The Inyo Craters are three north-south-aligned phreatic (steam) explosion craters on the summit and south flank of Deer Mountain. Deer Mountain is an approximately 115,000-year-old rhyolite dome in the west moat of the Long Valley Caldera that was erupted from the residual Long Valley Caldera magma. The two southernmost craters average about 660 ft in diameter and are about 200 ft deep. They both contain small lakes. The crater on the summit of Deer Mountain is smaller, irregular in outline, breached on its south side, and dry. The craters were formed in the mid-14th century shortly after the eruption of Deadman Creek, Obsidian, and Glass Creek domes in 1350 CE.
Despite apparent differences in shape and vegetation, all three craters formed at nearly the same time, probably within hours or days. They erupted in succession from north to south, as the light-colored deposits from the summit crater (Deer Mountain) underlie the darker ones of north crater, which in turn underlie the darkest ones of south crater - relations that are well exposed in the northeast wall of north crater and on the east flank of Deer Mountain (Mastin and Pollard, 1988).
The ground around the craters is broken by many north-trending faults and fissures that probably formed as a consequence of uplift and expansion above a dike (intruding magma) (Mastin and Pollard, 1988). The rising magma most likely encountered water causing explosive flashing of the water to steam. The steam formed a vent through the overlying andestic rocks, producing the Inyo explosion craters and surrounding phreatic deposits. Surprisingly no juvenile rhyolite magma reached the surface during the eruptions.
From the town of Mammoth Lakes take hwy 203/Minaret road west towards Mammoth Ski Mountain. Make a right onto the Mammoth scenic loop. Inyo craters are a few miles up on the left. If you continue on the scenic loop road for 6 miles you will connect with Hwy 395.
These craters are the most recently active eruptive centers of the Long Valley Caldera, dating from about 500 to 600 years ago. There are three main craters-North, South, and the crater at the summit of Deer Mountain.
The hike to the first two craters is a short scenic trail heavily shaded by a steady canopy of trees. The trail skirts the perimeter of the first crater, where you can step down to an observation area to take in the intense views of the turquoise water set against the backdrop of Mammoth Mountain. Continue following the trail as it makes a slight climb up to the second crater, which holds a smaller darker lake and is more obstructed by trees.
Retrace your steps to return to the parking lot.
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