The molten rock that feeds a volcano comes from the upper mantle and crust. It does not come from the core, as many people think. If the molten rock never makes it to the surface of the Earth it will take a long time to cool. When it takes a long time to cool, it will have large minerals and is referred to as a plutonic rock. Volcanic rocks have reached the surface and cool faster, with smaller minerals.

 If you can see a volcano as it is erupting, you can determine if the event is violent or quiet. However, most volcanic rocks that you find are part of ancient, extinct volcanoes. Is there a way to tell about the eruption from just a rock?