Obsidian Dome

The road to the top of the dome

Mammoth Mountain is as renowned for being part of a volcano as it is for its skiing. The Long Valley Caldera, a colossal volcano in the eastern Sierra Nevada region, is an area of extreme volcanism. Volcanoes have been erupting here since long before its last climactic eruption 750,000 years ago, and the last eruptions are as young as a few hundred years. This final eruption erupted approximately 600 cubic kilometers of material and resulted in the subsidence of the crust between 2 and 3 kilometers to create the current caldera. This particular feature, Obsidian Done, is related to the volcanics here but is from a different system: the Mono-Inyo crater chain. 

The last eruption from this chain was 250 years ago on Pahoa Island in Mono Lake.  Obsidian Dome and the nearby Glass Creek Dome erupted at about the same time, calculated through tree-ring studies to be late summer in 1350 CE.

They both started as an explosion crater, like the nearby Inyo Craters, and as the eruptions progressed, increased eruptions from steam explosions to tuff rings all ended in a slow extrusion of rhyolitic lava. This lava had a very high silica content, which makes it extremely viscous and thick, so it doesn't flow like basalt lavas do. These domes erupted very hot material, as evidenced by the lack of crystals in them. They are not homogenous masses of solid rock. Rather, they are loose piles of slowly extruded magma. Think of it like squeezing a toothpaste tube straight up through the ground. It won't flow, but rather create a pile on the surface. As a result, the ground is not solid. It is very rugged and difficult to cross. There are talus caves throughout the whole structure, but few can be followed more than fifteen or twenty feet underground. 

An investigation of rock from these reveals interesting things. The Obsidian here is usually not in large boulders but rather interlaced in matrices of pumice. Pumice is the sponge-form of obsidian, and other rocks that are not obsidian or pumice show evidence of magnesium oxidation and vitrified surfaces from remelting. The forest will eventually reclaim this feature of volcanism, and a few trees and shrubs are beginning to grow in unusual places. The dome itself was quarried for pumice until the 1980s.

The dome has very steep sides.

The surface of the quarried area is rather level

Terracing

From on top of a nearby pile. Different colors tell of different lava chemistries.

The pink on this boulder has been remelted.

The obsidian here is interlaced with pumice, showing that different lava
chemistries were mixing unevenly.

Most of the dome looks like this

Possible magnesium oxidation staining.

Talus cave into the depths.

From the moon to the Earth. The rugged surface of Obsidian Dome looking out toward Glass Creek Dome.