Rocks

Types of Rock

Rocks are made up of different minerals. These minerals are held together by a natural cement. Rocks differ from each other in different ways :

  • Hardness

  • Texture

  • Colour

  • Mineral Content

There are many different types of rocks but three groups :

  • Igneous

  • Metamorphic

  • Sedimentary

Igneous Rocks

Igneous Rocks are formed as a result of volcanic activity. When hot molten magma escapes from the mantle it cools and hardens. If it reaches the surface as lava, it cools and solidifies quickly whereas the magma inside the crust cools more slowly.

Basalt is an example of an igneous rock. It is dark grey or black in colour and has a medium grained texture. Basalt is an extrusive rock. It is formed when lava spreads out over the surface of the Earth’s crust due to volcanic activity. It then cools and hardens. Basalt has tiny crystals which cannot be seen without the use of a microscope. This is because the lava cooled and hardened very quickly in the open air when it reached the surface.

Basalt is used for road chippings due to its hardness.

An example of where basalt can be seen is the Giant’s causeway in Antrim.

Basalt Columns at The Giants Causeway

Sedimentary Rocks

Sedimentary Rocks are formed from particles of other rocks, as well as dead plants and animals. These sediments are deposited on sea and lake beds as well as on land. Layers of this material build up over time and are compressed and cemented, creating solid rock.

Limestone is an example of a sedimentary rock. It is light grey in colour with a coarse grained texture. The limestone in Ireland was formed over 350 million years ago when Ireland was located close to the equator. Limestone is made from particles of shells and the remains of skeletons of sea creatures. When these particles sink to the seabed they are called sediment. The sediment piled up at the bottom of a warm, shallow sea over millions of years. The weight of the upper layers of sediment compressed and cemented the lower layers into strata (layers) of solid rock. Limestone is permeable. This means that water can pass through it. It is also soluble, which means that it dissolves in water.

Limestone is often used to make monuments, in the construction of buildings and to make cement.

Limestone can be found in the Burren

Limestone at The Burren

Metamorphic Rocks

Metamorphic rocks are formed from rocks that already exist. They form when igneous or sedimentary rocks are changed into new harder rocks by great heat or pressure or sometimes both.

Marble is an example of a metamorphic rock. It can be white red green or black. It has a smooth texture. Marble is formed when limestone is put under great heat or pressure due to plate movement or when molten magma comes in contact with limestone. It is a very hard rock that can vary in colour.

Marble can be cut to whatever size is required and is often used for monuments, floor tiles, fireplaces and ornaments.

Marble is found in Antrim, Cork, Connemara and Kilkenny.

Marble Quarry


Uses of Rock