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Topic Area 6 (Mass Movement (Rockfalls (Talus Screeslope, They are usually…
Topic Area 6
Mass Movement
Rockfalls
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They are usually associated with near vertical slopes. only more resistant rock can sustain these angles on the coast. they are most commonly triggered by undercutting in the inter-tidal zone or freeze-thaw weathering.
Rotational Slumping
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Rainwater infiltrates the cliff through unconsolidated, porous material. Cracks may have developed in the cliff top during dry weathering, allowing rainwater into the cliff material. This then creates a slip plane. Marine processes erode and undermine the cliff base, causing toe erosion. This adds to the instability of the cliff. The weight of the saturated clay causes the material to slump along the slip plane. This is often triggered by undercutting by waves at the cliff base.
Transitional Slides
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A very low angle seaward dip in strata will prevent falls. In this case, material will tend to slide down the dipslope towards the sea.
Mudflow/ Flows
Flows are common in weak rocks such as clay or unconsolidated sands. These materials can become saturated, lose their cohesion and flow downslope. Heavy rainfall combined with high tides contribute to saturation.
Topple
Geological structure influences topple. Where rock strata have a very steep seaward dip, undercutting by erosion will quickly lead to instability and blocks of material toppling seaward.
Weathering
Mechanical
Salt Crystallisation
This is when salt crystals are deposited in cracks and, over time, the salt accumulates and applies pressure to the cracks.
Freeze Thaw
This is when water gets into into the cracks of the rock. The repeated freeze thaw action puts pressure on the rocks until they eventually crack and break.
Wetting and Drying
Clay rich rocks are prone to expand when they are wet and contract when they dry. This results in cracks which are vulnerable to both freeze thaw and salt crystallisation.
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Chemical
Oxidation
The addition of oxygen to minerals, especially iron compounds, which produces iron oxides and increases volume contributing to mechanical breakdown.
Carbonation
The slow dissolution of limestone due to rainfall (weak carbonic acid, pH 5.6) producing calcium bicarbonate in solution.