The Geology of the Mono Craters, California

The Mono Craters are a chain of extinct obsidian domes, coulees, and lapilli cones lying south of Mono Lake in eastern California. They are slightly more than ten miles long, are ranged along a curving fracture roughly parallel to the Sierra Nevada, and are divided into three nearly equal parts by two large obsidian flows, or coulees. In general, there has been a shift of activity from the center of the range to the two extremities. The volcanic vents in the Mono Craters, in the typical case, pass through a sequence in which activity commences with a series of explosions. Ash and lapilli are hurled out, and a funnel shaped explosion pit is formed. The explosions are succeeded by the rise of an obsidian dome near the center of the lapilli ring. The dome expands beyond the confines of the lapilli collar and produces a coulee if a sufficient volume of obsidian reaches the surface. Explosions of decreasing intensity may occur during any of the later stages in the sequence. The structure of the average obsidian dome is determined by the viscous nature of the semi-solid obsidian, and the narrow conduit through which it rises. The lava expands in a sheaf-like form upon reaching the surface. The obsidian is traversed by a multitude of joints on cooling, and rapidly disintegrates into angular blocks. The force responsible for the elevation of the obsidian is not known, but may well be exerted by the gas contained in the magma. The principal rock types in the Mono Craters are the frothy lapilli of the explosion vents; gray or purple, sanidine-bearing, pumiceous obsidian; black, vitreous obsidian; and the near-rhyolitic variety of "intrusive obsidian" found in the deeper parts of the volcanic conduits. Associated with the obsidian of the Mono Craters are two older rocks, the West Portal Rhyolite and the Black Butte Basalt. The nature of the West Portal Rhyolite, and its relation to the older glacial deposits, is clearly shown by the cross-section exposed in the water tunnel driven under the southern end of the craters by the Los Angeles Bureau of Water Works and Supply. The Mono Craters were active in the late Pleistocene. Although some ash is interstratified with deposits of Mono Lake, no shorelines cut the more recent craters. The Southern Coulee buries one of the earlier lateral moraines of one of the principal Sierra glaciers. Explosion pits at the southeastern end of the craters erupted through the floor of a late Pleistocene lake, part of a chain lying to the east of the main group of craters.