Obsidian

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?

Photo: Austin Halstead, 2021.

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

• This rock forms where felsic magma flows out of the earth and cools quickly.
• Obsidian can be found near volcanoes that erupt felsic (high silica) lavas.
• This includes, stratovolcanoes like Mt. St. Helens, or supervolcanoes like Yellowstone and the Newberry Complex in central Oregon.

How old is this rock?

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?

• This rock is made of volcanic glass
• There are no crystalline minerals in it, making shiny and reflective.
• This also causes it to break into sharp, curved pieces.
• Just like a broken glass that you accidentally drop on the floor.

Can you find the direction of flow?

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?

Photo: Austin Halstead, 2021.

• Obsidian breaks into curved shapes called conchoidal fractures.
• This happens because it is a glass, which means no mineral crystals formed when it hardened.
• Rocks that have REALLY small minerals in them can break like this too, but they are not as shiny and smooth as real volcanic glass.
• There are conchoidal fractures all over the rock, try to find one not pointed out in the picture!

What was this rock used for?

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

• This rock was used for thousands of years for stone tools.
• Rocks that broke conchoidally have a very sharp edge to them.
• Obsidian knives can be sharper than modern steel surgery scalpels.
• People all around the world used and traded obsidian for stone tools, up until the widespread use of metal tools.
• These obsidian tools from the end of the Pleistocene (over 10,000 years ago) were found at the Connley Caves in Oregon by the Oregon Museum of Natural and Cultural History.

Glossary

• Felsic: usually light-colored rocks (obsidian is an exception) with a high silica content, and low iron and magnesium.
• Conchoidal fracture: a rounded kind of fracture often found in brittle materials.

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.