|
Article |
|
Mount Rainier—Learning to Live
with Volcanic Risk |
|
Lahars
are the Greatest Hazard
Mount
Rainier—Far Greater Hazard
Past
Lahars...Future Hazards
Lahars
Form in Several Ways
Weakened
Rock and Rising Magma
Flanks
of Volcano
|
Lahars
in Multiple Valleys
Emergency
Planning
Lahar-warning
System
Lahar
Travel Times are Short
What
Can You Do?
Related
Links
|
|
|
|
Mount Rainier in Washington state is an active volcano reaching more
than 2.7 miles (14,410 feet) above sea level. Its majestic edifice
looms over expanding suburbs in the valleys that lead to nearby Puget
Sound. U.S. Geological Survey (USGS) research over the last several
decades indicates that Mount Rainier has been the source of many
volcanic mudflows (lahars) that buried areas now densely populated.
Now the USGS is working cooperatively with local communities to help
people live more safely with the volcano.
|
|
Figure 1. View across
Puyallup River valley toward Mount Rainier
Photo by David Wieprecht, U.S. Geological
Survey |
|
back
to top |
|
Lahars are the Greatest Hazard |
|
Mount Rainier (fig. 1) is an active volcano that is currently
at rest between eruptions. Its next eruption might produce volcanic
ash, lava flows, or pyroclastic flows. The latter can rapidly melt
snow and ice, and the resulting meltwater torrent could produce
lahars (a word of Indonesian origin that has come to mean volcanic
mudflow) that travel down valleys beyond the base of the volcano
to areas now densely populated. Lahars caused by large landslides
can also occur during non-eruptive times -- without the seismicity
and other warnings that normally precede eruptions.
Figure 2. Map showing
the annual probability that volcanic ash will be deposited to
a thickness of 1/3 inch or more from an eruption of Mount Rainier.
Volcanic ash, of this thickness or less, can cause disruption
of ground and air transportation, and can cause damage to electronics
and machinery. Lahars look and behave like flowing concrete, and their impact
forces destroy most man-made structures. At Mount Rainier, they
have traveled 45-50 miles per hour at depths of 100 feet or more
in confined valleys, slowing and thinning in the wide, now-populated
valleys.
At Mount Rainier, lahars are a greater hazard than other volcanic
products such as lava and poisonous gases that have been popularized
by TV and film. Lava flows and pyroclastic flows are unlikely to
extend more than a few miles beyond the National Park boundaries.
Volcanic ash (tephra) will be distributed downwind, 80 percent
of the time toward the east away from large populations (fig. 2).
The USGS, in cooperation with the University of Washington, monitors
many Cascade Range volcanoes, including Mount Rainier, to detect
precursors to eruptive activity. Mount Rainier last erupted in the
19th century; one or more small eruptions from one of the summit
craters produced local ashfall. |
|
back
to top |
|
Mount Rainier is one of the Most Hazardous
Volcanoes in the United States |
|
Although Mount Rainier has erupted less often and less explosively
in recent millennia than its neighbor, Mount St. Helens, the proximity
of large populations makes Mount Rainier a far greater hazard to
life and property.
- The population at risk --More than 150,000 people
reside on the deposits of previous lahars.
- The size and frequency of lahars --During the past
few millennia lahars that have reached the Puget Sound lowland
have occurred, on average, at least every 500 to 1,000 years.
Smaller flows not extending as far as the lowland occur more
frequently. If lahars of the future happen at rates similar to
those of the past, there is at least a one in seven chance of
a lahar reaching the Puget Sound lowland during an average human
life-span.
- We may not have advance warning --USGS research shows
that some lahars occur with little or no warning. Our only warning
could be a report that a flow is under way.
|
|
back
to top |
|
Past Lahars Indicate Future Hazards |
|
Geologists determine the size and timing of past lahars and
use this information to indicate future hazard potential. For
example, in figure 3, the areas inundated by the 2,300 year-old
National Lahar in the Nisqually Valley, and the 500 year-old
Electron Mudflow in the Puyallup Valley are superimposed on all
valleys surrounding Mount Rainier. They illustrate areas
that could be inundated if flows of those sizes occurred in each
valley.
A lahar flowing downvalley from Mount Rainier leaves a thick valley-bottom
deposit of boulders and hardened mud that may envelop stumps and
logs, forming a buried forest. Some of the deposits can be traced
upstream to the volcano's flanks, and all contain volcanic fragments
unique to Mount Rainier. Geologists map the deposits and determine
the tree ages to learn when the trees were engulfed and killed by
the lahar. Old-timers recall encountering huge buried stumps and
logs when plowing fields and digging wells. The youngest such forest
was buried about 500 years ago and uncovered during excavations for
new homes in the Puyallup River valley. |
|
back
to top |
|
Lahars Form in Several Ways |
|
Some of the largest lahars have originated by collapse of weakened
rock from the flanks of the volcano--a large landslide known as
a flank collapse. During eruptions, molten rock is injected into
cracks of the volcano and solidifies as slabs of rock called dikes.
The cooling magma releases gases and heat into groundwater, making
it hot and acidic. The hot, acidic waters convert hard volcanic
rock into soft, clay-rich rock by a process called hydrothermal
alteration. When masses of water-rich rock collapse, they transform
rapidly into a muddy slurry--a clay-rich (also called cohesive)
lahar that is funneled into one or more surrounding valleys.
Sand-rich lahars, also known as noncohesive lahars, form during
eruptions of Mount Rainier when hot pyroclastic flows melt snow
and ice. (Mount Rainier supports more than one cubic mile of glacial
ice--as much as all other Cascade Range volcanoes combined.) Because
sand-rich lahars occur during eruptive activity, they are likely
to be preceded by events that will warn of an impending eruption,
and thus of increased lahar potential.
Small lahars, traveling only a few miles, are caused by local
avalanches of rock debris, sudden releases of glacial meltwater,
or intense rainfall. These lahars occur many times each century.
New lahar deposits may get redistributed downstream over a period
of many years as the disrupted drainage network is re-established.
Thus, valley-floor areas that were not inundated by the initial lahar
deposits may suffer enhanced flooding and progressive burial by remobilized
sediment (zone of post-lahar sedimentation in figure 3). |
|
back
to top |
|
Weakened Rock and Rising Magma can Cause
Flank Collapse |
|
Flank collapses can be triggered when magma intrudes into a volcano
and destabilizes it, as happened at Mount Saint Helens in 1980.
A neighboring volcano-- Mount Baker --produced flank collapses in
the 1840's that were apparently triggered by steam explosions related
to volcanic activity. Steam explosions at Mount Rainier could trigger
flank collapses and lahars with little or no advance warning. Although
many flank collapses occur during eruptive periods, it is possible
for them to be triggered by earthquakes or they may be the result
of progressive weakening of the rock, saturation by groundwater and
the continuing pull of gravity. |
|
back
to top |
|
Flanks of Volcano can Unleash Flank Collapses
and Rockfalls |
|
The west
flank at the head of the Puyallup River valley has the greatest
potential for collapse because it has the largest area of weakened
clay-rich rock remaining at high altitude. The east and northeast
sides of the volcano can let loose large rockslides, such as that
of December 1963, because of fractures in the ridges. |
|
back
to top |
|
One Collapse can Yield Lahars in Multiple
Valleys |
|
Future
lahars will follow river valleys that drain Mount Rainier. Four
of the five major river systems flow westward into suburban areas
of Pierce County. These flow pathways are mapped by the USGS (fig.
3), just as flood-inundation maps show the areas at risk of flooding.
Lahars occurring during an eruption may affect valley areas miles
from the volcano, but a precursory warning should allow ample time
for evacuation. A catastrophic flow will likely spread into multiple
valleys. The largest known flow entered all five drainages, and
most of the known large flows have entered two or more. |
|
Figure 3. Hazard zones
for lahars, lava flows, and pyroclastic flows from Mount Rainier
(Hoblitt and others, 1998; U.S. Geological Survey Open-File Report
98-428). The map shows areas that could be inundated if events
similar in size to those of the past occurred today. Lahar hazard
is not equal in all valleys. Puyallup Valley is the valley most
susceptible to lahars caused by flank collapse. Risk to individual
drainages will be refined as scientists learn more about the
volcano. |
|
back
to top |
|
Monitoring and Emergency Planning are
Ongoing |
|
The USGS,
in cooperation with the University of Washington, monitors the
state of the volcano and assesses hazards from volcanic activity.
The lahar pathways mapped by the USGS guide the hazard-area regulations
of the comprehensive land-use plan for local counties. The plan's
urban growth boundary and its proposed land uses in unincorporated
areas are designed to minimize population growth, where possible,
within hazard zones. Local, county, state, and federal agencies
including the USGS have joined to develop a Mount Rainier volcanic
hazards response plan that addresses such issues as emergency-response
operations and strategies for expanded public awareness and mitigation
of volcanic hazard. |
|
back
to top |
|
Lahar-warning System Reduces Risk |
|
Because
of the higher level of risk from lahars in the Carbon and Puyallup
River valleys, the USGS and Pierce County Department of Emergency
Management have installed a lahar-detection and warning system.
The system consists of arrays of five acoustic flow monitors (AFM's)
that detect the ground vibrations of a lahar. Computerized evaluation
of data confirms the presence of a flowing lahar and issues an
automatic alert to emergency management agencies. Emergency managers
then can initiate response measures such as evacuations. This system
for automatic detection and notification of a lahar reduces, but
does not eliminate, risk in the lahar pathways. |
|
back
to top |
|
Lahar Travel Times are Short |
|
The estimated
time between detection of a lahar and its arrival in Orting is
about 40 minutes. Dispersed populations closer to Mount Rainier
would be affected sooner. Time is short, and successful evacuation
will depend on detection of the approaching lahar, effective notification
of people at risk, public understanding of the hazard, and prompt
response by citizens. Volcanoes often show signs that they are
getting ready to erupt days to weeks or months in advance. Scientists
of the U.S. Geological Survey and University of Washington evaluate
signs of unrest and look for increased seismic activity, increased
emission of volcanic gases and swelling of the volcano. When unrest
is detected, scientists will increase monitoring efforts and notify
emergency management officials. |
|
back
to top |
|
What Can You Do? |
|
Reducing
population growth in the paths of lahars, implementing a warning
system, and planning and practicing evacuations can lower the potential
loss of life and property during future eruptions and lahars. These
actions can reduce the risk from lahars and provide a measure of
safety for those who enjoy living, working, and playing in valleys
surrounding Mount Rainier. |
|
Learn—Determine whether you live,
work, or go to school in a lahar hazard zone. Learn about
all volcanic processes that could affect your community.
Inquire—Ask public officials to
advise you about how to respond during any emergency.
Plan—Develop an emergency plan
with your family so that you are prepared for natural hazards
and emergencies. |
|
|
back
to top |
|
Related Links |
|
|
|
|
|
Adapted
from Mount Rainier—Learning to Live with Volcanic Risk by
C.L. Driedger and K.M. Scott, U.S. Geological Survey Fact
Sheet 034-02, Online Version 1.0, May 20, 2004 |
|
back
to top |