Information Resource Center
History - Mt. St. Helens
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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.

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
Daniel Dzurisin).

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.

    Even heard in another Country:  Don Botten in
    Victoria B.C. said, "I heard it and thought it was the
    navy, firing ships guns, then I realized it was Mt. St.
    Helens.  When I phoned my father, Bill Botten, who
    lived on the waterfront, he told me that as he
    pushed open his front door, the shock wave
    pushed it back in his face.  I have often heard
    reports of the power of the mountain, but usually
    from the perspective of Washington state.  The
    sound waves carried much farther, between Puget
    Sound and Juan de Fuca Strait, they built
    considerable force."  This update recalled by Don
    during 30th anniversary, May 18, 2010

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 zone.
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.

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 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
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.

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 Forest Learning Center after being out of print. )

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 57 lives. 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' vicitims 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. However, many
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.

Panoramic view of Mounts St. Helens from Mount Margaret, about 9 miles north
(Photograph [montage] by Maleah Taubman in August 1979).

Mount St. Helens viewed from the same point after the May 18, 1980, eruption
(Photograph [montage] by James Hughes in 1982).

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.

Stand of timber in the "tree-down" zone north of Mount St. Helens devastated by the lateral blast. The downed trees were salvaged as
quickly as possible before the wood began to rot. Note the two people (circled) in lower right (Photograph by Lyn Topinka).

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.

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
winds followed other paths determined by
complex wind directions.
(Left) The advancing ash cloud from Mount St. Helens, as seen from the ground in
eastern Washington.

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
(08:32:21.0 PDT).
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.
Front-end loader removing
ash from Mount St. Helens
as part of the massive
cleanup effort in eastern
Washington (Copyrighted
photograph by Daryl
Gusey, credits given ).

Right: Patrol cars in Moses
Lake, WA over 300 miles
from the volcano.  
(Worldwide photo credit )
Patrol cars in Moses Lake, WA - ash busters