What rate does magma deep below Earth's surface cool What size of crystal results from magma cooling deep below Earth's surface?

Magma deep below Earth's surface cools slowly, often at rates of just a few degrees per million years. This slow cooling allows for the formation of larger crystals, as the minerals have more time to grow. As a result, the crystals in rocks formed from deep-seated magma, such as granite, are typically larger than those found in rocks formed from rapidly cooled lava, like basalt.