Geologic History

Medicine Lake Volcano

The Medicine Lake volcano began to form around one million years ago in the Pleistocene epoch. This process began after a massive eruption of basalt began to pile up and form the beginnings of the shield volcano. Over time the volcano has continued to produce a variety of lava that has contributed to the formation of the volcano and the area around it. During the Pleistocene time the volcano saw a wide array of eruptions that ranged in composition from basalt to rhyolite. The most recent eruption was less than 900 years ago and was composed of pumice and obsidian. This resulted in what is now known as Glass Mountain, a mountain that sits just south of Lava Beds in the Inyo National Forest. Geologists today are able to track the eruption patterns of Medicine Lake by studying the tectonic movement beneath the Cascade volcanos.

Lithospheric plates subducting into the asthenosphere
causing a volcanic arc to arise.

The plates that lie beneath the Pacific Ocean are considered a subduction zone where the lithospheric plates slide beneath continental plates, which causes the subducting plate to melt into magma. This then rises to the surface as lava creating powerful eruptions along the Cascade Mountain Range. While geologists don’t anticipate any eruptions in the near future, they have noted that there is still significant tectonic plate movement beneath the Cascade Mountain Range, so it is likely that the volcano will erupt again one day. Currently, it is possible to see over thirty different lava flows exposed at Lava Beds including the volcanic tuff at Gillem Bluff from the Neogene period, the basalt eruptions at Callahan Flow from Holocene epoch, and the liquid basalt flows from the Pleistocene epoch that cover around seventy percent of the park.