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Inyo lava flows erupt quietly after explosive activity

Obsidian Flow, a large circular-shaped lava flow covered in snow.
Obsidian Flow, a large circular-shaped lava flow, is part of the Mono-Inyo Chain. (Credit: Bailey, R. A.. Public domain.)

From almost any vantage point in Long Valley Caldera, the barren gray, craggy surfaces of three large circular lava flows are easily recognizeable - South Deadman, Obsidian, and Glass Creek flows. These flows were the last part of the molten rock to erupt onto the surface during the Inyo eruptions about 600 years ago. The volumes of the South Deadman and Obsidian flows were about 3 and 8 times larger than the volume of magma erupted explosively; the volume of the Glass Creek flow and its earlier explosive activity was about the same. The Obsidian flow (above) is about 1.5 x 2 km across.

Magma degases during ascent, erupts as lava flows

These flows oozed onto the ground only after the explosive eruption of pumice and ash had stopped. Clearly, by the time the molten rock had reached the surface, much of its dissolved gas had already been released into the atmosphere. Instead of being "trapped" in the magma until the build up of gas pressure became high enough to cause a sudden violent release of gas into the atmosphere (explosion), the dissolved water apparently formed gas bubbles that escaped during its ascent. The lava flows contain less than 0.5% dissolved water compared to 2-3% dissolved water of the earlier explosive magma. This small difference in water content (dissolved volatiles) of molten rock meant the difference between explosive eruptions and the quiet effusion of lava at the surface of the Inyo chain.

Lava flow examples

The edges of these lava flows are very steep and typically 30-50 m (100 - 165 ft) thick--testimony to the sluggish nature of rhyolite lava. Because rhyolite consists of 68-70% silica, it is the most viscous of all lava types. Consequently, it does not flow across the ground as easily as the other lavas and tends to form thick mound-shaped features called lava domes.

South Deadman Flow...
This view of South Deadman flow (vegetation free area, left center) is from the northwest rim of Long Valley caldera looking toward the south. Mammoth Mountain is the snow-covered peak on right skyline. (Credit: Brantley, S. R. Public domain.)
Aerial view of the Inyo Craters with Obsidian Flow.
View of Obsidian flow is toward the north; Wilson Butte lava dome (mound in left center) and the lava flows and domes of the Mono Craters are visible behind Obsidian Flow, Mono-Inyo Chain, California. (Credit: Brantley, S. R. Public domain.)
Glass Creek Dome seen from highway 395, Long Valley Caldera....
View of Glass Creek flow from Highway 395 looking toward the west; the crest of the Sierra Nevada towers above. The flow descends the steep northwest wall of the Long Valley caldera. (Credit: Venezky, D. Y. Public domain.)