Geology Overview
The local landscape, both in and out of the
National Monument, is defined by volcanic eruptions,
lava flows, granitic intrusions, and glacial activity.
It is an area of stark contrasts, with glacially-sculpted
peaks towering over equally impressive volcanic
outcrops.
The high volcanic activity within Devils Postpile
is due to its geographic location. The monument is
near the edge of the Long
Valley Caldera,
a 9- by 18-mile (15- by 30-km ) oval-shaped
depression located 12 miles (20 km) south of Mono
Lake along the east side of the Sierra Nevada in east-central
California. This area of eastern California has
produced numerous volcanic eruptions over the past
three million years, including the massive eruption
that formed the caldera 760,000 years ago.
Since the caldera-forming eruption, two major
glaciations shaped the region - the Tahoe and the
Tioga. The lava flow that produced the Devils
Postpile occurred somewhere in between these two
glacial periods that peaked at 130,000 and 20,000
years ago respectively. On top of the Postpile, the
glaciers left behind deep grooves, or striations, and
smooth glacial polish.
The latest period of caldera unrest began in May
of 1980 when a strong earthquake swarm struck the
southern margin of Long Valley Caldera. These 4
magnitude-6 earthquakes were associated with a 10-inch
(25-cm), dome-shaped uplift of the caldera floor.
Earthquake activity in the winter of l980 and the
spring and summer of 1981 dislodged numerous columns
at Devils Postpile. The U.S. Geological Survey
continues to closely monitor the caldera and expects
more activity in the future.
The National Monument itself was established because
of two important geophysical features, the
Devils Postpile
formation and the 101-foot Rainbow Falls. More in-depth information
may be found on a U.S. Geological Survey web
site
that tells the geologic story of Devils Postpile.
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