A 40,000-year history of eruptions
The Mono Craters are a 10.5-mile (17 km) chain of volcanic domes and craters that represent the youngest mountain range in North America. This volcanic field, located south of Mono Lake, consists of at least 27 distinct domes, three large flows of glassy lava called coulees, and numerous explosion pits. The entire system is part of the larger Mono-Inyo Craters volcanic chain, which stretches for 25 miles (40 km) from Mono Lake south to Mammoth Mountain.
Volcanic activity in the Mono Craters chain began around 40,000 years ago, with eruptions occurring every 250 to 700 years over the last 3,000 years. The youngest and most northern feature, Panum Crater, formed just 600 to 700 years ago. Its eruption was a multi-stage event. First, rising magma met groundwater, causing a massive steam explosion (a phreatic eruption) that blew out a wide crater and scattered old lakebed sediments. This was followed by a pyroclastic phase that ejected a fountain of cinders and ash, building the pumice ring visible today. Finally, thick, gas-poor rhyolitic lava oozed out to form the jagged glassy dome inside the crater rim. The highest dome in the chain, Crater Mountain, rises 2,400 feet (730 m) above the surrounding Pumice Valley.
The magma that feeds these eruptions is a crystal-poor, high-silica rhyolite, which is extremely viscous. This thickness is what allows it to form steep-sided domes rather than flowing away like more fluid lavas. A magma chamber estimated to contain 48 to 144 cubic miles of molten rock sits 5 to 6 miles directly beneath the craters.
Obsidian and human history
The rapid cooling of the Mono Craters' rhyolitic lava created vast quantities of obsidian, a type of volcanic glass. This obsidian is characteristically black and was highly valued by indigenous peoples for its superior tool-making properties. When fractured, obsidian creates an edge that is sharper than a steel surgical scalpel, measuring only about three nanometers thick at its cutting edge.
For centuries, the Mono Paiutes and other Native American groups gathered this raw material directly from the craters. They were masters of a technique called flintknapping, fashioning the glass into razor-sharp arrowheads, knives, and other tools. Unworked obsidian from the Mono Craters was a significant trade item. It was carried over the Sierra Nevada and traded with other groups across the continent, with archaeological evidence of its use found at ancient campsites far from its origin.
The geology of the craters shows a mix of this dense, black obsidian and light, porous pumice. Both are chemically identical, their different textures resulting from how much volcanic gas was trapped during cooling. Magma that lost its gas or cooled under high pressure became obsidian, while frothy, gas-rich magma solidified into pumice. At Panum Crater and Obsidian Dome, bands of pumice and obsidian are often swirled together in the same rock, a record of the turbulent flow of viscous lava.