Download The Cascades - Madrid Engineering Group

The Cascades
The Cascade Mountain
Range is about 700 miles
long,
extending
from
southwestern
British
Columbia, Canada south
through Washington and
Oregon, into north-central
California (see map). This
mountain chain consists of
a series of volcanoes and
innumerable smaller vents
and cones located about
80 to 150 miles inland
from the Pacific Ocean.
Four (4) national parks are
situated
within
the
Cascades, including North
Cascades National Park in
north-central Washington,
Mount Rainier in westcentral Washington, Crater
Lake
in
south-central
Oregon,
and
Lassen
National Park, centered on
Mount
Lassen,
the
southern-most
Cascade
peak, situated in northcentral California.
Figure 1 – Cascade Range
Many peaks exceed 10,000 feet (3,000 m), including Mount Hood (11,235 feet [3,424
m], highest point in Oregon) and Mount Rainier (14,410 feet [4,392 m], highest in
Washington and in the Cascade Range). Most of the summits are extinct volcanoes,
but Lassen Peak (10,457 feet [3,187 m]) and several others have erupted in the recent
past. Mount Baker (10,778 feet [3,285 m]) steamed heavily in 1975, and Mount St.
Helens (8,365 feet [2,550 m]) erupted in 1980 and again in 1981.
Page 1 of 11
The modern-day definition of the extent of the
Cascades is generally accepted to mean a
volcano between the end members (Mount
Lassen to the south and Mount Meager to the
north). There is also some discussion as to
whether “Three Sisters”, near the center of the
range is really 1 vent or 3 volcanoes very close
to one another.
Every orogenic belt, or mountain range
has their own unique method of
formation, and the Cascades are no
different. Off of the western coast of this
portion of the North American plate,
there is a “spreading center” along the
Juan de Fuca Ridge, where new ocean
crust is forming.
As this geologic
process continues, a relatively small
plate of the earth’s crust, the Juan De Fuca plate is pushed eastward, into the
western edge of the North American
plate. As the Juan de Fuca plate is
pushed beneath the North American
plate in a process referred to as
“subduction”, it is bent downward. Both
the plate and the sediments that are
riding on the plate begin to melt as it
descends into the mantle, releasing
steam and other gasses. Occasionally
the steam and melted rock are “vented” through an eruption of a volcano (see
Figure 2, right).
Figure 2 – Cascade Tectonic Overview
Page 2 of 11
Tectonic Setting
The tectonic setting along the margin between the eastern edge of the North Pacific
Ocean and the western cost of British Columbia, Washington, and Oregon is relatively
complex, and is bounded by another world-famous geologic feature, the San Andreas
Fault. The subduction zone that is the source of the material and energy for the
Cascades is essentially a 700-mile interruption of the San Andreas Fault, which extends
down to the south, to the Gulf of California.
It appears that the Cascade Range, or some early configuration of it began erupting
about 37 million years ago. Not all portions of the Range are equally as “active”. It appears that the central portion of the range is much more active, with more eruptions
and venting that the northern end. However, the older lavas appear to be found in the
northern portion of the Range.
The Cascade Range is also home to numerous alpine glaciers, with 25 glaciers on
Mount Rainier alone. Glaciers form when local annual snowfall exceeds that which
melts during the summer. At altitude, these icefields form against the mountains, and
will eventually begin to slide downhill under the effects of gravity.
Page 3 of 11
Unique Geologic Setting
At the southwestern-most corner of the three (3) tectonic plates that are being
subducted beneath the North American Plate there is what geologists refer to as a
“triple junction”, where three (3) tectonic boundaries converge (see below).
Location and Geometry of Mendocino Triple Junction
As shown, there is “right-lateral” movement across the San Andreas Fault, where the
Pacific Plate is grinding northward along the western margin of the North American
Plate. West of the Triple Junction there is also “right-lateral” movement of the Gorda
Plate as it passes to the east along the Mendocino Fault. North of the Triple Junction
lies the subduction zone where the Gorda Plate is descending beneath the North
American Plate. In this diagram are also representations of the three (3) types of
tectonic boundaries which include mid-ocean spreading centers, subduction zones, and
transform faults.
The location of the southern-most Cascade peak, Mount Lassen is shown by the yellow
arrow. There are no Cascade peaks south of the Mendocino Fault.
Page 4 of 11
Why are the Cascade Volcanoes Dangerous?
Eleven of the Cascades have erupted at least once in the past 4,000 years, and several
have done so in just the past 200 years (see Figure 3, below). Some volcanoes appear
to have erupted 100 times or more, while others are dormant, and have not erupted in
100,000 years. This variability in both timing and location only add to the dangers this
mountain range presents.
Lahar pathways from three (3) separate events on Mount Rainier (yellow arrow)
Page 5 of 11
In addition to the lahars, there is also a considerable risk of damage caused by the ash
and fallout that is typically ejected from a volcano during an eruption. Such pyroclastic
releases often shoot ash 30 miles (50 km) into the atmosphere, and cover broad areas.
Billowing eruption with pyroclastic and ash ejecta – Mount St. Helens, May 1980
Left: Extent of pyroclastic outfall from Mount Mazama
Right: Pyroclastic deposit at Mount Mazama
Page 6 of 11
Figure 3 – Recent Eruption Summary of the Cascade Range in the US
The most recent significant eruption in the Cascades happened in late May 1980 when
Mount St. Helens erupted. This event provided a great deal of data on the mechanics
of an eruption. When Cascade volcanoes do erupt, pyroclastic flows, lava flows, and
landslides can devastate areas more than 10 miles (>16 km) away; and huge mudflows
of volcanic ash and debris, called lahars, can inundate valleys more than 50 miles (>80
km) downstream. Falling ash from explosive eruptions can disrupt human activities
hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage
to jet aircraft even thousands of miles away.
Mount St. Helens has been among the most active of the Cascade peaks for a very long
time. Despite weeks of prolonged geologic activity, many people wanted to see an
eruption. On May 18 at 8:32 a.m., the northern slope of Mount St. Helens gave way in a
tremendous landslide (below, right), allowing the chamber beneath the mountain to
depressurize. The eruption killed 57 people and destroyed 250 homes, 47 bridges, and
185 miles of roadway. In the time since, a cone has begun to re-grow (photos, below).
Page 7 of 11
May 17, 1980
May 18, 1980
Mount St. Helens
“The glacier-covered summits of Mount Baker and other Cascade volcanoes greatly
increase the hazards of any eruption because of the possibility of a “lahar”, or volcanic mudflow, being triggered when snow and ice, suddenly transformed to water and steam,
mix with hot ash and other debris making a slurry. A lahar roars down a mountain
valley destroying everything in its path. The lahar set off by an explosive eruption at
Mount St. Helens was not as descriptive as it might have been because reservoir
(water) levels had been lowered when earthquake swarms and steam eruptions
indicated that the volcano might erupt”. Page 8 of 11
An excerpt from the USGS Volcano
Hazards Program: “Because of its
elevation (14,410 feet or 4,392 meters),
relief, hydrothermal alteration, icecap,
glacier-fed radial valleys, and proximity to
encroaching suburbs of the SeattleTacoma metropolis, Mount Rainier
(photo, left) is the most threatening
volcano in the Cascades.
Its next
eruption could produce volcanic ash, lava
flows, and avalanches of intensely hot
rock and volcanic gases, called
pyroclastic flows. Some of these events
swiftly melt snow and ice and could
produce torrents of meltwater that pick up
loose rock and become rapidly flowing
slurries of mud and boulders known as
lahars. The greatest risk at the volcano
comes from its potential for generating
huge lahars triggered by sector collapse
or magma- water-ice interaction rather
than from an eruption itself.”
Mount Rainier
55 miles southeast of Seattle
Modern-day measurements and computer modeling conducted by the USGS show that
there is a possibility that a lahar that is generated on the northwestern flank of Mount
Rainier would flow all the way to Puget Sound, a distance of about 50 miles.
In short, what makes Cascade Range volcanoes so dangerous is the combination of hot
rock and ejecta (tephra) that mix with meltwater from glaciers and snowfields, making a
debris flow that has the consistency of wet cement (lahar). As this heavy, wet slurry
runs downhill, it will collect trees, buildings, structures, and so forth. What makes them
even all the more dangerous is the proximity to population centers.
As discussed in The Geology of National Parks, “In the Cascades, volcanic eruptions
have been relatively infrequent in terms of human life spans. Before Mount St. Helens
erupted, many residents did not believe the geologists and other scientists who warned
of potential hazards. However, the 1980 explosion and subsequent less-intense
Page 9 of 11
eruptions made believers out of skeptics. What is not recognized is that major
evacuations are really the only adjustment that human beings can make when a
volcano becomes threatening”. Terminology
Lahar “Landslide or mudflow of pyroclastic material on the flank of a volcano. Lahars are described as being “wet” if they are mixed with water derived from heavy rain, escaping from a crater lake, or produced by melting snow” (see photo, below). They
typically have the consistency and texture of wet cement; they flow as a viscous liquid
but have a relatively high unit weight in terms of pounds per cubic foot.
Pyroclastic Flow “A general term applied to detrital volcanic materials that have been explosively or aerially ejected from a volcanic event”. These have been measured up to 700 deg. C.
Tephra
“A collective term for all clastic volcanic materials which during an eruption
are ejected from a crater or from some other type of vent and transported through the
air”. Page 10 of 11
References:
Dictionary of Geologic Terms, Revised Edition, 1976, American Geological Institute,
Anchor Press / Doubleday, 472 pp.
Encyclopedia Britannica (www.britannica.com)
Harris, A. G. and Tuttle, E., 1990, Geology of National Parks, Fourth Edition, Kendal /
Hunt Publishing Company, 652 pp.
United States Geological Survey (USGS); (www.usgs.gov)
Wikipedia; (www.wikipedia.org)
Wizard Island in Crater Lake National Park
Mount Mazama last erupted about 7,700 years ago
Estimates indicate it released 42 times the energy as the eruption from Mount St.
Helens in May 1980.
Page 11 of 11