What does crystal size tell us about igneous rocks?
Fast cooling magma/lava causes what is called extrusive igneous rocks. Very fine crystals means it cooled quickly and is called aphanitic texture. Flowing lava can get gas bubbles trapped as it cools leaving holes in the rock, this is called vesicular texture. If the crystals don’t have time to nucleate you get a glassy igneous rock like obsidian. All of those are extrusive. Magma that cools at depth forms larger visible crystals. This is called phaneritic texture. The larger the crystals the deeper and slower it cooled. If a rock has large crystals in a fine grained matrix it is called porphyritic texture. This shows multiple stages of cooling. It began to cool at depth and then was thrust to the surface in a dike or volcano and the rest cooled fast.
