Red Mountain, San Francisco volcanic field in summer 2018

Red Mountain is a large cinder cone of the west-central part of the San Francisco volcanic field. It is located just west of Highway 180 about 20 miles north of Snow Bowl. The origin of Red Mountain's prominent amphitheater remains a mystery. One possible explanation is put forward on a US Geological Survey fact sheet (https://pubs.usgs.gov/fs/2002/fs024-02/), 'An intriguing, though speculative, possibility is that one or more steam explosions created an amphitheater-shaped hole in the side of Red Mountain shortly after eruption ceased. Newly erupted cinders probably cooled to about 600° F as they fell back to earth, but then remained well above the boiling temperature of water for some time. Rainwater seeping into the cone and circulating through the still hot cinders may have quickly deposited a strong mineral cement that bound cinders together, creating the equivalent of a sealed "pressure cooker." Eventually, the pressure of the trapped superheated water may have exceeded the strength of that cooker, resulting in one or more steam explosions. Photo by pilot and photographer Ted Grussing.