Medicine Lake Caldera
Medicine Lake caldera is a 7 x 12 km (4.3 x 7.5 mi) depression in the summit area of the volcano. No single large eruption was responsible for the caldera formation; the only eruption recognized to have produced ash flow tuff occurred in late Pleistocene time, but this eruption was too small to fully account for formation of the caldera. Geologic evidence indicates that Medicine Lake caldera exists primarily because of subsidence in response to repeated extrusions (eruptions) of mostly mafic lava beginning early in the history of the volcano.
Seismic tomography identified a small silicic magma body under the eastern part of the caldera. It is located at 3 to as much as 7 km (1.8 to 4.3 mi) depth and has a volume of 10 to a few 10's of km3 (10 km2 = 3.8 mi2).
The center of the caldera is currently subsiding with respect to the margin of the volcano at a rate of 8.6 mm/yr (0.33 in/yr). The rate is 4.9 mm/yr (0.19 in/yr) for the center of the caldera relative to the caldera rim. The overall subsidence is explained as a combined result of east-west regional extension and the weight of the large edifice itself over a hot weak crust combined with the cooling of hot rock beneath the volcano.