Igneous rocks are '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 being thrown out by a volcanic eruption.
For example, basic lava that flows from constructive margins and forms shield volcanoes cools to form basalt rock. This has been eroded into hexagonal blocks at the Giant's Causeway in Northern Ireland.
Others are formed by magma cooling underground after having been 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 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.
Sediments are small particles of rock transported by water, ice and wind. Most eventually reach the sea bed where over the years 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. For example, the shells of sea creatures are made of calcium carbonate; when these animals die, masses of shells accumulate on the sea floor, building layers of limestone rock.
A lot of limestone was formed during the Carboniferous period (280-345 million years ago) because at that time much of Britain was a warm shallow sea, rich in plant and animal life.
Metamorphic rocks are those which 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 widely used in building and for floors in Mediterranean countries such as Italy.
Slate splits easily into sheets and, until recent times, was the main roofing material used in the UK.
Distribution of rock types within the UK
The geology of Highland Britain to the north and west of the Tees-Exe line is dominated by old and hard rocks. The majority are igneous and metamorphic rocks which have resisted erosion and therefore form the upland and mountainous parts of the country.
In Lowland Britain, to the south and east of the Tees-Exe line, the geology is dominated by younger sedimentary rocks. There is much low-lying and flat land, such as in the clay vales. Chalk, however, is more resistant to erosion than many of the other sedimentary rocks which surround it. This is why chalk ridges and scarps, such as the North and South Downs, appear to be high and steep, but they form lower, gentler and more rounded landscapes than the rocks in the uplands of Highland Britain.