VOLCANIC (EXTRUSIVE) IGNEOUS ROCKS

Form when magma cools outside on the Earth's crust. These have smaller crystals because lava cools quickly when exposed to the elements e.g. basalt

Basalt is a fine-to-medium-grained igneous rocks. It ranges in colour from dark grey to black. It is heavy and has small crystals (too small to be seen by the naked eye). It is the most common extrusive rock in Ireland.

Basalt is composed of feldspar and quartz. It is a hard, resistant rock and is therefore used for road construction. It is too dark to use for buildings. In the Antrim-Derry Plateau, lava poured out of fissures in the ground (60 million years ago). It cooled and contracted to form six-sided columns of basalt e.g. The Giant's Causeway

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notes on the rock cycle