JC Rock Types

The crust of the earth is made of solid rock. There are three types of rocks. There are igneous rocks, sedimentary rocks and metamorphic rocks.

Igneous Rocks

Igneous means ‘fire formed’. Igneous rocks form when hot molten magma cools. When magma cools ON the earth’s surface, basalt rock is made. Volcanic eruptions cause magma to come to the earth’s surface where it cools and hardens. This happened in the Giant’s Causeway where the basalt formed into columns.

When magma cools IN the earth’s crust, granite is formed. Magma can rise into the crust but does not come onto the surface of the earth. Sometimes this magma lies in a magma chamber which feeds volcanic eruptions. If the magma in the chamber cools and hardens, it turns into granite. Granite can be found in the Wicklow Mountains.

Sedimentary Rocks

Sedimentary Rocks are made from sediments. Sediments are tiny pieces of rock ( e.g. sand ) which were worn off other rocks by the weather. When these sediments are weathered, they may be carried by rivers to the sea where they settle on the ocean bed. Over time, these sediments build up in layers. As this happens, the lowest layers are compressed or squashed to form rock. Sandstone is an example of a sedimentary rock made when layers of sand were compressed on the ocean floor. Sandstone can be found in County Cork.

Limestone is another example of a sedimentary rock but is different to sandstone. Limestone is made up of the shells of dead sea creatures. When these creatures died, they fell to the ocean floor where they were covered by more shells and other sediments. Over time the shells were compressed and hardened into limestone rock. Limestone covers much of the Central Plain of Ireland.

Metamorphic Rock

Metamorphic means ‘changed’. Metamorphic rocks are other rocks which have been changed. Rocks can be changed by the influence of heat, pressure or heat and pressure working together. If, when magma rises into the earth’s crust it comes into contact with another rock it, will ‘cook’ the rock and change it into another rock. For example, Quartzite is a metamorphic rock which formed when sandstone was exposed to the heat of magma. Quartzite can be found on the Great Sugarloaf Mountain in County Wicklow. Similarly, great heat changes limestone to marble. Marble can be found in Connemara and in County Kilkenny.

Folding occurs when rock is squeezed due to the movement of the plates of the crust. When folding happens, rock is put under great pressure. When Shale (made from mud and silt) is put under pressure, it changes into Slate – the same stuff we put on our roofs.