glassy black rock

This rock is called obsidian. It is an igneous rock and forms when felsic (rich in silica) lava cools quickly on the earth’s surface—forming natural glass!

Where did this rock form?

Volcano erupting felsic magma

Photo: Austin Halstead, 2021.

map of the western U.S. with obsidian deposit locations

Map of obsidian deposits in the Western U.S. (Google Earth Pro); Photo: Austin Halstead, 2021.

How old is this rock?

Geologic time scale with red arrow pointing to Late Cretaceous.

Source of image: https://www.geosociety.org/GSA/Education_Careers/Geologic_Time_Scale/GSA/timescale/home.aspx; Photo: Austin Halstead, 2021.

We don’t know when this exact rock formed, but we know that the oldest obsidian found in nature formed in the Late Cretaceous (the last period of the dinosaurs).

This is because obsidian glass is unstable at Earth’s surface and weathers into a more stable mineral form over time with enough heat and or water.

What is this rock made of?

Can you find the direction of flow?

Lava flow lines perpendicular to vertical pencil.

Photo: Austin Halstead, 2021.

These stripes show that the lava was flowing perpendicular to the pencil when it hardened.

Can you find the conchoidal fractures?

Shell-shaped fractures on the rock surface.

Photo: Austin Halstead, 2021.

What was this rock used for?

Tools made of obsidian.

Source of image: https://mnch.uoregon.edu/collections-galleries/oregon-where-past-present; Photo: Austin Halstead, 2021.


About this page

This page was researched and written by Austin Halstead for the Regional Field Geology of the Pacific Northwest (GEOL 502) course in Fall 2021.