The twenty-one volcanic cones to the south of Mono Lake form the youngest mountain range in North America, the Mono Craters. While the earliest eruptions occurred about 35,000 years ago, the most recent activity dates back merely 600 years.

The most accessible cone is Panum Crater, located three miles east of Highway 395 on Highway 120. This 600 year-old volcano exhibits all of the characteristics of the textbook rhyolitic, plug-dome volcano.

Rhyolitic volcanoes are characterized by having large amounts of silica (quartz) in their lava. The content of silica at Panum is about 76 percent. It makes the lava very viscous, or thick, and very glassy. Products of this rhyolitic eruption are pumice, the rock that floats, and obsidian, the volcanic glass that Native Americans used to make arrow points and scrapers.

Panum Crater formed in a sequence of events. The first event was caused by magma rising from deep within the earth's crust. When this extremely hot, liquid rock made contact with water just below the surface, the water expanded into steam and a large, violent eruption occurred. So much debris was blown out that a gaping crater was left behind.

Once this debris was blown out, a fountain of cinders shot up a great distance into the sky. As this huge amount of ash and pumice began to fall back towards the earth, it formed a pumice ring, or cinder cone, about the original vent. You can still see this cinder cone today.

Following the violent eruptions of the first two phases, the remainder of the thick magma slowly rose to the surface in a series of domes. Each dome began with an outpouring of the viscous, rhyolitic lava which hardened and formed a cap over the vent. As magma continued to push up, the cap (or dome) shattered and fell to the outside of the newly formed dome. This happened so many times that a new mountain was created out of these broken pieces, called crumble breccia. The mountain continued to build in this manner until the force within the volcano weakened and no more new domes formed. The final one still stands today.

As the final dome hardened, a period of spire building began. Thick lava pushed up through cracks of the hardening dome and formed castle-like spires. If you can imagine toothpaste squeezing through the opening of a tube and forming a small tower before it topples over, you can imagine how these spires form. Most of the spires at Panum fell over and broke because of their rapid cooling and because of many small explosions at their bases. Most of the rocky debris you see at the top of the dome is the remains of spires that have crumbled.

-- Excerpt from:
U. S. Forest Service Website, Region 5, Volcanoes, Glaciers, and Plate TEctonics: The Geology of the Mono Basin, August 2001

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