Rocks found on the Earth's surface actually come from inside the Earth -
so they tell us a lot about the Earth's interior. They are classified
(organised) into three main groups: igneous rocks, sedimentary rocks and
metamorphic rocks.Rock Types
Igneous rocks are formed by magma
from the molten interior of the Earth. When magma erupts it cools to
form volcanic landforms. If magma cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering.Examples of igneous rocks include basalt and granite.
Metamorphic rocks have been subjected to tremendous heat and/or
pressure, causing them to change into another type of rock. They are
usually resistant to weathering and erosion and are therefore very
hard-wearing.Examples of metamorphic rocks include marble, which originates from limestone, slate, which originates from clay, and schists formed from sandstone or shale (sedimentary rocks).
Sedimentary rocks are formed from sediments that have settled at the
bottom of a lake, sea or ocean, and have been compressed over millions
of years. The sediment comes from eroded rocks carried there by rivers
or ice, and from the skeletons of sea creatures.Examples of sedimentary rocks include sandstone, limestone, chalk and clay.
The limestone solution process is also known as carbonation.
Limestone is made of calcium carbonate. When carbon dioxide is dissolved in rainwater, it makes a weak acid called carbonic acid.
carbonic acid comes into contact with limestone and passes through
joints and bedding planes, it reacts with the rock to form calcium bicarbonate.
The calcium bicarbonate is soluble and is carried away in solution, gradually weathering the limestone.
When rock is exposed to weather
conditions it becomes subject to processes, such as freeze-thaw
weathering, that cause it to change and break down.
The freeze-thaw weathering process is also known as frost shattering.
Water - eg from rainfall or melting snow and ice - becomes trapped in a crack or joint in the rock.
If the air temperature drops below freezing, the water will freeze and expand by 9-10 per cent putting pressure on the rock.
The ice will melt when the temperature rises above freezing.
If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments.
The fragments may then be deposited as scree at the foot of a slope.
It is most effective where the temperature fluctuates around 0°C, eg on north-facing high altitude slopes in Snowdonia.