Explosion Craters and Giant Gas Bubbles on Holocene Rhyolite Flows at Newberry Crater, Oregon

Robert A. Jensen, Engineering Geologist
Deschutes National Forest, Bend, Oregon 97701
Oregon Geology, Vol. 55, No. 1, January 1993

Placed on web - 8/16/96

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Forty-seven explosion craters pock mark the surface of the Big Obsidian Flow and Interlake Obsidian Flow within Newberry Crater in central Oregon. The craters range from 12 to 60 m in diameter and from 5 to 14 m deep. Discontinuous rings of rubble form rims around the craters. At the bottom of four of these are parts of spherical cavities or giant gas bubbles up to 15 m in diameter and filled to varying degrees with rubble that has collapsed from the crater walls above.

Rhyolite flows are thought to develop layers as gases exsolve from the flow. The surface layer is finely vesicular pumice; beneath this is a layer of obsidian underlain by coarsely vesicular pumice. Giant gas bubbles in the Newberry Crater flows probably form and grow beneath or within the coarsely vesicular pumice and rise upward into the obsidian layer within several meters of the surface, where they burst explosively. The blast creates a steep-walled crater rimmed with blocks of pumice and obsidian. Debris from the explosion falls back in, partially or completely obscuring the giant bubble.

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