Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock) or lava.

* The rate at which magma or lava cools determines the size of the mineral crystals that form within the rock.

* Slow cooling allows for larger crystals to form, leading to rocks like granite.

* Fast cooling results in smaller crystals or even a glassy texture, as seen in obsidian.

Let me know if you'd like to learn more about igneous rocks!