Formation of Igneous Rocks – Sample Answer

Formation of Igneous Rocks – Sample Answer

Igneous rocks are formed when magma is cooled, either inside the earth’s crust or on the earth’s surface. Plutonic igneous rocks are formed when molten rock cools and hardens below the surface e.g. granite. Volcanic igneous rocks are formed when molten rock cools and hardens on the surface, e.g. baslat. All igneous rocks have crystals. The smallest crystals are formed when the molten rock cools slowly deep in the crust. The smaller crystals are formed when the rocks are cooled quickly ont he surface.

Formation of Basalt

Basalt is a volcanic, igneous rock that is cooled quickly on or near the earth’s surface. It contains very small crystals as a result of rapid cooling. In Northern Ireland, magma poured out quietly from a crack and weakened the crust. The lava came into contact with the cold sea water and cooled rapidly in regularly spaced centres, forming hexagonal columns of basalt, which is now called the Giant’s Causeway.

Formation of Granite

Granite is an igneous, plutonic rock that cooled very slowly within the earth’s crust. It contains large crystals as a result of this slow cooling. It’s key mineral components are mica, feldspar and quartz which form granite. The overall colour of granite varies from pink with dark spots to black and white. Mica is composed of either black or white crystals, while feldspar is reflective and quarts is colourless or transparent.  Granite is widely used as an ornamental rock, for table tops and for headstones. Granite can be found in the Wicklow mountains in Ireland.


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