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What is the difference in appearance between the crystals in igneous rock that cooled slowly and those in igneous rock that cooled quickly?

Igneous rocks are one of the three types of rocks, the other two being sedimentary and metamorphic. Igneous rocks are formed when magma cools down and solidifies. The interior of Earth is very hot and temperatures are high enough to melt rocks. These molten rocks (known as magma) rise upwards and cool down either above the surface of Earth or below the surface.


When the magma cools down below the surface of Earth, it has plenty of time to do so and hence the cooling process is very slow. This allows sufficient time for crystal growth and hence these rocks have large crystal sizes. These igneous rocks are known as intrusive rocks or plutonic rocks. An example of an intrusive rock is granite.


When the magma cools down above the surface of Earth, it has to do so rapidly. This means less time for cooling and hence less time for crystal growth. Thus, the crystals of these rocks would be small in size. Such igneous rocks are known as extrusive rocks. Basalt is an example of an extrusive rock. 


Hope this helps. 

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