2B.3 Rates of Coastal Recession and Stability Flashcards
2B.3a Lithology
rock type: igneous
Granite, Basalt, Dolerite
very slow rate of erosions (less than 0.1 cm pa)
igneous rocks are crystalline; the interlocking crystals make for strong, hard, erosion-resistant rock
igneous rocks such as Granite often have few joints, so there are limited weaknesses that erosion can exploit
2B.3a Lithology
rock type: metamorphic
Slate, Schist, Marble
slow rate of erosion (0.1-0.2 pa)
crystalline metamorphic rocks are resistant to erosion
many metamorphic rocks exhibit a feature called foliation, where crystals are all orientated in one direction, which produces weaknesses
metamorphic rocks are often folded and heavily fractured, which are weaknesses that erosion can exploit
2B.3a Lithology
rock type: sedimentary
Sandstone, Limestone, Shale
moderate to fast rate of erosion (10cm pa)
most sedimentary rocks are clastic and erode faster than crystalline igneous and sedimentary rocks
the age of sedimentary rocks is important; geologically young rocks tend to be weaker
rocks with many bedding places and fractures, such as shale, are often most vulnerable to erosion
2B.3a Lithology
unconsolidated material: boulder clay
unconsolidated material is material such as sand, gravel clay and silt that has not been compacted and cemented to become sedimentary rock (it has undergone the process of lithification) and so is loose and easily eroded
very fast rate of erosion (1m pa)
2B.3b Differential Erosion
permeable/ impermeable
permeable rocks tend to be less resistant to weathering than impermeable rocks because water percolating comes into contact with a large surface area that can be chemically weathered
also, permeability is important because groundwater flow through rock layers can weaken rocks by removing the cement that binds sediment in the rock together
permeable rocks allow water to flow though them, and include many sandstones and limestones
impermeable rocks do not allow groundwater flow and include clays, mudstones and most igneous and metamorphic rocks
it can also create high pore water pressure within cliffs, which affects the stability, leading to slumping and sliding
water emerging from below ground on to a cliff face at a spring can run down the cliff face and cause surface runoff erosion (fluvial erosion), weakening the cliff
2B.3b Differential Erosion
resistant/ less resistant
less resistant strata erode and weather quickly, being cut back rapidly, wave cut notches may be formed
resistant strata erode and weather slowly, retreating less rapidly
they may form a ‘bench’ feature at the cliff base
higher up, they form overhanging sections until they collapse by mass movement
generally the overall rate of cliff recession is determined by the resistance of its weakest rock layer.
2B.3b Differential Erosion
complex cliff profiles
DIAGRAM IN TEXTBOOK
Groundwater flows through the permeable sands but cannot flow through the impermeable clay. Water flow along the sand/clay interface weakens the material leading to slumping.
Slumping of the weak unconsolidated layers.
Surface runoff erosion where groundwater emerges on to the cliff face.
More resistant siltstone overhands the coal below.
Wave-cut notch formed by erosion of the weak coal layer.
Resistant rock creates a ‘bench’ feature at the cliff base.
2B.3c Vegetation
dune successional development: halophytes
halophytes can tolerate salt water, either around their roots, being submerged in salt water (at high tide) or salt spray from the sea
2B.3c Vegetation
salt marsh successional development: xerophytes
xerophytes can tolerate very dry conditions, such as those found on coastal sand dunes where the sandy soil retains very little water due to drainage
2B.3c Vegetation
vegetation stabilising sediment
the roots of plants bind sediment particles together making them harder to erode
when submerged, plants growing in sediment provide a protective layer so the surface of he sediment is not directly exposed to moving water and therefore erosion
plants protect sediment from wind erosion by reducing wind speed at the surface due to friction with the vegetation
2B.3c Vegetation
dune successional development: process
a
2B.3c Vegetation
salt marsh successional development: process
a
2B.3a Lithology
how lithology affects resistances
mineral composition:
some rocks contain reactive minerals that are easily broken down by chemical weathering, some are more inert
rock class:
sedimentary/metamorphic/igneous
structure:
rocks with fissures or air spaces erode faster than those without
dip and permeability also affect resistance to erosion
