2b.3b Flashcards
Define impermeable
A rock that doesn’t allow water to pass through it.
Define permeable rock
rock that allows water to pass through it
Two types of alternating strata in cliff
- resistant and less resistant
- permeable and impermeable
What does differential erosion of alternating strata of rock produce
complex cliff profiles and influences recession rates
Define till
Deposits of angular rock fragments in a finer medium.
What happens if there is a cliff wiht permeable rock strata above the impermeable strata
Weakly consolidated rock is vulnerable to mass movement as rainwater percolating into the upper strata creates instability
What is the overall rate of cliff erosion determined by
determined by the resistance of its weakest rock layer
What happens to less resistant strata
Less resistant strata erode and weather quickly, being cut back rapidly, wave cut notches may be formed.
What happens to resistant strata
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
Why is rock permeable
they’re porous (chalk and poorly cemented sandstones)
they have numerous joints (e.g. carboniferous limestone)
Why is permeable rock less resistant to weathering
water percolating comes into contact with a large surface area that can be chemically weathered
e.g. Limestone weathered by carbonation converting calcium carbonate to soluble calcium bicarbonate.
Feldspar in granite weathered by hydrolysis into kaolin (china clay)
State impermeable rocks
clays, mudstones, igneous, metamorphic
How does saturation affect cliff profile
- Saturation promotes mass movement through lubrication and by adding weight.
- Leads to slumping in unconsolidated material and sliding in consolidated strata.
How does permeability create springs
Where a permeable rock overlays an impermeable stratum groundwater is unable to percolate down into the lower layer.
Water accumulates in the permeable layer, producing a saturated layer where the pores are full of water.
A spring will form on the cliff face at the top of the saturated layer.
As the stream flows down the cliff, fluvial erosion (surface run off erosion( will attack the saturated permeable bed and lower impermeable stratum, reducing the angle of the cliff profile.
How does groundwater flow remove cement
Water flows through the permeable (sands) but can’t flow through the impermable (clay), so flows along the interface. Groundwater flow through rock layers can weaken rock by removing the cement that binds them together. Weak, unconsolidated layers slump.
