Medicine Lake

Summary

Also known as the Medicine Lake Highlands, this volcanic region covers a total area of about 2,200 km² (850 mi²), extending approximately 80 km (50 mi) north-south and 50 km (30 mi) east-west. A shallow, but wide (7x12 km, 4.3x7.5 mi) caldera basin containing its namesake lake is located at the summit of the volcano. Intermittent, mostly effusive (non-explosive) eruptions over the last half-million years produced this large, shield-shaped volcano. In the last 13,000 years, effusive flank eruptions created expansive lava flows with single flows covering as much as 195 km² (75 mi²). Two-thirds of Lava Beds National Monument, on the north flank of the volcano, is covered by one such flow. Seven of the nine eruptions of the last 5,200 years, however, included an early explosive phase. The two youngest sent ash tens of miles downwind during their explosive phases before switching to slow effusion of thick, glassy-looking lava flows (obsidian) forming Little Glass Mountain about 1,000 years ago, and, Glass Mountain about 950 years ago. Exploratory drilling and geophysical studies reveal a high temperature geothermal system below Medicine Lake volcano fueled by a deeper zone of partially molten rock (magma). Ground surveys by USGS scientists show that the center of the volcano is slowly subsiding, due to motions on through-going regional faults and weakening of the rocks by high heat flow. Most notable seismically over the last three decades were a series of shallow earthquakes in 1988-89 (magnitudes of 4.1 and less).