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Grain Size & Cooling Rate

As magma cools, it begins to crystallise and form solid rock. Igneous rocks are made up of several different mineral crystals that grow within the melt as it cools.

The video below is a clip of crystals forming in solution. Crystals in magma grow in a similar way as the melt cools. (Video Credit: DCSF).

Volcanic Glass Sometimes, when cool, sticky magma erupts, the lava solidifies too rapidly for crystals to form and so volcanic glass (obsidian) is produced.
Basalt Lava If magma cools quickly, for example when basalt lava erupts from a volcano, then many crystals form very quickly, and the resulting rock is fine-grained, with crystals usually less than 1mm in size.
Igneous Intrusions - small If magma is trapped underground in an igneous intrusion, it cools slowly because it is insulated by the surrounding rock. Crystals have more time to grow to larger size.

In smaller intrusions, such as sills and dykes, medium-grained rocks are formed (crystals 2mm to 5 mm).

In large igneous intrusions, such as batholiths, coarse-grained rocks are formed, with crystals over 5mm in size.
Igneous Intrusions - large
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