2B.3 Rates of coastal recession and stability depend on lithology and other factors.

Mineral composition - Rate of Recession

* Some rocks contain **reactive minerals** easily broken down by chemical weathering, e.g. **calcite** in limestone. 
* Other minerals are more inert that chemically weather more slowly, if at all, e.g. quartz in sandstone. 

Rock class - Rate of Recession

* Sedimentary rocks, e.g. conglomerates, sandstones, limestones and clays, are **clastic** (made of **clasts** (sediment particles), cemented together)
* Many cements are **reactive** and easily **chemically weathered**, e.g. iron oxide and calcite. 
* Sedimentary rocks with very weak cementation, e.g. boulder clay, gravels and sands, are termed unconsolidated. 
* __Igneous rocks__, e.g. granite, and metamorphic rocks, e.g. marble, are **crystalline** with **strong chemical bonding.** 
* Rocky coastlines vary in resistance of geology.
* Granite erodes at a rate of 0.1 cm p.a.
* Carboniferous limestone at 1 cm p.a.
* Sandstone at 10 cm p.a.
* Boulder clay at 1 m p.a.

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Structure - Rate of Recession

Rocks with fissures (e.g. faults and joints) or air spaces (porous) rocks, weather and erode rapidly. 

Rate of recession definition

Rate of recession is the speed at which the coastline is moving inland.

Clastic rocks

Clastic rocks are those made of sediment particles cemented together

Crystalline rocks

Crystalline rocks are made of interlocking mineral crystals. 

Bedrock lithology - Rate of recession

Rate of recession is influenced by bedrock lithology (igneous, sedimentary or metamorphic) and the geology unconsolidated sediment. 

* How **reactive** minerals in the rock are when exposed to chemical weathering
* Whether rocks are **clastic** (less) or **crystalline** (more resistant)
* The degree to which rocks have **cracks**, **fractures**, and **fissures** (these weaknesses are exploited by weathering and erosion)

Cliff profiles

The height and angle of a cliff face as well as its features such as wave cut notches changes in slope angle.

horizontal strata

A horizontal strata produces steep cliffs

Why are complex cliff profiles produced?

Complex cliff profiles are produced where cliffs are composed of __strata of differing lithology__.

* **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.__

What determines rate of recession?

Generally the o__verall rate of cliff recession is determined by the resistance of its weakest rock layer.__

Permeable and non-permeable strata

Complex cliff profiles can also be produced when there are alternating permeable and non-permeable strata. 

* __Permeable__ rocks tend to be __less resistant__ to weathering because 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**.
* __Impermeable__ rocks __do not allow water__ to flow through them. 
* Clays, mudstones, and most igneous and metamorphic rocks are impermeable. 

A spring creating erosion

* 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.

Groundwater flow removing 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. 

Saturation leads to slumping and sliding

Saturation promotes mass movement through lubrication and by adding weight. \n Leads to slumping in unconsolidated material and sliding in consolidated strata - producing a complex cliff profile. 

What are the two main types of cliff

* Steep un-vegetated cliffs
* Shallow-angled vegetated cliffs

Steep, unvegetated cliffs

* ​Produced where marine erosion dominates


* There is little or no debris at the base because it is broken up by attrition and transported offshore or along the coast. 
* High energy coastlines
* Eroded so quickly there is barely any vegetation
* Certain erosional landforms such as
* wave cut notches
* undercut
* wave cut platforms
* Strong waves
* long fetch
* strong winds

Shallow-angled vegetated cliffs

* They have a convex profile (convex = curved like the interior of a circle)
* Debris at the base
* due to sub-aerial processes (weathering, mass movement and surface run-off erosion) slowly move sediment downslope, but marine erosion is unable to remove it from base 
* Produced where there is little active marine erosion
* Debris creates a natural rock armour, lessening wave energy
* Waves have shorter fetches

Pore water pressure

The pressure of groundwater held between soil or rocks in the gaps (or ‘pores’) between particles.

* It is affected by the soil type, water flow conditions and level of the water table

Importance of Vegetation

Vegetation is important in stabilising sandy coastlines through dune successional development on sandy coastlines and salt marsh successional development in estuarine areas. 

Vegetation can stabilise unconsolidated sediment and protect it from erosion. 

* Plant roots bind sediment together, making it harder to erode. 
* Plant stems and leaves covering the ground surface protect sediment from wave erosion and erosion form tidal or longshore currents when exposed at high tide.
* They also prevent sediment from wind erosion at low tide. 

Vegetation - Rate of sediment accumilation

* Plant stems and leaves interrupt the flow of wind and water, reducing their velocity and encouraging deposition. 
* When the vegetation dies it adds its organic matter (hummus) to the soil. 

Why are coasts such harsh environments for plants? (7)

* They're exposed to high wind speeds at low tide. 
* Lack of shade produces a high diurnal (daily) temperature range.  
* They're submerged in salty water for half the day. 
* The evaporated sea spray makes the sediment saline. 
* Salt is highly porous and permeable, so rain water drains quickly- so plants have little fresh water. 
* Submerged sediment has its pores saturated with salt water - there's no oxygen for plant roots to respire with. 
* Sand lacks nutrients.  

Pioneer Plants

These are the first plants to colonise freshly deposited sediment.  \n They modify the environment:


1. Stabilising sediment
2. Adding organic matter that retains moisture, contributes nutrients and provides shade. 
3. Reduce evaporation in sand. 

\n Now, slightly less hardy plants can colonise the sediment. They add more organic matter, stabilise existing sediment and trap more. 

Plant Succession

The changing structure of a plant community over time as an area of initially bare sediment is colonised.  \n The process by which a series of different plant communities occupy an area over time.  

Xerophytic plants

 Plants specially adapted to dry conditions to colonise bare sand. 

Psammosere.

Plant succession on sand

Xerophytic pioneer plants examples:

* Sea couch grass
* Lyme grass
* Saltwort 
* Sea rocket. 

Marram Grass

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* has **waxy leaves** to limit water loss through transpiration and resist wind-blown sand abrasion. 
* has roots that can grow to **3m** to reach down the water table and the stem can grow **1m** a year to avoid burial by deposited sand. 
* allows the dune to grow, rapidly forming a **yellow dune**

Grey dune plant examples:

* Red fescue
* Heather
* Creeping willow
* Gorse

Succesion of sand dunes

* __**Embryo Dunes**__ form when seaweed driftwood or litter provides a barrier or shelter to trap sand.
* colonised by pioneer plants
* pioneer plants stabilise the sand allowing marram grass to colonise
* allows the dune to grow, rapidly forming a __**yellow dune.**__
* As the marram grass and sedge grass dies, it adds hummus to the sand, creating soil. A __**grey dune**__ develops, with plants such as gorse. 
* The dune is now __above high tide level__, so rain washes salt from the soil, making it __less saline__. 
* The soil now has improved nutrients and moisture retention, allowing non-xerophytic plants to colonise the dunes until a **climax plant community** is reached, in equilibrium with the climate and soil conditions. 

Salt marshes

* Halophytic plants are specially adapted to saline conditions to colonise mud. 
* Estuarine areas are ideal for salt marshes because:
* they're sheltered from strong waves (so sediment like mud and silt can be deposited) 
* rivers transport a supply of sediment to the river mouth, which may be added to by sediment flowing into the estuary at high tide

Halosere

Halosere is plant succession in salty water. 

Salt marsh successional development

* The __mixing of fresh water and sea water__ in the estuary causes clay particles to stick together and sink - called __**flocculation**__ 

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* __Blue-green algae__ and __gut weed__ colonise mud, exposed at low tide for only a few hours. 
* The algae binds mud, adds organic matter, and traps sediment. 
* As the sediment thickens, water depth is reduced, and the mud is covered by tide for less time. 
* __Halophytic glasswort__ and __cord grass__ colonise as the next seral stage - the marsh is still low, and covered by high tide each day.
* An accumulation of organic matter and sediment raises the height of the marsh until it is only covered by spring tides. 
* The higher marsh is colonised by less hardy plants 
* sea aster
* sea lavender
* sea thrift
* scurvy grass
* Rainwater washes salt out of the high marsh's soil, allowing land plants to colonise. 
* This continues until climax community is reached. 
* In most of the UK, the climax community would be __deciduous oak forest__, or coniferous pine forest in north Scotland. 

Creeks

In a salt marsh, the river distributaries run through the marsh as creeks.

Mangroves

In tropical latitudes, mangrove swaps stabilise marine sediment rather than salt marsh.