One of the most unique geologic regions of the Eastern Sierras is Mammoth Mountain, a lava-dome complex that features an 11,053-foot-high volcano west of the structural rim of Long Valley Caldera and the Mono-Inyo Craters (which are a separate magmatic system).
The caldera was formed by Glass Mountain’s volcanic eruptions approximately 760,000 years ago, which created an oval-shaped depression 10 miles wide and 20 miles long. By contrast, the Mono-Inyo craters are part of the youngest volcanic chain in the Eastern Sierra and arose from a fissure system to the south of Mammoth Mountain. Major eruptions of high-silica rhyolite occurred around 600 years ago, while low-silica rhyolite eruptions, which formed the present craters, happened 500 years ago.
Eruptions tend to occur every 250 to 700 years, and the last one was on Paoha Island at Mono Lake’s center, some 250 years ago. Over the past four decades, geologic unrest has characterized Mammoth Mountain and the Long Valley Caldera, with a dome-like uplift occurring that raised the floor by around a foot. The ground continues to swell, accompanied by earthquake swarms, which indicates the rise of fresh magma under the surface. With increased CO2 gas accompanying this seismic activity and causing local trees to die, scientists believe that an eruption is imminent.