|1  3 Mount St. Helens History
|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
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 volcanic 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
The climactic eruption in full fury in the late morning of May 18, 1980 (Photograph by Joseph Rosenbaum).
May 18, a Sunday, dawned bright and clear. At 7 a.m. 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 laser-beam 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 57 people, including volcanologist Johnston.
Aerial views of the volcano at the moment the summit collapse (see text) triggered the debris avalanche and
associated catastrophic eruption (Photographs selected from the copyrighted sequence taken by Keith and
Dorothy Stoffel). The tail of the plane can be seen in the upper right-hand corner of the lower picture, as the
Stoffels took a final backward look while escaping.
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 110 to 155 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.
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.
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 (Photograph by
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. Helens--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.
Border area of the lateral-blast zone. Dead trees of the "seared zone" (middle ground) stand between the flatteded
trees of the "tree-down zone" (foreground) and unaffected forest (upper right) (Photograph by Lyn Topinka in April
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:
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
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."
Seared zone, also called the "standing dead" 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.
Generalized map showing the lateral-blast zones.
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
The early form of the May 18 eruption plume, which was not photographed, probably resembled the
mushroom-shaped ash cloud of the July 22, 1980, eruption shown here (Photograph by James Vallence).
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
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.
The generalized map shows the distribution of ash fallout from the May 18 eruption.
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.
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.
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 flows--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
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.
Explosion pits were formed by "secondary" eruptions when the hot volcanic debris came into contact with water or
moist ground. This picture also shows an eruption in progress (lower center) (Photograph by Daniel Dzurisin).
Pyroclastic flows were first directly observed shortly after noon, although they probably began to form a short time
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° to 785°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
Volcanic debris flows--mobile mixtures of volcanic debris and water popularly called mudflows--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. Such flows are also called lahars, a term borrowed from
Indonesia, where volcanic eruptions have produced many such deposits.
Mudflows were observed as early as 8:50 a.m. 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 Rivers. 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 drainages--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.
Generalized geologic map showing the impact and deposits of the climactic eruption in the vicinity of the volcano.
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° to 91°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.
Mudflow-damaged house along the Toutle River. The height of the mudflow is shown by the "bathtub-ring" mudlines
seen on the tree trunks and the house itself (Photograph by Dwight Crandell).
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 in 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
eruptions. The times indicated are in hours, minutes, and seconds (Pacific Daylight Time).
These photographs were selected from the sequence taken by Gary Rosenquist (©copyright Gary Rosenquist
1980 ). ( These photo postcard strips are now available at Hoffstadt Bluffs Visitor Center after being out of print
until 2004's renewed eruptive activity.)
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.
FUN FACTS ABOUT Mount St. Helens
Schematic cross sections of Mount St. Helens showing
the three major blocks that collapsed to form the debris
avalanche (After USGS Professional Paper 1250).
Compare with photographs in "The Catastrophic First
A, The volcano in the early morning of May 18, 1980;
the bulging of the north flank is clearly shown by the
pre-1980 and pre-collapse profiles.
B and C, (within about 30 seconds after the collapse)
show the progressive development of the debris
avalanche and the beginning of both the lateral blast
and vertical eruption, as the cryptodome was exposed;
the Bulge block was the first to slide, followed by the
D, (about another 30 seconds later), by now the Summit
block had slid and the lateral blast had stopped; the
vertical eruption was now in full fury.
View up the North Fork Toutle River toward
Mount St. Helens (upper right) showing the
valley choked with the hummocky deposits of the
debris avalanche (Photograph by Austin Post).
The splintered and charred remains of a tree
removed in the direct blast zone. In this picture,
the direction of the blast was from right to left.
Tree trunk was originally about 2 feet in diameter
(Photograph by Robert Smith).
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
A strong, vertically directed explosion of ash
characteristic mushroom-shaped ash cloud.
Near the volcano, the swirling ash particles
Near the volcano, the swirling ash particles
in the atmosphere generated lightning, in
the atmosphere generated lightning, which
in turn started many forest fires. As which in
turn started many forest fires. As the
eruption roared on, the major part of the the
eruption roared on, the major part of the
ash cloud drifted downwind in an 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
(Left) The advancing ash cloud from Mount St. Helens, as seen from the ground in
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
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.
08:32:53.3 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.
08:32:47.0 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
08:33:03.7 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
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.
08:33:18.8 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.