Napa Valley - volcanic rock (obsidian)

Obsidian, or volcanic glass, from Napa Glass Mountain.

The mountains along the east side of Napa Valley are primarily made up of Pliocene volcanic rocks of the Sonoma Volcanics (shown as Tpmv on the geologic map). One peak in that range, called Napa Glass Mountain, is made up of rocks that contain large pods of volcanic glass, or obsidian.

Obsidian is a volcanic rock. Volcanic rocks are formed when a body of melted rock (magma) deep under the Earth's surface is forced to the surface, where it either oozes out as a lava flow, or explodes out. Napa Glass Mountain is formed of a type of the exploded type of volcanic rocks, a flow of very hot volcanic ash and lava. The lava in this flow cooled so fast that no crystals were formed, creating the volcanic glass. You can tell a rock is volcanic because it is made, at least mostly, from tiny crystals too small to see, or glass without crystals at all.