The key difference between intrusive and extrusive igneous rocks the size of crystals making them up is related to how rapidly melted rock cools. The longer melted rock has to cool, the larger the crystals within it can become. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet.

You have answered it Alright!