Question

Give a detailed explanation of how the rate of cooling affects the size of crystals in igneous rocks. Use the terminology extrusive and intrusive in your answer

Answer 1

Answer:

Sometimes, when cool, sticky magma erupts, the lava solidifies too rapidly for crystals to form and so volcanic glass (obsidian) is produced.

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.

 

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.