US History/US History (Bureau Development Inc.)(1991).ISO/dp/0029/00294.txt

Text File | 1990-12-23 | 49KB | 785 lines

$Unique_ID{USH00294}
$Pretitle{26}
$Title{Eruptions of Mount St. Helens:  Past, Present and Future
History of Mount St. Helens}
$Subtitle{}
$Author{Tilling, Robert I.}
$Affiliation{US Geological Survey}
$Subject{ash
st
mount
helens
eruption
volcano
debris
miles
blast
north}
$Volume{}
$Date{1987}
$Log{St. Helens Lava Dome*0029401.scf
Before the Eruption*0029402.scf
The Eruption*0029403.scf
St. Helens Area*0029404.scf
Ash Cloud*0029405.scf
Debris River*0029406.scf
Blasted Forest*0029407.scf
}
Book:        Eruptions of Mount St. Helens:  Past, Present and Future
Author:      Tilling, Robert I.
Affiliation: US Geological Survey
Date:        1987

     Overview of Eruptions of Mount St. Helens:  Past, Present and Future

On May 18, 1980, Mount St. Helens Volcano in Washington exploded after two
months of intense earthquake activity and intermittent, relatively weak
eruptions.  This book chronicles -- in both pictures and words -- the
processes, effects and products of one of the most significant geologic events
in the U.S. in the 20th century.

History of Mount St. Helens

Introduction

     Mount St. Helens, located in southwestern Washington about 50 miles
northeast of Portland, Oregon, is one of several lofty volcanic peaks that
dominate the Cascade Range of the Pacific Northwest; the range extends from
Meager Mountain in British Columbia, Canada, to Lassen Peak in northern
California.  Geologists call Mount St. Helens a stratovolcano or composite
volcano, a term for steep-sided, often symmetrical cones constructed of
alternating layers of lava flows, ash, and other volcanic debris.  Composite
volcanoes tend to erupt explosively and pose considerable danger to nearby
life and property.  In contrast, the gently sloping shield volcanoes, such as
those in Hawaii, typically erupt nonexplosively, producing fluid lavas that
can flow great distances from the active vents.  Although Hawaiian-type
eruptions may destroy property, they rarely cause death or injury.

[See St. Helens Lava Dome: Steam plume rises from lava dome growing inside the
large crater formed during the catastrophic eruption of May 18, 1980. Photo
taken in May, 1982.]

     Before 1980, snow-capped, gracefully symmetrical Mount St. Helens was
known as the "Fujiyama of America."  Mount St. Helens, other active Cascaan
volcanoes, and those of Alaska comprise the North American segment of the
circum-Pacific "Ring of Fire," a notorious zone that produces frequent, often
destructive, earthquake and volcanic activity.

     Some Indians of the Pacific Northwest called Mount St. Helens
"Louwala-Clough," or "smoking mountain."  The modern name, Mount St. Helens,
was given to the volcanic peak in 1792 by Captain George Vancouver of the
British Royal Navy, a seafarer and explorer.  He named it in honor of a fellow
countryman, Alleyne Fitzherbert, who held the title Baron St. Helens and who
was at the time the British Ambassador to Spain.  Vancouver also named three
other volcanoes in the Cascade - Mounts Baker, Hood, and Rainier - for British
naval officers.

[See Before the Eruption: View of Mount St. Helens from north on May 17, 1980.]

     The local Indians and early settlers in the then sparsely populated
region witnessed the occasional violent outbursts of Mount St. Helens.  The
volcano was particularly restless in the mid-19th century, when it was
intermittently active for at least a 26-year span from 1831 to 1857.  Some
scientists suspect that Mount St. Helens also may have been active
sporadically, but weakly, during the three decades before 1831.  Although
minor steam explosions may have occurred in 1898, 1903, and 1921, the mountain
gave little or no evidence of being a volcanic hazard for more than a century
after 1857.  Consequently, the majority of 20th-century residents and visitors
thought of Mount St. Helens not as a menace, but as a serene, beautiful
mountain playground teeming with wildlife and available for leisure activities
throughout the year.  At the base of the volcano's northern flank, Spirit
Lake, with its clear, refreshing water and wooded shores, was especially
popular as a recreational area for hiking, camping, fishing, swimming and
boating.

[See The Eruption: The climactic eruption of May 18, 1980, at about noon. The
maximum height of the ash and gas column was about 12 miles.]

     The tranquility of the Mount St. Helens region was shattered in the
spring of 1980, however, when the volcano stirred from its long repose, shook,
swelled, and exploded back to life.  The local people rediscovered that they
had an active volcano in their midst, and millions of people in North America
were reminded that the active and potentially dangerous volcanoes of the
United States are not restricted to Alaska and Hawaii.

[See St. Helens Area: Sketch map showing the location of Mount St. Helens and
the principal drainages and places mentioned in the text.]

Previous Eruptive History

     The story of Mount St. Helens is woven from geologic evidence gathered
during studies that began with Lieutenant Charles Wilkes' U.S. Exploring
Expedition in 1841.  Many geologists have studied Mount St. Helens, but the
work of Dwight R. Crandell, Donal R. Mullineaux, Clifford P. Hopson, and their
associates, who began their studies in the late 1950's, has particularly
advanced knowledge of Mount St. Helens.  Their systematic studies of the
volcanic deposits, laboratory investigations of rock and ash samples, and
radiocarbon (carbon-14) dating of plant remains buried in or beneath the ash
layers and other volcanic products enabled them to reconstruct a remarkably
complete record of the prehistoric eruptive behavior of Mount St. Helens.

     Ancestral Mount St. Helens began to grow before the last major glaciation
of the Ice Age had ended about 10,000 years ago.  The oldest ash deposits were
erupted at least 40,000 years ago onto an eroded surface of still older
volcanic and sedimentary rocks.  Intermittent volcanism continued after the
glaciers disappeared and nine main pulses of pre-1980 volcanic activity
("eruptive periods") have been recognized.  These periods lasted from about
5,000 years to less than 100 years each and were separated by dormant
intervals of about 15,000 years to only 200 years.  A forerunner of Spirit
Lake was born about 3,500 years ago, or possibly earlier, when eruption debris
formed a natural dam across the valley of the North Fork of the Toutle River.
The most recent of the pre-1980 eruptive periods began about A.D. 1800 with an
explosive eruption, followed by several additional minor explosions and
extrusions of lava, and ended with the formation of the Goat Rocks lava dome
by 1857.

     Mount St. Helens is the youngest of the major Cascade volcanoes, in the
sense that its visible cone was entirely formed during the past 2,500 years,
well after the melting of the last of the Ice Age glaciers about 10,000 years
ago.  Mount St. Helens' smooth, symmetrical slopes are little affected by
erosion as compared with its older, more glacially scarred neighbors - Mount
Rainier and Mount Adams in Washington, and Mount Hood in Oregon.  As geologic
studies progressed and the eruptive history of Mount St. Helens became better
known, scientists became increasingly concerned about possible renewed
eruptions.  The late William T. Pecora, a former Director of the USGS, was
quoted in a May 10, 1968, newspaper article in the Christian Science Monitor
as being "especially worried about snow-covered Mt. St. Helens."

     On the basis of its youth and its high frequency of eruptions over the
past 4,500 years, Crandell, Mullineaux, and their colleague Meyer Rubin
published in February 1975 that Mount St. Helens was the one volcano in the
conterminous United States most likely to reawaken and to erupt "perhaps
before the end of this century."  This prophetic conclusion was followed in
1978 by a more detailed report, in which Crandell and Mullineaux elaborated
their earlier conclusion and analyzed, with maps and scenarios, the kinds,
magnitudes, and areal extents of potential volcanic hazards that might be
expected from future eruptions of Mount St. Helens.  Collectively, these two
publications contain one of the most accurate forecasts of a violent geologic
event.

Reawakening and Initial Activity

     A magnitude 4.2 (Richter Scale) earthquake on March 20, 1980, at 3:47
p.m. Pacific Standard Time (PST), preceded by several much smaller earthquakes
beginning as early as March 16, was the first substantial indication of Mount
St. Helens' awakening from its 123-year sleep.  Earthquake activity increased
during the following week, gradually at first and then rather dramatically at
about noon on March 25.  The number of earthquakes recorded daily reached peak
levels in the next 2 days, during which 174 shocks with magnitudes greater
than 2.6 were recorded.  Many hundreds of smaller earthquakes accompanied
these larger events, the largest of which were felt by people living close to
the volcano.  Aerial observations of Mount St. Helens during the week of
seismic buildup revealed small earthquake-induced avalanches of snow and ice,
but no sign of an eruption.

     With a thunderous explosion, or possibly two nearly simultaneous ones,
widely heard in the region at about 12:36 p.m. PST on March 27, Mount St.
Helens began to spew ash and steam, marking the first significant eruption in
the conterminous United States since that of Lassen Peak, California, from
1914 to 1917.  The crown of the ash column rose to about 6,000 feet above the
volcano.  The initial explosions formed a 250-foot-wide crater within the
larger, preexisting snow- and ice-filled summit crater, and new fractures
broke across the summit area.

     Through April 21, Mount St. Helens intermittently ejected ash and steam
in bursts lasting from a few seconds to several tens of minutes.  The first
crater was joined on the west by a second, slightly larger crater, and as the
activity continued, both craters enlarged and ultimately merged.  Several
avalanches of snow and ice, darkened by ash, formed prominent streaks down the
mountain's slopes.  The effect of the prevailing easterly wind was striking
during the March-April eruptive activity, transforming the snow-covered Mount
St. Helens into a "two-tone" mountain.

     The ash blown out between March 27 and May 18 was derived entirely from
the 350-year-old summit dome, shattered and pulverized by phreatic
(steam-blast) processes driven by the explosively expanding, high-temperature
steam and other gases.  No magma (molten rock and contained gases) was tapped
during the initial eruptions.

     Intense earthquake activity persisted at the volcano during and between
visible eruptive activity.  As early as March 31, seismographs also began
recording occasional spasms of harmonic tremor a type of continuous, rhythmic
ground shaking different from the discrete sharp jolts characteristic of
earthquakes.  Such continuous ground vibrations, commonly associated with
eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are
interpreted to reflect subsurface movement of fluids, either gas or magma.
The combination of sustained strong earthquake activity and harmonic tremor at
Mount St. Helens suggested to scientists that magma and associated gases were
on the move within the volcano, thereby increasing the probability of magma
eruption.

     Visible eruptive activity ceased temporarily in late April and early May.
Small steam-blast eruptions resumed on May 7, continued intermittently for the
next several days, and ceased again by May 16.  During this interval, the
forceful intrusion of magma into the volcano continued with no respite, as was
shown by intense seismic activity and visible swelling and cracking of the
volcano.  The swelling was easily measurable and affected a large area on the
north face of Mount St. Helens; this area became known as the "bulge," the
initial growth of which probably began during the first eruption (March 27) or
perhaps even a few days before. Through mid-May about 10,000 earthquakes were
recorded.  The earthquake activity was concentrated in a small zone less than
1.6 miles directly beneath the bulge on the north flank of Mount St. Helens.

     A comparison of aerial photographs taken in the summer of 1979 with those
taken during and after April 1980 showed that by May 12 certain parts of the
bulge near the summit were more than 450 feet higher than before the magma
intrusion began.  Repeated measurements begun in late April with precise
electronic instruments that shoot a laser beam to reflector targets placed on
and around the bulge showed that it was growing northward at an astonishing
rate of about 5 feet per day.  The movement was predominantly horizontal -
clear evidence that the bulge was not simply slipping down the volcano's steep
slope.  As the bulge moved northward, the summit area behind it progressively
sank, forming a complex down-dropped block called a graben.  These changes in
the volcano's shape were related to the overall deformation that increased the
volume of the mountain by 0.03 cubic mile by mid-May.  This volume increase
presumably corresponded to the volume of magma that pushed into the volcano
and deformed its surface.  Because the intruded magma remained below ground
and was not directly visible, it was called a cryptodome, in contrast to a
true volcanic dome exposed at the surface.

     In summary, during late March to mid-May 1980, Mount St. Helens was
shaken by hundreds of earthquakes, intermittently erupted ash and debris
derived by steam blast reaming out of its preexisting summit dome, and
experienced extremely large and rapid deformation caused by magma intrusion.
The hot intruding magma provided the thermal energy to heat groundwater, which
explosively flashed to generate and sustain the observed steam-blast
eruptions.  For 2 months the volcano was literally being wedged apart,
creating a highly unstable and dangerous situation.  The eventual collapse of
the bulge on the north flank triggered the chain of catastrophic events that
took place on May 18, 1980.

The Climactic Eruption of May 18, 1980

     May 18, a Sunday, dawned bright and clear.  At 7 am. Pacific Daylight
Time (PDT), USGS volcanologist David A. Johnston, who had Saturday night duty
at an observation post about 6 miles north of the volcano, radioed in the
results of some laserbeam measurements he had made moments earlier that
morning.  Even considering these measurements, the status of Mount St. Helens'
activity that day showed no change from the pattern of the preceding month.
Volcano-monitoring data - seismic, rate of bulge movement, sulfur-dioxide gas
emission, and ground temperature revealed no unusual changes that could be
taken as warning signals for the catastrophe that would strike about an hour
and a half later.  About 20 seconds after 8:32 a.m. PDT, apparently in
response to a magnitude 5.1 earthquake about 1 mile beneath the volcano, the
bulged, unstable north flank of Mount St. Helens suddenly began to collapse,
triggering a rapid and tragic train of events that resulted in widespread
devastation and the loss of 60 people, including volcanologist Johnston.

[See Ash Cloud: The climactic eruption in full fury in the late morning of May
18, 1980.]

Debris avalanche

     Although the triggering earthquake was of slightly greater magnitude than
any of the shocks recorded earlier at the volcano, it was not unusual in any
other way.  What happened within the next few seconds was described by
geologists Keith and Dorothy Stoffel, who at the time were in a small plane
over the volcano's summit.  Among the events they witnessed, they


noticed landsliding of rock and ice debris inward into the crater . . . the
south-facing wall of the north side of the main crater was especially active.
Within a matter of seconds, perhaps 15 seconds, the whole north side of the
summit crater began to move instantaneously.  The nature of movement was
eerie. . . .  The entire mass began to ripple and churn up, without moving
laterally.  Then the entire north side of the summit began sliding to the
north along a deep-seated slide plane.  I [Keith Stoffel] was amazed and
excited with the realization that we were watching this landslide of
unbelievable proportions. . . .  We took pictures of this slide sequence
occurring, but before we could snap off more than a few pictures, a huge
explosion blasted out of the detachment plane.  We neither felt nor heard a
thing, even though we were just east of the summit at this time.


     Realizing their dangerous situation, the pilot put the plane into a steep
dive to gain speed, and thus was able to outrun the rapidly mushrooming
eruption cloud that threatened to engulf them.  The Stoffels were fortunate to
escape, and other scientists were fortunate to have their eyewitness account
to help unscramble the sequence and timing of the quick succession of events
that initiated the May 18 eruption.

     The collapse of the north flank produced the largest landslide-debris
avalanche recorded in historic time.  Detailed analysis of photographs and
other data shows that an estimated 7-20 seconds (about 10 seconds seems most
reasonable) elapsed between the triggering earthquake and the onset of the
flank collapse.  During the next 15 seconds, first one large block slid away,
then another large block began to move, only to be followed by still another
block.  The series of slide blocks merged downslope into a gigantic debris
avalanche, which moved northward at speeds of 155 to 180 miles an hour.  Part
of the avalanche surged into and across Spirit Lake, but most of it flowed
westward into the upper reaches of the North Fork of the Toutle River.  At one
location, about 4 miles north of the summit, the advancing front of the
avalanche still had sufficient momentum to flow over a ridge more than 1,150
feet high.  The resulting hummocky avalanche deposit consisted of intermixed
volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake.
Covering an area of about 24 square miles, the debris avalanche advanced more
than 13 miles down the North Fork of the Toutle River and filled the valley to
an average depth of about 150 feet; the total volume of the deposit was about
0.7 cubic mile.  The dumping of avalanche debris into Spirit Lake raised its
bottom by about 295 feet and its water level by about 200 feet.

[See Debris River: View up the North Fork Toutle River toward Mount St. Helens
showing the valley choked with the hummocky deposits of the debris avalanche.]

Lateral "blast"

     Within a few seconds after the onset and mobilization of the debris
avalanche, the climactic eruptions of May 18 began as the sudden unloading of
much of the volcano's north flank abruptly released the pent-up pressure of
the volcanic system.  The sudden removal of the upper part of the volcano by
the landslides triggered the almost instantaneous expansion (explosion) of
high temperature-high pressure steam present in cracks and voids in the
volcano and of gases dissolved in the magma that caused the bulge of the
cryptodome.  The abrupt pressure release, or "uncorking," of the volcano by
the debris avalanche can be compared in some ways to the sudden removal of the
cap or a thumb from a vigorously shaken bottle of soda pop, or to punching a
hole in a boiler tank under high pressure.

     At Mount St. Helens, the "uncorking" unleashed a tremendous,
northward-directed lateral blast of rock, ash, and hot gases that devastated
an area of about 230 square miles in a fan-shaped sector north of the volcano.
To the south, the devastated area was much less, extending only a small
distance downslope from the summit.  Along with older volcanic debris, the
blast also included the first magmatic material erupted by Mount St. Helens,
indicating that the landslides and the ensuing blast had exposed the
cryptodome magma.

[See Blasted Forest: What appear to be blades of mown grass are actually large
trees, some over 100 feet tall, flattened by the tremendous force of the
lateral blast, even out to distances as far as 19 miles from the volcano.]

     Although the lateral blast began some seconds later than the debris
avalanche, the blast's velocity was much greater, so that it soon overtook the
avalanche.  Calculations have shown that the blast's initial velocity of about
220 miles an hour quickly increased to about 670 miles an hour.  The average
velocity did not surpass the speed of sound in the atmosphere (about 735 miles
an hour).  This observation is consistent with the lack of reports of loud
atmospheric shocks or "sonic booms" from nearby observers such as Keith and
Dorothy Stoffel in the light plane or survivors on the ground.  In some areas
near the blast front, however, the velocity may have approached, or even
exceeded, the supersonic rate for a few moments.

     The blast was widely heard hundreds of miles away in the Pacific
Northwest, including parts of British Columbia, Montana, Idaho, and northern
California.  Yet, in many areas much closer to Mount St. Helen - for example,
Portland, Oregon, only 50 miles away - the blast was not heard.  Subsequent
studies by the Oregon Museum of Science and Industry demonstrated a so-called
"quiet zone" around Mount St. Helens, extending radially a few tens of miles,
in which the eruption was not heard.  The creation of the "quiet zone" and the
degree to which the eruption was heard elsewhere depended on the complex
response of the eruption sound waves to differences in temperature and air
motion of the atmospheric layers and, to a lesser extent, local topography.

     The near-supersonic lateral blast, loaded with volcanic debris, caused
widespread devastation as far as 19 miles from the volcano.  The area affected
by the blast can be subdivided into three roughly concentric zones:

     (1) Direct blast zone, the innermost zone, averaged about 8 miles in
radius, an area in which virtually everything, natural or manmade, was
obliterated or carried away.  For this reason, this zone also has been called
the "tree-removal zone."  The flow of the material carried by the blast was
not deflected by topographic features in this zone.

     (2) Channelized blast zone, an intermediate zone, extended out to
distances as far as 19 miles from the volcano, an area in which the flow
flattened everything in its path and was channeled to some extent by
topography.  In this zone, the force and direction of the blast are strikingly
demonstrated by the parallel alignment of toppled large trees, broken off at
the base of the trunk as if they were blades of grass mown by a scythe.  This
zone was also known as the "tree-down zone."

     (3) Seared zone, the outermost fringe of the impacted area, a zone in
which trees remained standing, but singed brown by the hot gases of the blast.

     A similar, but narrower and northeast-trending, strong laterally directed
explosion occurred at Mount St. Helens about 1,100 years ago.  The blast of
May 18, 1980, however, traveled at least three times as far as the
1,100-year-old blast.  Thus, the occurrence of a lateral blast such as that of
May 18 was not the first in Mount St. Helens' history, but its power and
resulting destruction were unprecedented.  The lateral blast, debris
avalanche, and associated mudflows and floods caused most of the casualties
and destruction on May 18; the adverse impact of volcanic ash fallout downwind
was minor by comparison.

Ash eruption and fallout

     A strong, vertically directed explosion of ash and steam began very
shortly after the lateral blast.  The resulting eruptive column rose very
quickly.  In less than 10 minutes, the ash column reached an altitude of more
than 12 miles and began to expand into a characteristic mushroom-shaped ash
cloud.  Near the volcano, the swirling ash particles in the atmosphere
generated lightning, which in turn started many forest fires.  As the eruption
roared on, the major part of the ash cloud drifted downwind in an
east-northeasterly direction, although ash that rose above the high-speed
(jet-stream) winds followed other paths determined by complex wind directions.

     Clear skies permitted tracking the advance of the drifting cloud by
satellite imagery.  Moving at an average speed of about 60 miles an hour, the
cloud reached Yakima, Washington, by 9:45 a.m. PDT and Spokane, Washington, by
11:45 a.m.  The ash cloud was dense enough to screen out nearly all sunlight,
activating darkness-sensitive switches on street lights in Yakima and Spokane.
Street lights remained on for the rest of the darkened day, as the eruption
continued vigorously for more than 9 hours, pumping ash into the atmosphere
and feeding the drifting ash cloud.

     The eruptive column fluctuated in height through the day, but the
eruption subsided by late afternoon on May 18.  By early May 19, the eruption
had stopped.  By that time, the ash cloud had spread to the central United
States.  Two days later, even though the ash cloud had become more diffuse,
fine ash was detected by systems used to monitor air pollution in several
cities of the northeastern United States.  Some of the ash drifted around the
globe within about 2 weeks.  After circling many more times, most of the ash
settled to the Earth's surface, but some of the smallest fragments and
aerosols are likely to remain suspended in the upper atmosphere for years.

     Prevailing winds distributed the fallout from the ash cloud over a wide
region.  Light ash falls were reported in most of the Rocky Mountain States,
including northern New Mexico, and fine ash dusted a few scattered areas
farther east and northeast of the main path.  The heaviest ash deposition
occurred in a 60-mile-long swath immediately downwind of the volcano.  Another
area of thick ash deposition, however, occurred near Ritzville in eastern
Washington, about 195 miles from Mount St. Helens, where nearly 2 inches of
ash blanketed the ground, more than twice as much as at Yakima, which is only
about half as far from the volcano.  Scientists believe that this unexpected
variation in ash thickness may reflect differences in wind velocity and
direction with altitude, fluctuations in the height of the ash column during
the 9 hours of activity, and the effect of localized clumping of fine ash
particles leading to preferential fallout of the large particle clumps.

     During the 9 hours of vigorous eruptive activity, about 540 million tons
of ash fell over an area of more than 22,000 square miles.  The total volume
of the ash before its compaction by rainfall was about 0.3 cubic mile,
equivalent to an area the size of a football field piled about 150 miles high
with fluffy ash.  The volume of the uncompacted ash is equivalent to about
0.05 cubic mile of solid rock, or only about 7 percent of the amount of
material that slid off in the debris avalanche.  The eruption of ash also
further enlarged the depression formed initially by the debris avalanche and
lateral blast, and helped to create a great amphitheater shaped crater open to
the north.  This new crater was about 1 mile by 2 miles wide and about 2,100
feet deep from its rim to its lowest point.  The area of this crater roughly
encompassed that of the former bulge on the north flank of the volcano and the
former summit dome.  After the eruption, the highest point on the volcano was
about 8,364 feet, or 1,313 feet lower than the former summit elevation.

Pyroclastic flows

     The term "pyroclastic" derived from the Greek words pyro (fire) and
klastos (broken) describes materials formed by the fragmentation of magma and
rock by explosive volcanic activity.  Most volcanic ash is basically
fine-grained pyroclastic material composed of tiny particles of explosively
disintegrated old volcanic rock or new magma.  Larger sized pyroclastic
fragments are called lapilli blocks, or bombs.  Pyroclastic flow - sometimes
called nuees ardentes (French for "glowing clouds") - are hot, often
incandescent mixtures of volcanic fragments and gases that sweep along close
to the ground.  Depending on the volume of material, proportion of solids to
gas, temperature, and slope gradient, the flows can travel at velocities as
great as 450 miles an hour.  Pyroclastic flows can be extremely destructive
and deadly because of their high temperature and mobility.  During the 1902
eruption of Mont Pelee (Martinique, West Indies), for example, a nuee ardente
demolished the coastal city of St. Pierre, killing nearly 30,000 inhabitants.

     Pyroclastic flows commonly are produced either by the fallback and
downslope movement of fragments from an eruption column or by the direct
frothing over at the vent of magma undergoing rapid gas loss.  Volcanic froth
so formed is called pumice.  Pyroclastic flows originated in both ways at
Mount St. Helens on May 18, but flows of mappable volume were of the latter
type.  The flows were entirely restricted to a small fan-shaped zone that
flares northward from the summit crater.

     Pyroclastic flows were first directly observed at 12:17 p.m. PDT,
although they probably began to form shortly after the lateral blast.  They
continued to occur intermittently during the next 5 hours of strong eruptive
activity.  Eyewitness accounts indicated that the more voluminous pyroclastic
flows originated by the upwelling of volcanic ejecta to heights below the rim
of the crater, followed by lateral flow northward through the breach of the
crater.  One scientist likened this process to a "pot of oatmeal boiling
over."  Most of the rock in these flows was pumice.  A few smaller pyroclastic
flows were observed to form by gravitational collapse of parts of the high
eruption column.  The successive outpourings of pyroclastic material consisted
mainly of new magmatic debris rather than fragments of preexisting volcanic
rocks.  The resulting deposits formed a fan-like pattern of overlapping
sheets, tongues, and lobes.  At least 17 separate pyroclastic flows occurred
during the May 18 eruption, and their aggregate volume was about 0.05 cubic
mile.

     When temperature measurements could safely be made in the pyroclastic
flows 2 weeks after they were erupted, the deposits ranged in temperature from
about 570 degrees to 785 degrees F.  As might be expected, when the hot
material of the debris avalanche and the even hotter pyroclastic flows
encountered bodies of water or moist ground, the water flashed explosively to
steam; the resulting phreatic (steam-blast) explosions sent plumes of ash and
steam as high as 1.2 miles above the ground.  These "secondary" or "rootless"
steam-blast eruptions formed many explosion pits on the northern margin of the
pyroclastic flow deposits, at the south shore of Spirit Lake, and along the
upper part of the North Fork of the Toutle River.  These steam-blast
explosions continued sporadically for weeks or months after the emplacement of
pyroclastic flows, and at least one occurred about a year later, on May 16,
1981.

Mudflows and floods

     Mudflows - mobile mixtures of volcanic debris and water - often accompany
pyroclastic eruptions, if water is available to erode and transport the loose
pyroclastic deposits on the steep slopes of stratovolcanoes.  Destructive
mudflows and debris flows began within minutes of the onset of the May 18
eruption, as the hot pyroclastic materials in the debris avalanche, lateral
blast, and ash falls melted snow and glacial ice on the upper slopes of Mount
St. Helens.  Mudflows are also called Iharas, a term borrows from Indonesia,
where volcanic eruptions have produced many such deposits.

     Mudflows here observed as early as 8:50 am. PDT in the upper reaches of
the South Fork of the Toutle River.  The largest and most destructive
mudflows, however, were those that developed several hours later in the North
Fork of the Toutle River, when the water-saturated parts of the massive debris
avalanche deposits began to slump and flow.  The mudflows in the Toutle River
drainage area ultimately dumped more than 65 million cubic yards of sediment
along the lower Cowlitz and Columbia River.  The water-carrying capacity of
the Cowlitz River was reduced by 85 percent, and the depth of the Columbia
River navigational channel was decreased from 39 feet to less than 13 feet,
disrupting river traffic and choking off ocean shipping.  Mudflows also swept
down the southeast flank of the volcano - along the Swift Creek, Pine Creek,
and Muddy River drainage - and emptied nearly 18 million cubic yards of water,
mud, and debris into the Swift Reservoir.  The water level of the reservoir
had been purposely kept low as a precaution to minimize the possibility that
the reservoir could be overtopped by the additional water-mud-debris load to
cause flooding of the valley downstream.  Fortunately, the volume of the
additional load was insufficient to cause overtopping even if the reservoir
had been full.

     On the upper steep slopes of the volcano, the mudflows traveled as fast
as 90 miles an hour; the velocity then progressively slowed to about 3 miles
an hour as the flows encountered the flatter and wider parts of the Toutle
River drainage.  Even after traveling many tens of miles from the volcano and
mixing with cold waters, the mudflows maintained temperatures in the range of
about 84 degrees to 91 degrees F.; they undoubtedly had higher temperatures
closer to the eruption source.  Shortly before 3 p.m., the mud and
debris-choked Toutle River crested about 21 feet above normal at a point just
south of the confluence of the North and South Forks.  Another stream gage at
Castle Rock, about 3 miles downstream from where the Toutle joins the Cowlitz,
indicated a high-water (and mud) mark also about 20 feet above normal at
midnight of May 18.  Locally the mudflows surged up the valley walls as much
as 360 feet and over hills as high as 250 feet.  From the evidence left by the
"bathtub-ring" mudlines, the larger mudflows at their peak averaged from 33 to
66 feet deep.  The actual deposits left behind after the passage of the
mudflow crests, however, were considerably thinner, commonly less than 10
percent of their depth during peak flow.  For example, the mudflow deposits
along much of the Toutle River averaged less than 3 feet thick.

The catastrophic first minute

     During the initial hours of the May 18 activity, people were obviously
confused about the nature and sequence of the phenomena taking place.  Did the
eruption trigger the 5.1 magnitude earthquake or did the earthquake trigger
the eruption?  Or were both associated with some other, but unknown, cause or
causes?  At first, these questions and others could not be answered because of
the rapidity of developments and the initial lack of firsthand observations by
people who were close to the mountain and who survived the catastrophe.  It
was not until many hours, indeed days, later that scientists were able to
reconstruct clearly the sequence of events.  The reconstruction was aided by
eyewitness accounts.  Geologists Keith and Dorothy Stoffel, flying over the
volcano in a small plane when the earthquake struck, observed "minor
landsliding of rock and ice debris" into the crater.  Within the next 15
seconds, the north flank of the volcano "began to ripple and churn up, without
moving laterally."  At the same time the Stoffels were witnessing from the air
the developing debris avalanche, a remarkable series of ground-based
photographs was being taken by Keith Ronnholm and Gary Rosenquist from Bear
Meadows, a camping area located about 11 miles northeast of Mount St. Helens.
Seconds after the earthquake, William Dilly, a member of the Rosenquist party,
noticed through binoculars that the north flank was becoming "fuzzy, like
there was dust being thrown down the side" and shouted that the "mountain was
going."  Within seconds Rosenquist began taking photographs .n rapid
succession.

     Frame-by-frame analysis of the Rosenquist photographs, taken within a
span of about 40 seconds, together with seismic and other evidence,
established the following sequence of events during the first minute of the
climactic eruption.  The times indicated are in hours, minutes, and seconds
(Pacific Daylight Time).

     08:27 (approximate) Pre-earthquake view of the bulge on the volcano's
north flank produced by the growing cryptodome of magma intruded since March
20.  About 5 minutes later (08:32:11.4 PDT), a 5.1 magnitude earthquake struck
beneath the mountain at shallow depth.

     08:32:47.0 Estimate of the time of the first photograph in Rosenquist's
sequence that shows movement of the mountain.  By this time, the first slide
block had already dropped about 2,300 feet and a second block behind it had
slid 330 feet.  The beginning of the north flank's collapse and downward
movement to initiate the debris avalanche was estimated to be 26 seconds
earlier (08:32:21.0 PDT).

     08:32:49.2 A little more than 2 seconds later, as the slide blocks
continued to move, the initial explosions of the vertical eruption column as
well as the lateral blast, although obscure, had already begun.

     08:32:53.3 The first slide block now had dropped sufficiently to expose
more of the cryptodome magma, accelerating the explosive expansion of gases in
the magma and the eruption of the first magmatic material of the 1980
eruptions.

     08:33:03.7 The continuing movement of the slide blocks and explosions had
now thoroughly "uncorked" the magmatic system of the cryptodome, and old and
new (magmatic) debris were blasted outward by increasingly more powerful
explosions.  The high-velocity lateral blast cloud, with its clearly visible
trajectory trails of large blocks, was overtaking the slower moving debris
avalanche.

     8:33:18.8 Less than a minute after the start of the debris avalanche, the
eruption of Mount St. Helens was in full fury, further enlarging the crater as
smaller slide blocks fell into the vent and were blasted away.  The leading
front of the lateral blast now had completely overtaken the debris avalanche.

     The lateral blast at the vent probably lasted no more than about 30
seconds, but the northward radiating and expanding blast cloud continued for
about another minute, extending to areas more than 16 miles from the volcano.
Shortly after the blast shot out laterally, the vertically directed ash column
rose to an altitude of about 16 miles in less than 15 minutes and the vigorous
emission of ash continued for the next 9 hours. The eruption column began to
decline at about 5:30 p.m. and diminished to a very low level by early morning
of May 19.

     The extraordinary photographic documentation of the first minute enabled
scientists to reconstruct accurately what had happened.  The 5.1 magnitude
earthquake caused the gravitational collapse of Mount St. Helens' north flank,
which produced the debris avalanche and triggered the ensuing violent lateral
and vertical eruptions.  From a scientific perspective, it was fortunate that
the initial May 18 events occurred during daylight hours under cloudless
conditions; otherwise, the sequence of events during that crucial first minute
following the earthquake would have been difficult to reconstruct precisely.

Impact and aftermath

     The May 18, 1980, eruption was the most destructive in the history of the
United States.  Novarupta (Katmai) Volcano, Alaska, erupted considerably more
material in 1912, but owing to the isolation and sparse population of the
region affected, there were no human deaths and little property damage.  In
contrast, Mount St. Helens' eruption in a matter of hours caused loss of lives
and widespread destruction of valuable property, primarily by the debris
avalanche, the lateral blast, and the mudflows.

     Landscape changes caused by the May 18 eruption were readily seen on
high-altitude photographs.  Such images, however, cannot reveal the impacts of
the devastation on people and their works.  The May 18 eruption resulted in
scores of injuries and the loss of about 60 lives (35 known deaths and 25
missing persons).  Within the United States before May 18, 1980, only two
known casualties had been attributed to volcanic activity - a photographer was
struck by falling rocks during the explosive eruption of Kilauea Volcano,
Hawaii, in 1924; and an Army sergeant who disappeared during the 1944 eruption
of Cleveland Volcano, Chuginadak Island, Aleutians.  Autopsies indicated that
most of Mount St. Helens' victims died by asphyxiation from inhaling hot
volcanic ash, and some by thermal and other injuries.

     The lateral blast, debris avalanche, mudflows, and flooding caused
extensive damage to land and civil works.  All buildings and related manmade
structures in the vicinity of Spirit Lake were buried.  More than 200 houses
and cabins were destroyed and many more were damaged in Skamania and Cowlitz
Counties, leaving many people homeless.  Many tens of thousands of acres of
prime forest, as well as recreational sites, bridges, roads, and trails, were
destroyed or heavily damaged.  More than 185 miles of highways and roads and
15 miles of railways were destroyed or extensively damaged.  Trees amounting
to more than 4 billion board feet of salable timber were damaged or destroyed,
primarily by the lateral blast.  At least 25 percent of the destroyed timber
has been salvaged since September 1980.  Hundreds of loggers have been
involved in the timber-salvage operations, and, during peak summer months,
more than 600 truckloads of salvaged timber were retrieved each day.

     Wildlife in the Mount St. Helens area also suffered heavily.  The
Washington State Department of Game estimated that nearly 7,000 big game
animals (deer, elk, and bear) perished in the area most affected by the
eruption, as well as all birds and most small mammals.  A few small animals,
chiefly burrowing rodents, frogs, salamanders, and crawfish, managed to
survive because they were below ground level or water surface when the
disaster struck.  The Washington Department of Fisheries estimated that 12
million Chinook and Coho salmon fingerlings were killed when hatcheries were
destroyed; these might have developed into about 360,000 adult salmon.
Another estimated 40,000 young salmon were lost when they were forced to swim
through the turbine blades of hydroelectric generators because the levels of
the reservoirs along the Lewis River south of Mount St. Helens were kept low
to accommodate possible mudflows and flooding.

     Downwind of the volcano, in areas of thick ash accumulation, many
agricultural crops, such as wheat, apples, potatoes, and alfalfa, were
destroyed.  Many crops survived, however, in areas blanketed by only a thin
covering of ash.  In fact, the apple and wheat production in 1980 was higher
than normal due to greater-than-average summer precipitation.  The crusting of
ash also helped to retain soil moisture through the summer.  Moreover, in the
long term, the ash may provide beneficial chemical nutrients to the soils of
eastern Washington, which themselves were formed of older glacial deposits
that contain a significant ash component.  Effects of the ash fall on the
water quality of streams, lakes, and rivers were short-lived and minor.

     The ash fall, however, did pose some temporary major problems for
transportation operations and for sewage-disposal and water-treatment systems.
Because visibility was greatly decreased during the ash fall, many highways
and roads were closed to traffic, some only for a few hours, but others for
weeks.  Interstate 90 from Seattle to Spokane, Washington, was closed for a
week.  Air transportation was disrupted for a few days to 2 weeks as several
airports in eastern Washington shut down due to ash accumulation and attendant
poor visibility.  Over a thousand commercial flights were canceled following
airport closures.

     The fine-grained, gritty ash caused substantial problems for internal-
combustion engines and other mechanical and electrical equipment.  The ash
contaminated oil systems, clogged air filters, and scratched moving surfaces.
Fine ash caused short circuits in electrical transformers, which in turn
caused power blackouts.  The sewage-disposal systems of several municipalities
that received about half an inch or more of ash, such as Moses Lake and
Yakima, Washington, were plagued by ash clogging and damage to pumps, filters,
and other equipment.  Fortunately, as these same cities used deep wells and
closed storage, their water-supply systems were only minimally affected.

     The removal and disposal of ash from highways, roads, buildings, and
airport runways were monumental tasks for some eastern Washington communities.
State and Federal agencies estimated that over 2.4 million cubic yards of ash
- equivalent to about 900,000 tons in weight - were removed from highways and
airports in Washington State.  Ash removal cost $2.2 million and took 10 weeks
in Yakima.  The need to remove ash quickly from transportation routes and
civil works dictated the selection of some disposal sites.  Some cities used
old quarries and existing sanitary landfills; others created dumpsites
wherever expedient.  To minimize wind reworking of ash dumps, the surfaces of
some disposal sites have been covered with topsoil and seeded with grass.
About 250,000 cubic yards of ash have been stockpiled at five sites and can be
retrieved easily for constructional or industrial use at some future date if
economic factors are favorable.

     What was the cost of the destruction and damage caused by the May 18
eruption?  Accurate cost figures remain difficult to determine.  Early
estimates were too high and ranged from $2 to $3 billion, primarily reflecting
the timber, civil works, and agricultural losses.  A refined estimate of $1.1
billion was determined in a study by the International Trade Commission at the
request of Congress.  A supplemental appropriation of $951 million for
disaster relief was voted by Congress, of which the largest share went to the
Small Business Administration, U.S. Army Corps of Engineers, and the Federal
Emergency Management Agency.

     There were indirect and intangible costs of the eruption as well.
Unemployment in the immediate region of Mount St. Helens rose tenfold in the
weeks immediately following the eruption and then nearly returned to normal
once timber salvaging and ash cleanup operations were underway.  Only a small
percentage of residents left the region because of lost jobs owing to the
eruption.  Several months after May 18, a few residents reported suffering
stress and emotional problems, even though they had coped successfully during
the crisis.  The counties in the region requested funding for mental health
programs to assist such people.

     Initial public reaction to the May 18 eruption nearly dealt a crippling
blow to tourism, an important industry in Washington.  Not only was tourism
down in the Mount St. Helens-Gifford Pinchot National Forest area, but
conventions, meetings, and social gatherings also were canceled or postponed
at cities and resorts elsewhere in Washington and neighboring Oregon not
affected by the eruption.  The negative impact on tourism and
conventioneering, however, proved only temporary.  Mount St. Helens, perhaps
because of its eruptive activity, has regained its appeal for tourists.  The
U.S. Forest Service (USFS) and State of Washington opened visitor centers and
provided access for people to view firsthand the volcano's awesome
devastation.

     The spectacular eruption impressed upon the people in the Pacific
Northwest that they share their lands with both active and potentially active
volcanoes.  With the passage of time, the damaged forests, streams, and fields
will heal, and the memory of the 1980 eruption and its impacts will fade in
future generations.  The Mount St. Helens experience has been so thoroughly
documented, however, that it likely will be a reminder for decades in the
future of the possibility of renewed volcanic activity and destruction.