Geologic Background:
Medicine Lake is a large
Pleistocene-to-Holocene, basaltic-to-rhyolitic shield
volcano east of the main axis of the Cascade Range.
Medicine Lake volcanism, similar in style to that of
Newberry volcano in Oregon, began less than one million
years ago. A roughly 7 x 12 km caldera truncating the
summit contains a lake that gives the volcano its name. A
series of young eruptions lasting a few hundred years
began about 10,500 years before present (BP) and produced
5 cu km of basaltic lava. Eruptive activity resumed 6,000
years later, producing a chemically varied group of
basaltic lava flows from flank vents and silicic obsidian
flows from vents within the caldera and on the upper
flanks. The last eruption produced the massive Glass
Mountain obsidian flow on the east flank about 900 years
BP. The volcano has produced a wide range of volcanic
products from basaltic spatter cones to obsidian flows of
rhyolitic composition.
A portion of the lower north flank is within the Lava
Beds National Monument.
Historic Activity:
Recent Activity:
- On Sept. 29, 1988, 47 earthquakes were detected at
Medicine Lake volcano which had experienced only 1
earthquake in the past 8 years. Activity increased on
Sept. 30, when a total of 146 earthquakes were registered
(66 during a 1-hour period). After Sept. 30, seismicity
declined to about 10 events/day until Oct. 4 when there
were 33 events. Activity again declined with sporadic
events continuing through early Dec. when several
locatable events per week were occurring. Most events
were less than magnitude 1. Releveling of level line
across caldera floor shows about 175 mm of central
caldera subsidence since 1954. On Dec. 13 and 14 another
small swarm of 17 events (M1.9) occurred over a 5 hour
period.
- Twelve earthquakes were detected in the Medicine Lake
area (about 900 km NNW of the epicenter) in the 30
minutes after the June 28, 1992, M 7.5 earthquake in
southern California.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 13, No. 9-11)
- Global Volcanism Network (V. 17, No. 6).
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