text: The larger crystals in granite tell us that it formed deep underground.  Geologists call such rocks intrusive igneous rocks.  Insulated from the coolness of earth's surface, the molten rock (magma--molten rock underground) cooled slowly and the crystals had time to grow large.  Basalt flowed onto earth's surface as molten rock (lava--molten rock on earth's surface) and there it cooled quickly, so the crystals had little time to grow.  Geologists call such rocks extrusive igneous rocks.\n\nThere are other differences between granites and basalts, but I think speed of cooling, related to where the molten rock cooled, answers your question.