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History - Mt. St. Helens
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 Mount
Garibaldi in British Columbia, Canada, to Lassen Peak in northern California. Geologists call Mount St. Helens a
composite volcano (or stratovolcano), a term for steepsided, 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. Before 1980, snow-capped, gracefully symmetrical
Mount St. Helens was known as the "Fujiyama of America." Mount St. Helens, other active Cascade volcanoes, and
those of Alaska form 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 variously 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 Cascades--Mounts Baker, Hood, and Rainier--for British naval officers.
Indians on the Cowlitz River watching an eruption of Mount St. Helens, as painted by Canadian artist Paul Kane
following a visit to the volcano in 1847 (Photograph courtesy of the Royal Ontario Museum).
Indian Legend of Mount St. Helens Eruption
Before Mt. St. Helens blew its top is was a beautifully symmetric rounded snow-capped mountain that stood between
two powerfully jagged peaks Mt. Hood ( which Indians called Wy'east) and Mt. Adams ( which Indians called Klickitat).
According to one Indian legend, the mountain was once a beautiful maiden, "Loowit". When two sons of the Great
Spirit "Sahale" fell in love with her, she could not choose between them. The two braves, Wyeast and Klickitat fought
over her, burying villages and forests in the process ( hurling rocks as they erupted?). Sahale was furious. He smote
the three lovers and erected a mighty mountain peak where each fell. Because Loowit was beautiful, her mountain
(Mount St. Helens) was a beautiful, symmetrical cone of dazzling white. Wyeast (Mount Hood) lifts his head in pride, but
Klickitat (Mount Adams) wept to see the beautiful maiden wrapped in snow, so he bends his head as he gazes on St.
Helens. This is one of many indian legends involving Mount St. Helens. See Bridge of the Gods also.
Witnessed Eruptions of Mount St. Helens
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 was active sporadically
during the three decades before 1831, including a major explosive eruption in 1800. 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.
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.
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-l4) 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 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.
The post-A.D. 1400 segment of the 50,000-year eruptive history of Mount St. Helens (after USGS Bulletin 1383-C).
Mount St. Helens is the youngest of the major Cascade volcanoes, in the sense that its visible cone was entirely
formed during the past 2,200 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 4,000 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 view to the north of the "two-tone" mountain--an appearance produced by prevailing easterly winds during the initial
activity of Mount St. Helens. Mount Rainier is visible in background (Photograph by C. Dan Miller).
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.
View of the "bulge" on the north face of Mount St. Helens, from a measurement site about 2 miles to the northeast
(Photograph by Peter Lipman). The drawing above the photograph illustrates, in a highy exaggerated fashion, the
nearly horizontal movement--about 85 feet in 20 days--of one of the measured points on the "bulge."
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 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.
Debris avalanche
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.
PRE-1980-HISTORY-PDF
lofty volcanic peaks that dominate the Cascade Range of the Pacific Northwest; the range extends from Mount
Garibaldi in British Columbia, Canada, to Lassen Peak in northern California. Geologists call Mount St. Helens a
composite volcano (or stratovolcano), a term for steepsided, 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. Before 1980, snow-capped, gracefully symmetrical
Mount St. Helens was known as the "Fujiyama of America." Mount St. Helens, other active Cascade volcanoes, and
those of Alaska form 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 variously 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 Cascades--Mounts Baker, Hood, and Rainier--for British naval officers.
Indians on the Cowlitz River watching an eruption of Mount St. Helens, as painted by Canadian artist Paul Kane
following a visit to the volcano in 1847 (Photograph courtesy of the Royal Ontario Museum).
Indian Legend of Mount St. Helens Eruption
Before Mt. St. Helens blew its top is was a beautifully symmetric rounded snow-capped mountain that stood between
two powerfully jagged peaks Mt. Hood ( which Indians called Wy'east) and Mt. Adams ( which Indians called Klickitat).
According to one Indian legend, the mountain was once a beautiful maiden, "Loowit". When two sons of the Great
Spirit "Sahale" fell in love with her, she could not choose between them. The two braves, Wyeast and Klickitat fought
over her, burying villages and forests in the process ( hurling rocks as they erupted?). Sahale was furious. He smote
the three lovers and erected a mighty mountain peak where each fell. Because Loowit was beautiful, her mountain
(Mount St. Helens) was a beautiful, symmetrical cone of dazzling white. Wyeast (Mount Hood) lifts his head in pride, but
Klickitat (Mount Adams) wept to see the beautiful maiden wrapped in snow, so he bends his head as he gazes on St.
Helens. This is one of many indian legends involving Mount St. Helens. See Bridge of the Gods also.
Witnessed Eruptions of Mount St. Helens
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 was active sporadically
during the three decades before 1831, including a major explosive eruption in 1800. 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.
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.
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-l4) 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 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.
The post-A.D. 1400 segment of the 50,000-year eruptive history of Mount St. Helens (after USGS Bulletin 1383-C).
Mount St. Helens is the youngest of the major Cascade volcanoes, in the sense that its visible cone was entirely
formed during the past 2,200 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 4,000 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 view to the north of the "two-tone" mountain--an appearance produced by prevailing easterly winds during the initial
activity of Mount St. Helens. Mount Rainier is visible in background (Photograph by C. Dan Miller).
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.
View of the "bulge" on the north face of Mount St. Helens, from a measurement site about 2 miles to the northeast
(Photograph by Peter Lipman). The drawing above the photograph illustrates, in a highy exaggerated fashion, the
nearly horizontal movement--about 85 feet in 20 days--of one of the measured points on the "bulge."
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 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.
Debris avalanche
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.
PRE-1980-HISTORY-PDF
Schematic cross sections of Mount St. Helens showing
the cryptodome of magma that produced the bulge and
the three major blocks that collapsed to form the debris
avalanche (After USGS Professional Paper 1250).
Compare with photographs in "The Catastrophic First
Minute."
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
Graben block.
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.
the cryptodome of magma that produced the bulge and
the three major blocks that collapsed to form the debris
avalanche (After USGS Professional Paper 1250).
Compare with photographs in "The Catastrophic First
Minute."
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
Graben block.
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).
Mount St. Helens (upper right) showing the
valley choked with the hummocky deposits of the
debris avalanche (Photograph by Austin Post).