Lithology

Lithology: different rock types

There are three major rock types: igneous, sedimentary, and metamorphic. They all erode at different rates. Let's look at the bedrock lithology of these rocks.

Sedimentary rocks

Sedimentary rocks include rocks like limestone, sandstone, chalk, and shale, which are formed in strata (layers). Lithification has, over time, compacted and hardened these rocks. They are, in general, not crystalline. Sedimentary rocks are more vulnerable to weathering and erosion. They erode very quickly at an erosion rate of about two to six centimetres per year.

• Jointed sedimentary is limestone and sandstone, which are permeable.
• Chalk has air spaces between the particles, which makes it porous.
• Shale is finely grained, which makes it impermeable.

Igneous

Igneous rock includes basalt, granite, and dolerite. They are crystalline, and the interlocking crystals make them very strong. Crystals in the rock increase its strength, thereby reducing the lines of weakness that can be exploited.

As a result, they are resistant to erosion and weathering and impermeable. Igneous rocks have an erosion rate of about 0.1 to 0.5cm, so very slow.

Metamorphic

Metamorphic rocks include slate, marble, and schist. Crystalline metamorphic rocks are resistant to erosion. Many metamorphic rocks have a feature known as foliation: this is where all the crystals originate in one direction, which produces weaknesses. Metamorphic rocks are often folded and heavily fractured, and this creates weaknesses that can be exploited. They have a slow erosion rate of 1mm to 10cm per year.

An example of rock strata. Image: D.M. Vernon, Wikimedia Commons, Public Domain

Lithology and permeability

Permeable rocks allow water to pass through them. They contain voids called pores. Examples include chalk and poorly cemented sandstones. The flow of water through the rock can create high pore water pressure within cliffs, affecting their stability. Permeable rocks are susceptible to the processes of weathering.

• Where a porous rock overlays an impermeable stratum, groundwater cannot percolate down into the lower layer. Instead, water accumulates in the porous layer, producing a saturated layer because the pores are full of water. As a result, a spring will form on the cliff face at the top of the saturated layer. As the water flows down the cliff, fluvial erosion (surface runoff erosion) will attack the saturated permeable bed and lower impermeable layer. Eventually, the angle of the cliff profile will be reduced.
• Water flows through the porous sands but can't rush through the impermeable clay, so it flows along the interface. Groundwater flow (where water moves downwards and sideways) through rock layers can weaken rock by removing the cement that binds them together. As a result, weak, unconsolidated layers slump.
• Saturation, slumping in unconsolidated material and sliding in consolidated strata encourage mass movement, producing a complex cliff profile.

Lithology and coastal vegetation

Rocks and sediment play a significant role in influencing the shape of the coastal landscape. However, vegetation is essential in stabilising any landforms from further change. Vegetation helps to stabilise:

• Sand dunes.

• Coastal salt marshes, found in many river estuaries.

• Coastal mangrove swamps, which are found in tropical coastlines.

Vegetation helps to stabilise coastal sediment in many ways:

• The soil is held together by the roots of the plants, which helps to reduce erosion.

• When completely submerged, plants provide a protective layer. The surface is not directly exposed to the moving water and is, therefore, less easily eroded.

• Plants reduce the wind speed at the surface, so less wind erosion occurs.

• Plant interrupt wind and water, reducing their velocity and encouraging deposition.

• Organic matter (humus) is added to the soil when vegetation dies.

Plants grow in different coastal environments and are either halophytes or xerophytes.

• Halophytes (or brackish) - plants that are tolerant of salty conditions.

• Xerophytes - plants that are tolerant of dry conditions.

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A combination of saltwater, fierce winds, tides, and waves makes the coast an extreme environment for plants; as a result, only specially adapted plants can live here.

Plant succession

This is a long-term change in an area of initially bare sediment by colonised plants. This is especially important on coasts because of their role in coastal accretion (growth). On coasts where a supply of sediment and deposition occurs, pioneer plants begin to grow in bare mud and sand. Each stage is plant succession is known as a seral stage.

Pioneer Plants

Pioneering plants colonise freshly deposited sediment; they modify the environment by

• They stabilise the sediment.
• They add organic matter, which retains moisture, contributes nutrients, and provides shade.
• They reduce evaporation in the sand.

Now, slightly hardy plants can colonise the sediment. They add more organic matter and stabilise the existing sediments. Over time pioneer plants alter the environmental conditions from harsh and salty to an environment where other plants could survive. New species of plants can now colonise the area, which will change the setting progressively.

The result of plant succession is called a climatic climax community.

Dune succession

Sand dune succession, known as psammosere, requires very specialist plants. Dunes are a very dynamic environment; Xerophytic plants are ideal for colonising bare sand. Marram grass is a perfect example of a pioneer plant:

• It is tough and flexible, so it can cope when being blasted with sand,
• It has adapted to reduce water loss through transpiration.
• Their roots grow up to 3 meters deep.
• can tolerate temperatures of up to 60oC
• Sand dune succession takes places as follows

Embryo dunes are formed around driftwood or litter, providing a barrier or shelter to trap sand. As the dune grows, it is colonised by xerophytic pioneer plants, like sea couch grass and sea rocket. The embryo dunes alter the conditions allowing other plants to colonise and form a foredune . For example, pioneer plants stabilise the sand allowing marram grass to occupy.

This allows the dune to grow, rapidly forming a yellow dune; the surface is mainly sand, not soil, which is why it's called a yellow dune. The colonised plants begin to add humus, which in turn creates soil. Consequently, a grey dune develops, where plants such as gorse grow. The height of the dune is now above high tide, rain washes the salt from the soil, making it less saline. The soil now has improved moisture retention and more nutrients; this allows non-xerophytic plants to become established. Eventually, climax plant community is reached,

Saltmarsh succession

Plants that are able to live in saline conditions are known as Halopytic plants, and they are ideal for colonising mud. On a salt marsh, the bare mud is exposed at low tide and submerged at high tide. Estuaries are perfect as salt marshes because:

• Sediment like mud and silt can be deposited because they are sheltered from the forces of strong waves.
• Rivers transport a sediment supply to the river mouth, which may be added to by sediment flowing into the estuary at high tide.

Plant succession in salty water is known as Halosere.