical Weathering) Acidic rainfall penetrating cliff faces, and seawater that has become acidic in rock pools due to organisms secreting organic acids, are responsible for the chemical/solution weathering of rocks Oxidation (Chemical Weathering) Many rocks contain iron compounds and the oxidation of these compounds into iron oxide (rust) weakens and degrades the rocks on the cliff face Did You Know? Seabirds nesting on cliff ledges also have an extremely corrosive guano (excrement) that helps to chemically weather rocks 4/5/2019

Sub-Aerial Processes: Types of Mass Movement (also covered at AS) Weathering weakens the rocks of the cliff face, while marine erosion processes at the base of the cliff undermine it. At the same time, rainwater seeps into (percolates) and out of the cliff face as throughflow and groundwater flow, lubricating cracks, joints and bedding planes, and weakening the internal cohesion of the cliff face rocks. Together, these processes lead to a variety of mass movements – rock falls, landslides, rotational slipping/slumping, mudflows, soil creep – all contributing to cliff retreat In cliffs made of hard rock, rock falls are the main form of mass movement, often occurring after heavy rain In cliffs made of unconsolidated material, such as boulder clay, rotational slipping/slumping is the main form of mass movement The likelihood of a cliff face failing is determined by the relative resistance of the slope compared with the force that is trying to move it. Gravity, slope angle and pore pressure are the key factors at play. For example, water lubricates particles and may fill the spaces between them. This forces them apart under pressure (pore pressure), decreasing the frictional strength of the solid material and greatly increasing the ability of the material to move 4/5/2019

Sub-Aerial Processes: Types of Mass Movement Landslides occur on cliffs where there is weak rock, steep slopes and active undercutting by marine erosion. Often, an increase in water content facilitates the slide Rock falls occur on cliffs with very steep slopes (>40 degrees) that have bare rock faces with exposed joints. The initial cause is often weathering e.g. freeze-thaw Slumps/slips occur on weaker rocks and have a rotational movement along a curved slip plane. Clay absorbs water, becomes saturated and exceeds its liquid limit. It then flows along a slip plane. Typically, the base of the cliff has been undercut and weakened by marine erosion, thereby reducing its strength 4/5/2019

Assessment Time: Tasks 1. Outline the similarities and differences between freeze-thaw weathering and salt weathering 2. What factors are required for biological weathering to be an important sub-aerial process? 3. Describe two different examples of chemical weathering and explain how they help to change coastal landforms 4. How and why does water play a key role in mass movements? 5. Outline the key differences between rock falls, slumps/slips, landslides and mudflows 6. If sub-aerial processes operated but there was no marine erosion, suggest what would happen to the cliff profile over time 4/5/2019

Assessment Time: Past Exam Questions on Marine & Sub-Aerial Processes ANSWER one 10 & one 15 MARK EXAM QUESTION: Choose carefully - some questions require more knowledge of coastal landforms – therefore are more suited to being answered later in the unit when we have covered all landforms Nov 2017 (22) Discuss the relative importance of mass movements in the evolution of cliffs. [15] Nov 2016 (21) Describe the marine erosion processes of hydraulic action, wave quarrying and corrasion/abrasion. Explain the factors that make these processes effective in the development of cliffs. [10] Nov 2016 (22) To what extent does the interaction of marine processes and sub-aerial processes determine the morphology (shape) of coastal landforms? [15] Nov 2015 (21) Explain the factors that determine the effectiveness of marine erosion processes. [10] June 2015 (22) Using examples, explain the extent to which marine erosion and sub-aerial processes can influence the development of landforms along a stretch of coastline. [15] 4/5/2019

Assessment Time: Past Exam Questions (continued) June 2017 (21) Photograph A shows some effects of marine erosion on the Yorkshire coast, UK. Explain the processes of marine erosion and mass movement. Assess the effects these processes have upon cliff profiles, such as the one shown in Photograph A. [15] 4/5/2019

Assessment Time: Past Exam Questions (continued) Nov 2015 (23) Fig. 2 shows factors and processes affecting the development of coastal landforms. Evaluate the roles of factors and processes in the development of different cliff profiles and other coastal erosion features along a stretch, or stretches, of coastline. [15] 4/5/2019