The Inyo Craters are three north-south-aligned phreatic (steam) explosion craters on the summit and south flank of Deer Mountain near Mammoth Lakes, California.  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.  This is the middle crater averaging about 660 ft in diameter and 200 ft deep.  A small lake forms at the bottom.  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: https://www.fs.usda.gov/recarea/inyo/recarea/?recid=20410