Glass Mountain is the largest of several pumice-erupting cones along a northwest-trending fissure, which showered the surrounding area with pumice. Tree growth indicates that the first pumice deposit occurred some 1,700 years ago, the second about 1,400 years ago. (There was a report of light ash fall from Glass Mountain in 1910.) Glass Mountain has the largest flow of finely vesicular glass, extending some 5.6 kilometers (3.5 miles) easterly and dividing into two tongues. In the other cones the walls were breached and the flows went only short distances.

At the extremities, the tongues are dacite with some olivine basalt; the dacite makes a sharp transition to rhyolitic obsidian near the mountain. At the end of the eruption, the lava was so viscous that it pushed up into a small dome. Later activity resulted in a flow that partially covered the first pumiceous and scoriaceous surface phases, with dense, glassy interiors characterizing both flows. In the final stage a dome arose consisting of microvesicular rhyolitic glass, 400 meters (0.25 mile) in diameter and 45 meters (150 feet) high, covered with spines, some collapsed.

The surrounding vegetation is a pine woodland, with ponderosa, Pinus ponderosa, predominating. The rare Penstemon cinicola occurs in the area.

Integrity: There are pumice mines on the edges of the flow.

Use: Research, educational, observational.

Ref: Chesterman, C. W. 1955. Age of the Obsidian Flow at Glass Mountain, Siskiyou County, California. Am. Jour. Sci. Vol. 253, No. 7, pp. 418-424.

October 1976

Siskiyou
Inventory of California Natural Areas
Revision © 2005 Steven Louis Hartman