The most prominent geological feature in the area, 11,053-foot Mammoth Mountain is a dormant composite volcano formed by a series of eruptions between 220,000 and 57,000 years ago. Steam-blast eruptions occurred from vents on the north side of the mountain as recently as 700 years ago. Since 1990, increases in earthquake swarms, gas venting and tree kills have led to heightened monitoring by the US Geological Survey. Extensive facilities for fee-based downhill skiing and snowboarding, as well as mountain biking in summer and an interpretive center on the summit, are operated under a lease with the US Forest Service.
