5. Rocks 5.4 Rock Groups


Igneous Rocks As shown in figure 4, igneous rocks are formed from the magma found in the mantle just below the crust. When magma is forced up into the crust and becomes trapped, it begins to cool very slowly. Rocks formed in this way are known as intrusive igneous rock. Igneous rocks that form when lava reaches the earth’s surface are called extrusive. As the lava spreads out on the surface and meets the air, it cools and hardens very quickly.


Granite Formation


Extrusive igneous rock


Intrusive igneous rock 4. Igneous rock formation


Granite is an intrusive igneous rock. As it cooled very slowly over millions of years, the minerals in the magma formed very large crystals . These crystals include quartz, mica and feldspar. Granite is a hard rock and has a rough texture. Granite is multi-coloured.


How it is used It is used for building monuments and gravestones, kitchen countertops and fireplaces.


Location


It can be found in the Dublin–Wicklow and Mourne Mountains.


Granite countertop


Granite


Basalt Formation


Basalt is an extrusive igneous rock. Lava cools very quickly when it reaches the surface. Because it cooled so fast, basalt has very small crystals that can only be seen using a microscope. Basalt is grey to black in colour. It is very hard and has a smooth texture.


How it is used Crushed basalt is used in road-building. Location


Basalt can be found in the Antrim– Derry plateau. The Giant’s Causeway in Antrim consists of columns of basalt.


The world-famous Giant’s Causeway in Antrim


49


FUN FACT! The Giant’s Causeway is


Northern Ireland’s most popular tourist destination.


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