|View south from top of Obsidian Dome; Glass Creek Dome mid photo, Sierra Nevada on skyline.
I was intent on visiting a rhyolite dome or coulée (lava flow), landforms that had puzzled Russell during his fieldwork in the Mono Basin in 1883. "These outbursts of acidic lava are in strong contrast with the overflows of basic rock with which geologists are most familiar ... [which] are frequently quite liquid at first, flow rapidly, and reach a distance of many miles before congealing sufficiently to check their progress." Not so the Mono coulées. They barely reached beyond the foot of their cones.
|Mono Craters, added arrows point to coulées—short thick lava flows with rugged surfaces and steep sides (from USGS 1971).
Time has proven Russell wrong in thinking the Mono coulées unique; similar flows have been found around the world. But he was spot on about geological interest. Rhyolite volcanoes are relatively uncommon, so their abundance in eastern California makes the area attractive to volcanologists. Especially intriguing is their youth. Most erupted in the last 10,000 years, several in the last thousand. And they may be only sleeping.
|Between Mammoth Mountain and Mono Lake "virtually every hill is a young volcano" (Sharp & Glazner 1997; NASA photo 2000).
|Flow front, North Coulée, Mono Craters.
|Obsidian Dome, the cowpie coulée. Google Earth 2019.
|Looking ± northeast at Obsidian Dome; blue line is gated access road. Gray area is an old pumice quarry, the reason for the road. Google Earth 2019.
|Formidable slope of broken glass, slightly worried field assistant for scale.
|But the ascent turned out to be a stroll :)
|Chunks of black glass were beautiful to the eye but challenging for the camera's light meter.
Whatever the exact nature of glass, the volcanoes that extruded these domes and coulées were well equipped to produce it. The magma was >70% silica; at such a high concentration silica tetrahedra (molecules) bind tightly to each other, making super viscous lava. Not only did it barely flow, it was so viscous that other kinds of atoms couldn't move around and bond with their brethren to form crystals. When crystal-poor lava such as this cools rapidly, for example by being carried to the surface in a volcanic eruption, obsidian and other forms of volcanic glass result.
|The tiny silica tetrahedron plays a big role in volcanoes (source).
|Obsidian with scattered small crystals in a matrix of glass.
|What happened here?!
|Stony rhyolite can be as beautiful as obsidian.
|Coarsely vesicular obsidian, part of a squeeze-up (read more here).
|The Inyo Dike, suspected source of Obsidian Dome and its neighbors (Reches & Fink 1988).