Lithology & Vegetation

Geography: Edexcel A Level, topic 3.1.3

Lithology and Coastal Recession

Lithology plays an important role in the rate of coastal recession.

When Geographers talk about the physical properties of different rocks, this is known as bedrock lithology.
The existence of sedimentary, igneous and metamorphic rocks affects the rate of recession along a coastline.

Igneous rocks, such as Basalt and Granite, form from the crystallisation of magma. They are more resistant to erosion and weathering.
This is because igneous rocks contain crystals which increase the strength of the rocks and reduce the number of lines of weaknesses that would be exploited by physical processes.
Igneous rocks can either be intrusive (formed from magma inside the Earth) or extrusive (formed when magma escapes through vents on the Earth’s surface).
The average rate of erosion of igneous rocks is 0.1-0.5 cm per year.

Sedimentary rocks, such as Limestone and Sandstone, form from the build-up, compacting and hardening of sediments into layers over time by lithification.
Sedimentary rocks that have been formed in a shorter geological time scale tend to be more susceptible to erosion and weathering and erode at a faster rate compared with those formed over a longer geological time.
E.g. limestone and chalk.
The average rate of erosion of sedimentary rocks is 2-6 cm per year.

Metamorphic rocks, such as slate and marble, are formed from the change in the structure of sedimentary and igneous rocks, caused by variations in heat and pressure.
The process of change is referred to as the rocks having been metamorphosed.
Marble is an example of a metamorphic rock that has been formed from the changing structure of limestone caused by the re-crystallisation of calcite.
Metamorphic rocks are resistant to erosion.

Vegetation is important in supporting the development of sandy coastlines and so plays an important role in the rate of coastal recession.

Sand dunes need a continuous supply of sand, powerful winds to transport sand and obstacles to trap the sand, like plants.
A typical sand dune transect goes from the mobile dunes nearest the sea (embryo, fore and yellow) to the fixed dunes nearest the backshore (grey dunes and dune slack).
Sand dunes contain two types of plants:
Xerophytes, which can withstand periods of dry weather.
Halophytes, plants that can withstand high concentrations of salt from seawater.

Typical plants that colonise embryo and foredunes are sea rocket and sea crouch. These plants have deep roots and can tolerate high concentrations of salt.
Yellow dunes tend to be colonised by marram grass. These plants also have long deep roots and are salt tolerant.
Heathers are commonly found in grey dunes because of the higher rates of humus.

How do igneous rocks form?
Which of the following is an example of a metamorphic rock?
Which type of dune plant can withstand high concentrations of salt from seawater?
Which of the following are mobile dunes nearer the sea?