Formation of igneous rocks

Igneous rocks

formed when magma from the mantle cools and solidifies, either within the earth's crust (intrusive) or on the earth's surface (extrusive).

Basalt

Basalt is an igneous rock. It is a volcanic

(extrusive) rock formed at divergent plate boundaries. It can be found at the Giant's Causeway in Co Antrim. It was formed approximately 65 million years ago when the Eurasian and North

• American plates separated.

As the plates moved apart, fissures were created on the earth's crust, allowing lava with low silica content to pour out slowly onto the earth's surface. The lava cooled and hardened rapidly as it met the cold air, forming a 'basic' igneous rock.

Only small crystals of quartz had time to form in the rock due to the rapid cooling of the lava.

Basalt at the Giant's Causeway contracted and cracked into hexagonal (six-sided) columns due to the rapid cooling of the rock in a river valley.

Granite

Granite is an igneous rock. It is a plutonic (intrusive) rock formed at convergent plate boundaries. Granite is found in the Wicklow mountains. It was formed at the Wicklow Mountains approximately 400 million years ago during the Caledonian mountain building period when the Eurasian and North American crustal plates collided.

During the folding process, molten magma with silica content close to 70% intruded into a large space in the upfolds beneath the earth's surface. The magma cooled slowly as it was protected from the elements of weather by overlying layers of slate and quartzite metamorphic rocks.

Over time, it formed a large domed mass of granite known today as the Leinster Batholith. Large crystals of minerals such as feldspar, quartz and mica were formed in the granite as a result of different minerals cooling at different temperatures.