Igneous rocks

There are two types of igneous rocks;

  1. Extrusive/volcanic - these are formed at constructive/ divergent plate margins/boundaries when lava and magma cools and solidifies quickly on or near the earths surface respectively. Basalt accounts for over 90% of the worlds extrusive/volcanic igneous rock. It is black or dark-grey in colour with small crystals because of the rapid cooling. Basalt is fine-grained, hard and erosion-resistant rock, amking it suitable for road construction. However, its lack of luster means it is unsuitable for building construction. When it is weathered, it produces deep fertile soils. It can be found in the Antrim-Derry Plateau and the Giants Causeway.

  1. Intrusive/ plutonic - these are formed at destructive/ convergent plate margins/boundaries when magma cools and solidifies slowly far below the earths surface, often taking thousands of years to fully harden. Granite, from the Latin word ‘grain’, is the most common intrusive/plutonic rock. It was formed slowly underground, when magma intruded into gaps in the earths crust eg, batholiths, allowing it time to form large crystals. It consists of three minerals; mica, feldspar and quartz. It is normally dark grey; however variations in the mineral content can alter and influence its colour. Granite is a coarse-grained, hard and erosion-resistant rock; because of this, it is used to face buildings and for the construction of bridges. When it is weathered, it produces kaolin/ china clay which is used in the production of porcelain. It can be found in the Wicklow Mountains and the Leinster Batholith