2B.3A: Lithology and Rate of Coastal Recession. 

Bedrock lithology (igneous, sedimentary, metamorphic) and unconsolidated material geology are important in understanding rates of coastal recession. 

How Lithology Affects Resistance:

1. Mineral composition
   1. Some rocks contain reactive minerals easily broken down by chemical weathering, e.g. calcite in limestone. 
   2. Other minerals are more inert that chemically weather more slowly, if at all, e.g. quartz in sandstone. 
2. Rock class
   1. Sedimentary rocks, e.g. conglomerates, sandstones, limestones and clays, are clastic (made of clasts (sediment particles), cemented together)
   2. Many cements are reactive and easily chemically weathered, e.g. iron oxide and calcite. 
   3. Sedimentary rocks with very weak cementation, e.g. boulder clay, gravels and sands, are termed unconsolidated. 
   4. Igneous rocks, e.g. granite, and metamorphic rocks, e.g. marble, are crystalline with strong chemical bonding. 
   5. Rocky coastlines vary in resistance of geology.
      1. Granite erodes at a rate of 0.1 cm p.a.
      2. Carboniferous limestone at 1 cm p.a.
      3. Sandstone at 10 cm p.a.
      4. Boulder clay at 1 m p.a.
3. Structure
   1. Rocks with fissures (e.g. faults and joints) or air spaces (porous) rocks, weather and erode rapidly. 
4. Dip of rock strata and juxtaposition of permeable and impermeable rocks can also affect resistance. 

 First, some rather important definitions:

• Lithology is rock type.
• Rate of recession is the speed at which the coastline is moving inland.
• Clastic rocks are those made of sediment particles cemented together
• Crystalline rocks are made of interlocking mineral crystals. 

 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)

Igneous

• Igneous rocks are formed from solidified lava or magma. (Lava is just magma on the surface of the Earth)
• Some examples are: granite, dolerite, basalt and pumice. 
• They erode and weather very slowly, producing very slow rates of coastal recession, because:​
  • It's composed of interlocking crystals, forming hard, resistant rock. (so less recession)
  • In addition, although they may contain cooling joints, they usually have fewer joints (e.g. granite) and weaknesses than other rock groups, slowing the rate of recession. 
• ​Igneous coasts recede at less than 0.1 cm p.a.
• However, there are exceptions:
  • ​Newly formed lava and solidified ash layers (tuff) erode easily.
  • Newly formed volcanic islands can exhibit very rapid erosion rates of 40m p.a.​

Metamorphic 

• They are formed by the recrystallisation of sedimentary and igneous rocks through heat and pressure.
• Some examples are: slate, schist, marble, gneiss
• It's hard and resistant because:
  • ​It has a crystalline structure
• ​However, it is less resistant than sedimentary rock because:
  • ​​Their crystals are often orientated in the same direction (foliation) making them weaker than the interlocking crystals of igneous rocks.
  • Often heavily folded and faulted
• ​Metamorphic coasts often recede slowly about 0.1-0.3 cm p.a.

Sedimentary

• They are formed by the compaction and cementation of deposited material, or sediment.
• E.g. sandstone, limestone, shale

1. Usually the sediment is fragments of other rock (clasts)
   1. Conglomerates
   2. Sandstones
   3. Mudstones
2. ​Sometimes the sediment is a chemical precipitate.
   1. ​E.g. limestone
3. ​Sometimes the sediment is composed of dead organic matter.
   1. ​Coal
   2. Chalk

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They are less resistant than metamorphic or igneous rocks, due to: 

• Weak bedding planes
• ​They're clastic
• Often heavily jointed as a result of compaction and pressure release.
• Rocks like shale may have many bedding planes and fractures. 

However, the rate of erosion varies from slow (0.5 cm) to fast (10 cm):

• Carboniferous limestone 1 cm p.a.
• Young sandstones 10 cm p.a.

This is because:

• Older sedimentary rock is buried deeper and is subject to more intense compaction with stronger sedimentation - so tend to be more resistant than younger sedimentary rocks.

Unconsolidated Sediment​ - sediment that has not yet been cemented to form solid rock (lithification)
Drift geology is recently deposited unconsolidated sediment that usually overlies the solid geology of the bedrock. 

Some examples of unconsolidated sediment are: (or sand)

• Fluvial alluvium 
• Glacial boulder clay
• Aeolian loess

They are very easily eroded, resulting in very fast rates of recession

• The boulder clay of Holderness coast in Yorkshire retreats at 2-10 m p.a.

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