Volcanoes of the Cascade Range erupt far less frequently than Kilauea
and Mauna Loa, but they are more dangerous because of their violently
explosive behavior and their proximity to populated and cultivated areas
in Washington, Oregon, and California. The 1980 eruption of Mount St.
Helens in southwest Washington dramatically illustrated the type of
volcanic activity and destruction these volcanoes can produce.
Scientific studies of the eruption of Mount St. Helens and the eruptive
histories of other Cascade volcanoes continue to improve public
awareness and understanding of these potentially dangerous peaks.
In contrast to Kilauea, Cascade volcanoes erupt a variety of magma types
that generate a wide range of eruptive behavior and build steep-sided
cones known as _composite volcanoes_. In addition to basalt, andesite
and dacite magmas are common.
[Illustration: Cascade Volcanoes.]
EXPLANATION
Volcano active during past 2,000 years
Potentially active volcano
Area of potential volcanic activity
Population centers
40,000 to 100,000
100,000 to 350,000
350,000 to 1,000,000
Greater than 1,000,000
_PACIFIC OCEAN_
WASHINGTON
Mount Baker
Glacier Park
Seattle
Spokane
Mount Rainier
Mount St. Helens
Mount Adams
MONTANA
Great Falls
Billings
IDAHO
Boise
Craters of the Moon
OREGON
Portland
Mount Hood
Mount Jefferson
Three Sisters
Eugene
Newberry Crater
Crater Lake
WYOMING
Yellowstone
Casper
Cheyenne
CALIFORNIA
Medicine Lake
Mount Shasta
Lassen Peak
Clear Lake
Sacramento
San Francisco
Long Valley Caldera
Coso
Los Angeles
San Diego
NEVADA
Reno
Las Vegas
UTAH
Salt Lake City
COLORADO
Denver
ARIZONA
San Francisco Field
Phoenix
Tucson
NEW MEXICO
Albuquerque
Bandera Field
These magmas are so highly viscous, or sticky, that expanding volcanic
gases cannot easily escape from them. This causes a tremendous build-up
in pressure, often leading to extremely explosive eruptions. During such
eruptions, magma is shattered into tiny fragments (chiefly ash and
pumice) and ejected thousands of meters into the atmosphere or even the
stratosphere. Under the force of gravity, sometimes these fragments
sweep down a volcano’s flanks at speeds of more than 100 kilometers per
hour, mixing with air and volcanic gases to form _pyroclastic flows_.
Rock fragments can also mix with water in river valleys to form _lahars_
(volcanic debris flows and mudflows) that destroy everything in their
paths.
Andesite and dacite magmas also erupt to form lava flows. Because these
lavas are more viscous (“stickier”) than basalt, they tend to form
thicker flows that travel shorter distances from the vent; consequently,
andesite and dacite lavas typically build tall cones with steep slopes
of more than 20 degrees.
[Illustration: Mt. Hood, Oregon. Eruptions from the volcano about 1,800
and 200 years ago from the Crater Rock lava dome formed a broad apron of
rock debris on the volcano’s south side. (_Photograph by Lyn Topinka._)]
Mount Baker, Washington.
Eyewitness reports of small ashy plumes and active steam vents on Mount
Baker dating as far back as the mid-1800’s were clear evidence that the
ice-covered volcano had one of the most active geothermal systems among
Cascade volcanoes. When new fumaroles and unusually dark vapor plumes
appeared abruptly in March 1975, however, people in the Northwest became
concerned about an impending eruption and possible avalanches and lahars
from Sherman Crater, a vent just south of Mount Baker’s summit. Despite
a tenfold increase in the release of heat by the volcano during the next
12 months, which resulted in extensive changes to the ice cover in
Sherman Crater and produced minor releases of ash, no eruption occurred.
The thermal activity was not accompanied by earthquakes, which generally
precede most eruptions, and since 1976, the volcano has not showed
additional signs of activity.
[Illustration: Mount Baker viewed to the west. Increased fumarolic
activity occurred in Sherman Crater (left of summit) during the
mid-1970’s.]
The increased thermal activity between 1975 and 1976 prompted public
officials and Puget Power to temporarily close public access to the
popular Baker Lake recreation area and to lower the reservoir’s water
level by 10 meters. Significant avalanches of debris from the Sherman
Crater area could have swept directly into the reservoir, triggering a
disastrous wave that would have caused loss of life and damage to the
reservoir.
[Illustration: Mount Rainier towers 3,000 meters above the surrounding
valleys, all of which have been swept by lahars during the past 10,000
years. Future eruptions will probably trigger similar lahars.
(_Photograph by David Wieprecht._)]
Mount Rainier, Washington.
Mount Rainier has not produced a significant eruption in the past 500
years, but scientists consider it to be one of the most hazardous
volcanoes in the Cascades. Mount Rainier has 26 glaciers containing more
than five times as much snow and ice as all the other Cascade volcanoes
combined. If only a small part of this ice were melted by volcanic
activity, it would yield enough water to trigger enormous lahars.
Mount Rainier’s potential for generating destructive mudflows is
enhanced by its great height above surrounding valleys and its “soft”
interior. The volcano stands about 3,000 meters above river valleys
leading from its base. Volcanic heat and ground water have turned some
of the volcano’s originally hard lava into soft clay minerals, thereby
weakening its internal structure. These conditions make Mount Rainier
extremely susceptible to large landslides. Several have occurred in the
past few thousand years, one as recently as about 600 years ago. These
landslides, apparently containing great volumes of water, quickly turned
into lahars as they rushed down river valleys.
Mount St. Helens, Washington.
The catastrophic eruption on May 18, 1980, was preceded by 2 months of
intense activity that included more than 10,000 earthquakes, hundreds of
small _phreatic_ (steam-blast) explosions, and the outward growth of the
volcano’s entire north flank by more than 80 meters. A magnitude 5.1
earthquake struck beneath the volcano at 08:32 on May 18, setting in
motion the devastating eruption.
[Illustration: Mount St. Helens crater and lava dome viewed from the
north, 1990. Inset: Close view of lava dome with new lava extrusion on
top (snow-free part) from the south, 1986. (_Photographs by Lyn
Topinka._)]
Within seconds of the earthquake, the volcano’s bulging north flank slid
away in the largest landslide in recorded history, triggering a
destructive, lethal lateral blast of hot gas, steam, and rock debris
that swept across the landscape as fast as 1,100 kilometers per hour.
Temperatures within the blast reached as high as 300 degrees Celsius.
Snow and ice on the volcano melted, forming torrents of water and rock
debris that swept down river valleys leading from the volcano. Within
minutes, a massive plume of ash thrust 19 kilometers into the sky, where
the prevailing wind carried about 520 million tons of ash across 57,000
square kilometers of the Western United States.
[Illustration: Stand of timber in the process of being harvested was
instead knocked over by the lateral blast. An estimated 4 million board
feet of timber was destroyed. (_Photograph by H.H. Kieffer._)]
[Illustration: Small eruption of gas and ash from the lava dome caused
by violent release of volcanic gas or the geyser-like flashing of
superhot ground water to steam. (_Photograph by Dan Dzurisin._)]
[Illustration: Mount St. Helens towers above the chaotic landslide
deposit that fills a former valley to a depth of as much as 195 meters.
Note many small hills atop the landslide, called “hummocks” by
geologists. (_Photograph by Lyn Topinka._)]
The well-documented landslide at Mount St. Helens has helped geologists
to recognize more than 200 similar deposits at other volcanoes in the
world, including several other Cascade peaks. Geologists now realize
that large landslides from volcanoes are far more common than previously
thought—seventeen such volcanic landslides have occurred worldwide in
the past 400 years. Consequently, when scientists evaluate the types of
volcanic activity that may endanger people, giant landslides are now
included, in addition to other types of volcanic activity such as lava
flows, pyroclastic flows, lahars, and falling ash.
Following the 1980 explosive eruption, more than a dozen extrusions of
thick, pasty lava built a mound-shaped lava dome in the new crater. The
dome is about 1,100 meters in diameter and 250 meters tall.
[Illustration: Giant mushroom-shaped ash cloud of May 22, 1915, viewed
from 80 kilometers west of Lassen Peak. (_Photograph provided by
National Park Service._)]
Lassen Peak, California.
Long before the recent activity of Mount St. Helens, a series of
spectacular eruptions from Lassen Peak between 1914 and 1917
demonstrated the explosive potential of Cascade volcanoes. Small
phreatic explosions began on May 30, 1914, and were followed during the
next 12 months by more than 150 explosions that sent clouds of ash as
high as 3 kilometers above the peak. The activity changed character in
May 1915, when a lava flow was observed in the summit crater. A deep red
glow from the hot lava was visible at night 34 kilometers away. On May
19, an avalanche of hot rocks from the lava spilled onto snow and
triggered a lahar that extended more than 15 kilometers from the
volcano.
The most destructive explosion occurred on May 21, when a pyroclastic
flow devastated forests as far as 6.5 kilometers northeast of the summit
and lahars swept down several valleys radiating from the volcano. An
enormous ash plume rose more than 9 kilometers above the peak, and the
prevailing winds scattered the ash across Nevada as far as 500
kilometers to the east. Lassen Peak continued to produce smaller
eruptions until about the middle of 1917.