More specifically, it's an extrusive igneous rock because it formed from the rapid cooling of lava on the Earth's surface. The rapid cooling prevented the formation of crystals, resulting in a glassy texture.

Here's a breakdown of why it's classified as igneous:

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

* Extrusive igneous rocks solidify on the Earth's surface.

* Volcanic glass is a specific type of extrusive igneous rock that forms when lava cools so quickly that mineral crystals don't have time to form.

Some common examples of siliceous volcanic glass include:

* Obsidian: A dark, glassy rock with a conchoidal fracture.

* Pumice: A light, porous rock with a frothy texture.

* Perlite: A glassy rock that expands significantly when heated.