Igneous Rock

The first form of rock that ever existed on the planet were igneous rocks. It forms 95% of the earth's crust and there are over 700 different types of igneous rocks on the planet. Igneous rocks, derived from the latin word ignus meaning fire, were formed when magma was forced to the earths surface (crust)then cooled and solidified thousands of years ago. Igneous rocks can be further divided into two groups, intrusive and extrusive.

Intrusive rocks were formed within the earth's crust and were surrounded by a pre- existing rock called a 'country rock' therefore they cooled slowly. Due to this, they formed large crystals. An example of an intrusive igneous rock commonly found in Ireland is granite such as that found in the Wicklow Mountains.

Extrusive rocks were formed when molten rock from the earth's mantle extrudes onto the surface as lava. The magma cools much quicker in the open air, or in most cases in the sea water and due to this one of the features of an extrusive rock is it has fine crystals. An example of an extrusive igneous rock found in Ireland would be basalt such as that that formed the Giant's Causeway in Northern Ireland.

Pumice is argueably one of the most unusual type of igneous rock. It is lightweight and formed when molten rock rapidly explodes out of a volcano. It quickly loses pressure and heat, causing it to form small bubbles on the surface giving it it's "spongey" appearance. Tuff is another strange rock formed from volcanic ash. Obsidian is a volcanic glass that forms very quickly and is used as a cutting tool.

This is granite , and this is basalt