What can be said about an igneous rock with larger crystals compared to one with smaller crystals?

1 Answer
Oct 5, 2017

Answer:

It has had more time to crystallise

Explanation:

Intrusive igneous rocks are as you described, magma that sits deep in the crust and slowly crystallises.

The main control on crystal growth is time, if a lava is erupted onto the Earth's surface then it has little time to crystallise and therefore the grains are smaller. This could be something like a basalt (which is an extrusive igneous rock):

http://geology.com/rocks/basalt.shtml

But if you leave the magma in the crust and give it time to crystallise, then the grains will be much bigger like that of a granite:

http://geology.com/rocks/granite.shtml