Little Glass Mountain Geologic Area

A view of Little Glass Mountain from Mount Hoffman with Mount Shasta in the background. Little Glass Mountain Geologic Area offers a rare glimpse of a young volcanic glass flow. It covers over one square mile, mostly on Shasta–Trinity National Forest, with about 100 acres on Klamath National Forest. The best exposures of Little Glass Mountain are accessible by road on the Klamath. Please use extreme caution when walking around volcanic glass. It is sharp and it commonly shatters.


Geologic Background

Just over 1,000 years ago, Little Glass Mountain was formed as viscous lava oozed out of one of the vents along the southwest flank of Medicine Lake Volcano and moved slowly down slope, building up the spectacular high flow front of broken obsidian and pumice you see from the road today. Glass flows are an uncommon and intriguing phenomena. Because the eruption at Little Glass Mountain was so recent, we are afforded the unique view of an intact flow.


Geologic Processes at Work

Forest road 43N46 follows the northern front of the lava flow known as Little Glass Mountain. You will note that the flow is thick (nearly 100 feet high) and that the "toe" is covered by talus. You may be surprised to see that such a variety of rock colors and textures were produced by just one eruption.

The black glass is called obsidian, and the frothy, airy rock is called pumice. Red hues are the result of weathering or oxidation during eruption.

Obsidian boulders from the glass flow. The red banding accentuates the sense of flow. The first eruptions from the vent were explosive and produced tephra, a deposit of small fragments of pumice, glass, ash, and broken rock fragments from the older, underlying lava flows. When lava began to ooze out of the vent, some of it contained large gas bubbles. Pumice formed at the top of the flow as the lava continued to release gas before it solidified. Solid obsidian also formed as it cooled. On the top and bottom of the flow, volcanic rocks were broken up as the flow advanced, forming breccia.


Cultural Significance

Obsidian within the Medicine Lake Highlands has been used for thousands of years by numerous American Indian tribes. With a cutting edge sharper than a surgical scalpel, obsidian was formed into a variety of everyday tools. Often these tools, and the flakes from their manufacture and maintenance, are the only traces left of past human activities on the landscape. Obsidian from the Medicine Lake Highlands was a principal trade item and has been found as far away as California's northwestern coast and central valley. Obsidian and obsidian source locations have spiritual as well as functional value for American Indians; as such these areas continue to be of special interest to today's American Indians.

Collection of obsidian from Little Glass Mountain Geologic Area is prohibited. Natural features and archeological and historical objects are protected by federal law. You can help preserve America's past by leaving archaeological and historic remains undisturbed, encouraging others to do the same, and reporting information about disturbances of these remains to National Forest personnel.

 Generalized map of Little Glass Mountain.


Fink, J.H., 1981., Surface structure of Little Glass Mountain, in Johnston, D.A., and Donnelly–Nolan, J.M. editors, Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California: U.S. Geological Survey Circular 838, p. 171–176.


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