Mammoth Mountain, a lava-dome complex, lies on the southwest topographic rim of Long Valley Caldera. The 3,369-m (11,053 ft) high volcano lies west of the structural rim of the caldera and is considered to represent a magmatic system distinct from Long Valley Caldera and the Mono-Inyo Craters (Hildreth, 2004). The latest magmatic eruptions at Mammoth Mountain took place about 57,000 years ago. Mammoth Mountain is surrounded by at least 35 mafic vents that are part of the same magmatic system and include Red Cones, two closely spaced basaltic cinder cones located southwest of Mammoth Mountain and southeast of Devils Postpile National Monument. The cones, whose name derives from colorful mantling scoria deposits, are unglaciated and were radiocarbon dated at about 8,900 years ago. Phreatic eruptions, distinct from those at South Inyo Craters, took place about 700 years ago from vents on the north side of Mammoth Mountain. Recent volcanic unrest, including seismicity, gas emission, and tree kill, is thought to be related to a dike intrusion beneath Mammoth Mountain in 1989.
Visit the Mammoth Mountain section of this website to read more about its geology and volcanic history.