Glassy igneous rocks form when magma cools too quickly for mineral crystals to form. Here's why:

* Crystallization: Minerals form within magma as it cools. The slower the cooling, the more time atoms have to arrange themselves into crystal lattices. This results in large, visible crystals.

* Rapid Cooling: When magma cools very quickly (like when it erupts as lava and flows into water or air), atoms don't have time to arrange themselves into organized structures. Instead, they solidify in a disordered, amorphous state, resembling glass.

Examples of glassy igneous rocks:

* Obsidian: A dark, volcanic glass with a smooth, conchoidal fracture.

* Pumice: A light, porous glass formed from frothy lava.

* Tachylite: A dark, glassy basalt.

Let me know if you have any other questions!