As the speed of cooling of igneous rocks increases, what happens to the size of the crystals that form?

As the speed of cooling of igneous rocks increases, the size of the crystals that form decreases. When magma or lava cools quickly, there is less time for the crystals to grow, resulting in smaller crystals. This is commonly observed in volcanic rocks, which cool rapidly upon eruption and often exhibit a fine-grained texture with small or even microscopic crystals.

In contrast, when igneous rocks cool slowly, usually beneath the Earth's surface, there is ample time for the mineral grains to grow larger, leading to a more coarse-grained texture. Therefore, the relationship between cooling rate and crystal size is fundamental in understanding igneous rock formation and texture.