Between what years was the most recent eruptive episode of the craters that show no effect of wave erosion?
Multiple eruptions of silica-rich rhyolite from 40,000 to 600 years ago built the Mono Craters. Black Point, today on the north shore of Mono Lake, is a flattened volcanic cone of basaltic debris that formed under the surface of a much deeper Mono Lake about 13,300 years ago, during the most-recent glacial period. Several eruptive episodes from 1,600 to 270 years before present in Mono Lake formed Negit Island. The magma reservoir feeding the Mono Lake Volcanic Field is unrelated to the Mono Craters magma reservoir.Basaltic andesite lava built the Red Cones, two small cinder cones 6.2 miles (10 km) southwest of Mammoth Lakes, around 8,500 before present. The five Mammoth Mountain Craters are a set of explosion pits that trend west-north-west for 1.6 miles (2.5 km) near the northern flank of Mammoth Mountain.None of the Mono Craters near the lake show the effects of wave erosion, but a hill at the southern end of the field shows what Israel Russell called a "beach line". The present elevation of this beach line is the level of the Mono Lake high stand before the formation of the northern Mono Craters, plus any surface deformation that has happened since that time. Stream-rounded stones are found on the volcanoes, and were lifted up as the volcanoes grew. Although glaciers were present throughout the Sierra Nevada, they did not reach as far down as the Mono Craters.
The most recent eruptive episode on the Mono Craters occurred sometime between the years 1325 and 1365. A vertical sheet-like mass of magma, called a dike, caused groundwater to explosively flash to steam, creating a line of vents 4 miles (6 km) long. A mix of ash and pulverized rock, called tephra, covered about 3,000 square miles (8,000 km2) of the Mono Lake region. The tephra were carried by the wind and deposited in a layer 8 inches (20 cm) deep 20 miles (32 km) from the vents and 2 inches (5 cm) deep 50 miles (80 km) away.Pyroclastic flows of hot clouds of gas, ash and pulverized lava erupted from these vents in narrow tongues that extended up to 5 miles (8 km) away and covered 38 square miles (100 km2). Rhyolite lava oozed out of the vents to form several steep-sided domes, including Panum Dome and the much larger North Coulee flow. The youngest domes and coulees are 600 to 700 years old and are, therefore, the youngest mountains in North America.
1325 1365