There are many different rock types in the earth’s crust but they can be divided into three main groups.
1. Igneous 2. Sedimentary 3. Metamorphic
Limestone Sandstone
Shales and sandstone Shales
Basalt Granite
Quartzite and other metamorphic rocks such as schist
Fig. 1 Geological map of Ireland 5.1 Igneous rocks
Igneous rocks are formed from magma which cools and solidifies either below ground, forming intrusive igneous rock, or on the earth’s surface, forming volcanic (extrusive) igneous rock. These rocks are made of crystals, which may be large or small depending on how fast the magma cools.
Intrusive igneous rock
Intrusive igneous rock can be divided into two groups: 1. Plutonic rocks form as magma cools very slowly (taking thousands of years) deep inside the earth’s crust. They have large crystals of quartz, felspar and mica. Granite is a common example. It is often grey in colour. When very large masses of magma are injected into the crust
Fig. 2 Granite is composed of three minerals: feldspar, quartz and mica. The white crystals are feldspar, the black ones are mica and the grey crystals are quartz.
they slowly cool to form large granite structures called batholiths. Weathering and erosion can remove the rock above these structures so that today we can see the batholiths in places such as the Wicklow Mountains.
2. Intermediate (hypabyssal) rocks form when magma makes its way closer to the earth’s surface and cools more quickly than plutonic rock so that the crystals are smaller. Dolerite is a common example. It is a dull grey colour and is found in the Fair Head Sill, County Antrim.