Download Cascade Volcanoes - Fort Hays State University

Cascade
Volcanoes
Mount St. Helens NVM
Washington
Crater Lake NP
Oregon
Lassen Volcanic NP
California
Department of Biological Sciences, Fort Hays State University
Instructor: Mark Eberle
Course Homepage
Prior to its most recent eruption, the conical shape of Mount Saint Helens did not bear the
marks of glaciers, which indicated that it was active more recently than other Cascade
volcanoes, such as Mount Rainier. Mount Saint Helens erupted violently in May 1980, when a
landslide on the north flank allowed the pent-up pressure of steam in the magma rising within
the volcano to escape. The montane forest and riparian communities destroyed in the
surrounding blast zone are slowly recovering. Trees in these forests live for centuries, so the
30+ years that have elapsed since the eruption represent a relatively short amount of time in
the life of a forest community. Those of us with lifespans measured in decades tend to be a
little impatient with nature.
Crater (left) and Surrounding Blast Zone (right), Mount St. Helens NVM, Washington
Photographs by Mark Eberle, August 1998
Prior to the eruption of Mount Saint Helens, the most recent volcanic eruptions in the
Cascades occurred from 1914 through 1917 at Lassen Peak in northern California. The most
explosive of these eruptions occurred on 22 May 1915. Volcanic activity in the Lassen Peak
area has occurred for the last 825,000 years. Prior to the eruptions about 100 years ago,
Cinder Cone in the northeastern part of the park erupted about 350 years ago. Current
hydrothermal features in Lassen Volcanic National Park include fumaroles (vents of steam
and volcanic gases), mudpots, and hot springs. Lassen Volcanic National Park includes all 4
types of volcanoes: shield volcano, composite volcano, cinder cone, and plug dome. The park
and its active hydrothermal features were added to the most recent fieldtrip. The trail to
Bumpass Hell to observe the hydrothermal features is 1½ miles (returning the same way) and
fairly level until the descent into the basin (photo below); the elevation is about 8,000 feet.
Thermal Features at Bumpass Hell, Lassen Volcanic NP, California
Photograph by Mark Eberle, July 2009
One beautiful example of a Cascade volcano we are unable to visit (because we are
driving down the Pacific coast) is Crater Lake, Oregon. Crater Lake fills the caldera created
following a cataclysmic eruption of Mount Mazama about 7,700 years ago (the last volcanic
activity was about 5,000 years ago). An average of 13.54 m (533 inches) of snow each winter
fill the lake and compensate for losses through evaporation and seepage. No streams flow into
or out of the lake. Crater Lake is the deepest lake in the USA (592 m = 1,943 feet) and the 9th
deepest lake in the world. Lake Tahoe, which we visit on our return trip in the Sierra Nevada,
is the second deepest lake in the USA (maximum depth of 501 m = 1,645 feet) and the 16th
deepest lake in the world. Unlike the volcanic origin of Crater Lake, Lake Tahoe was created
more than 2 million years ago by uplifted blocks that now form the Sierra Nevada on the west
and the Carson Range on the east, while blocks dropped between them to form the Tahoe
Basin. More information about Lake Tahoe is provided in the summary for the Sierra Nevada.
Crater Lake and Wizard Island, Crater Lake NP, Oregon
Photograph by Mark Eberle, July 2009
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