Rising up to 2400 feet above the surrounding Mono Basin, the Mono Craters first began erupting as early as 100,000 years ago. Eruptions were well under way by 40,000 years ago and most of what we see today as the Mono Craters was formed within the last 10,000 years. The Mono Craters are not really craters at all in the classic sense. They are a range of 30 or more overlapping rhyolite domes and explosion pits. Much of the visible surface of the Mono Craters is composed of pumice and obsidian.
Pumice is formed when the pressure on the magma is reduced as it rises to the surface.
Some of the molten material expands and explodes much like opening a shaken can of soda. Gas bubbles permeate the molten material during this process and when it cools it becomes pumice. When the molten material moves to the surface more slowly in large blocks and is of a more viscous nature, it is quickly cooled when it becomes exposed to the air and stops moving forming a skin of solid glass which is later broken off by further activity. This greyish-black volcanic glass is known as obsidian. Local Indians would later use obsidian to fashion arrowheads and cutting tools.