Types of Rock and their distribution within the UK

Learning Objectives:

•To know that there are 3 main rock groups found in the UK.

•To understand how each rock type was formed.

•To be able to name locations of each rock type in the UK.

Types of Rock

Although there are many types of rock on the Earth’s surface, there are only three groups of rocks.

Rocks are either igneous, sedimentary or metamorphic.

Igneous Sedimentary Metamorphic

Brief Definition

Where were they formed?

How they were formed?

Rock types found in the UK

Main UK areas where rock outcrops occur

Igneous Rocks

‘Formed by fire’; they begin as magma in the interior of the earth. Some are formed by lava cooling on the earth’s surface after a volcanic eruption. For example, the basic lava that flows from tensional (constructive) margins and forms shield volcanoes cools to form basalt rock. The Giants Causeway in N. Ireland is eroded basalt rock.

Others are formed by magma cooling underground having being intruded into other rocks without reaching the surface. Granite is an example of this type of igneous rock.

It is often intruded during the building of fold mountains along compress ional (destructive) plate boundaries. Granite outcrops on the surface after erosion of the rocks above it over millions of years.

Today it is exposed in many places in Scotland, and forms most of the moorlands of Devon and Cornwall as well as the dramatic cliffs at Land’s End.

Sedimentary Rocks

Sediments are small particles of rock transported by water, ice and wind. Most eventually reach the sea bed where over the years of successive layers of sediments accumulate. The weight of materials above compresses the sediments below into sedimentary rocks. These rocks are laid down in layers, or beds, with lines of weakness, or bedding planes, between layers

When sand is compressed, sandstone rock is formed.

Clay forms from the accumulation and compression of deposits of mud.

Limestone and chalk consist of calcium carbonate which comes from the remains of plants and animals.

A lot of limestone was formed during the Carboniferous period (280-345 million years ago) because at that time Britain was a warm shallow sea, rich in plant and animal life

Metamorphic Rocks

These are rocks that have been changed in shape or form. They begin as either igneous or sedimentary rocks but are later altered by heat or pressure. This happens, for example, along destructive plate boundaries and fault lines.

Heat and pressure change limestone into marble and clay into slate. Both marble and slate are harder forms of the original rocks, and have greater economic value. Marble is used in buildings and floors. Slate, until recently, was the main roofing material in the UK.

Dartmoor

Land’s End

South Downs

North Downs

Yorkshire Dales

London Basin

Peak District

Tees Exe Line

Granite – An Igneous Rock

What is Granite?

Granite is an igneous rock. That means it was made from molten rock from inside the Earth (the mantle, in fact). If the magma doesn’t quite make it out of the crust, but cools down inside the crust, it makes granite. Where the magma intrudes (goes into) the crust it makes granite intrusions.

Diagram to show different types of granite intrusions

1. The top of the granite might be exposed if the rock on top gets eroded away. Granite is quite hard to erode, so it gets left sticking up.

2. This is what has happened in Devon and Cornwall – Dartmoor, Bodmin Moor, and Land’s End are all part of a huge batholith.

3. The top of the batholith isn’t smooth, there are bits that stick up more. These bits are called tors.

What’s a Tor Like?Granite is a jointed rock. That means it has small crack is it. These joints are lines of weakness. Granite doesn’t get weathered or eroded very easily, but these joints can be weathered by frost-shattering.