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| Mount St Helens |
Credit & Thanks to Totally Explained for the use of their Public Domain Resources
Mount St. Helens' an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest
region of the United States. It's south of Seattle and northeast of Portland, Oregon. Mount St. Helens takes its
English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey
of the area in the late 18th century. The volcano's located in the Cascade Range and is part of the Cascade
Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano's well
known for its ash explosions and pyroclastic flows.
Mount St. Helens' most famous for its catastrophic eruption on May 18, 1980, at 8:32am PDT which was the
deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people
were killed; 250 homes, 47 bridges, of railways, and of highway were destroyed. The eruption caused a massive
debris avalanche, reducing the elevation of the mountain's summit from to and replacing it with a wide horseshoe-
shaped crater. The debris avalanche was up to in volume. The Mount St. Helens National Volcanic Monument was
created to preserve the volcano and allow for its aftermath to be scientifically studied.
As with most other volcanoes in the Cascade Range, Mount St. Helens' a large eruptive cone consisting of lava
rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite
through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous
summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.
Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain
collapsed in a massive rock debris avalanche. Nearly of forest was blown down or buried beneath volcanic
deposits. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted
downwind, turning day into night as dark, Grey ash fell over eastern Washington and beyond. The eruption lasted
9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.
Geographic setting and description
General
Mount St. Helens' west of Mount Adams, in the western part of the Cascade Range. These "sister and brother"
volcanic mountains are approximately from Mount Rainier, the highest of Cascade volcanoes. Mount Hood, the
nearest major volcanic peak in Oregon,'s southeast of Mount St. Helens.
Mount St. Helens' geologically young compared to the other major Cascade volcanoes. It formed only within the
past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago. The volcano's considered
the most active in the Cascades within the Holocene epoch (the last 10,000 or so years).
Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington. It stood out prominently
from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone,
earning it the nickname "Fuji-san of America" ("Mount Fuji of America"). The peak rose more than above its base,
where the lower flanks merge with adjacent ridges. The mountain's across at its base, which is at an altitude of
on the northeastern side and elsewhere. At the pre-eruption tree line, the width of the cone was. Streams that
originate on the volcano enter three main river systems: the Toutle River on the north and northwest, the Kalama
River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow.
The average annual rainfall's, and the snow pack on the mountain's upper slopes can reach .
With the recent volcanic activity starting in 2004, the glacier lobes were pushed aside and upward by the growth of
new volcanic domes. The surface of the glacier, once mostly without crevasses, turned into a chaotic jumble of
icefalls heavily criss-crossed with crevasses and seracs caused by movement of the crater floor. The new domes
have almost separated the Crater Glacier into an eastern and western lobe. Despite the volcanic activity, the
termini of the glacier have still advanced, with a slight advance on the western lobe and a more considerable
advance on the more shaded eastern lobe. Due to the advance, two lobes of the glacier joined together in late-
May 2008 and thus the glacier completely surrounds the lava domes. In addition, since 2004, new glaciers have
formed on the crater wall above Crater Glacier feeding rock and ice onto its surface below; there are two rock
glaciers to the north of the eastern lobe of Crater Glacier.
Human history
Importance to Native Americans
Traces of ancient campsites have been found in the Gifford Pinchot National Forest, which surrounds the
monument. Dating of these sites reveals that people have lived in this area for at least 6,500 years. Throughout
human history, Mount St. Helens eruptions have had a dramatic effect on the lives of local inhabitants. Work by
archaeologists has shown that a massive eruption 3,500 years ago buried native settlements with a thick layer of
pumice. As a result, people abandoned the area for nearly 2,000 years.
They came upon an area that's now called The Dalles and thought they'd never seen a land so beautiful. The
sons quarrelled over the land, so to solve the dispute their father shot two arrows from his mighty bow — one to
the north and the other to the south. Pahto followed the arrow to the north and settled there while Wy'east did the
same for the arrow to the south. Saghalie then built Tanmahawis, the Bridge of the Gods, so his family could meet
periodically.
For punishment, Saghalie struck down each of the lovers and transformed them into great mountains where they
fell. Wy'east, with his head lifted in pride, became the volcano known today as Mount Hood. Pahto, with his head
bent toward his fallen love, was turned into Mount Adams. The fair Loowit became Mount St. Helens, known to the
Klickitats as Louwala-Clough, which means "smoking or fire mountain" in their language (the Sahaptin called the
mountain Loowit).
Exploration by Europeans
Royal Navy Commander George Vancouver and the officers of HMS Discovery made the Europeans' first recorded
sighting of Mount St. Helens on May 19, 1792, while surveying the northern Pacific Ocean coast. Vancouver named
the mountain for British diplomat Alleyne Fitzherbert, 1st Baron St Helens on October 20, 1792, They did however
report the presence of quicksand and clogged channel conditions at the mouth of the Sandy River near Portland,
suggesting an eruption by Mount Hood sometime in the previous decades.
In 1829 Hall J. Kelley led a campaign to rename the Cascade Range as the President's Range and also to
rename each major Cascade mountain after a former President of the United States. In his scheme Mount St.
Helens was to be renamed Mount Washington.
European settlement and use of the area
The area's first non-aboriginal inhabitants were European fur traders and trappers. Most of these men worked for
the fur trading enterprise of the British-owned Hudson's Bay Company. In the early 1890s, Ole' Peterson set up
housekeeping at Cougar Flats, on the Upper Lewis River. He sent an account to the Edinburgh New
Philosophical Journal, which published his letter in January 1836. James Dwight Dana of Yale University, while
sailing with the United States Exploring Expedition, saw the quiescent peak from off the mouth of the Columbia
River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base.
In late fall or early winter of 1842, nearby settlers and missionaries witnessed the so-called "Great Eruption." This
small-volume outburst created large ash clouds, and mild explosions followed for 15 years. The eruptions of this
period were likely phreatic (steam explosions). Josiah Parrish in Champoeg, Oregon witnessed Mount St. Helens
in eruption on November 22, 1842. Ash from this eruption may have reached The Dalles, Oregon, 48 miles (80
km) southeast of the volcano. British lieutenant Henry J. Warre sketched the eruption in 1845, and two years later
Canadian painter Paul Kane created watercolours of the gently smoking mountain. Warre's work showed erupting
material from a vent about a third of the way down from the summit on the mountain's west or northwest side
(possibly at Goat Rocks), and one of Kane's field sketches shows smoke emanating from about the same
location.
On April 17, 1857, the Republican, a Steilacoom, Washington newspaper, reported that "Mount St. Helens, or
some other mount to the southward,'s seen ... to be in a state of eruption". The lack of a significant ash layer
associated with this event indicates that it was a small eruption. This was the first reported volcanic activity since
1854. The newly exposed hot and pressurised rock in the volcano responded by producing the largest historic
volcanic eruption in the 48 contiguous U.S. states.
Among the victims of the 1980 eruption was 30-year-old volcanologist David A. Johnston, who was stationed on
the nearby Coldwater Ridge. Moments before his position was hit by the hot ash cloud, Johnston uttered his
famous last words: "Vancouver! Vancouver! This is it!" Johnston's body was never found. U.S. President Jimmy
Carter surveyed the damage and said "Someone said this area looked like a moonscape. But the moon looks
more like a golf course compared to what's up there." A film crew, led by Seattle filmmaker Otto Seiber, was
dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, spun in
circles and they quickly became lost. A second eruption occurred on May 25, but the crew survived and was
rescued two days later by National Guard helicopter pilots. Their film, The Eruption of Mount St. Helens, later
became a popular documentary.
Protection and later history
In 1982, President Ronald Reagan and the U.S. Congress established the Mount St. Helens National Volcanic
Monument, a area around the mountain and within the Gifford Pinchot National Forest.
Following the 1980 eruption, the area was left to gradually return to its natural state. In 1987, the U.S. Forest
Service reopened the mountain to climbing. It remained open until 2004 when renewed activity caused the closure
of the area around the mountain (see Geology section for more detail).
Most notable was the closure of the Monitor Ridge trail, which previously let up to 100 permitted hikers per day
climb to the summit. However, on July 21, 2006, the mountain was again opened to climbers.
Geologic history
Ancestral stages of eruptive activity
The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage" (around 40–35,000 years
ago), the "Cougar Stage" (ca. 20–18,000 years ago), and the "Swift Creek Stage" (roughly 13–8,000 years ago).
The modern period, since about 2500 BCE,'s called the "Spirit Lake Stage." Collectively, the pre-Spirit Lake stages
are known as the "ancestral stages." The ancestral and modern stages differ primarily in the composition of the
erupted lavas; ancestral lavas consisted of a characteristic mixture of dacite and andesite, while modern lava's
very diverse (ranging from olivine basalt to andesite and dacite).
St. Helens started its growth in the Pleistocene 37,600 years ago, during the Ape Canyon stage, with dacite and
andesite eruptions of hot pumice and ash. All told there may have been up to of material ejected in this cycle. The
pre-1980 summit cone started to form during the Castle Creek period. Significant lava flows in addition to the
previously much more common fragmented and pulverised lavas and rocks (tephra) distinguished this period.
Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 BCE
that travelled all the way into the Lewis and Kalama river valleys. In 1482, another large eruption rivalling the 1980
eruption in volume's known to have occurred. Large parts of the dome's sides broke away and mantled parts of
the volcano's cone with talus. Lateral explosions excavated a notch in the southeast crater wall. St. Helens
reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended,
about 1647. By the end of April, the north side of the mountain started to bulge. With little warning, a second
earthquake of magnitude 5.1 May 18 triggered a massive collapse of the north face of the mountain. It was the
largest known debris avalanche in recorded history. The magma inside of St. Helens burst forth into a large-scale
pyroclastic flow that flattened vegetation and buildings over . Over 1.5 million metric tons of sulphur dioxide were
released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a five (a Plinian
eruption).
The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic
mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber
camps. A total of material was transported south into the Columbia River by the mudflows.
For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27 km)
above sea level. The plume moved eastward at an average speed of with ash reaching Idaho by noon. Ashes
from the eruption were found collecting on top of cars and roofs next morning, as far as the city of Edmonton in
Alberta, Canada. By about 5:30 p.m. on May 18, the vertical ash column declined in stature, and less severe
outbursts continued through the night and for the next several days. The St. Helens May 18 eruption released 24
megatons of thermal energy; it ejected more than of material.
2004 to 2008 activity
Magma reached the surface of the volcano about October 11, 2004, resulting in the building of a new lava dome
on the existing dome's south side. This new dome continued to grow throughout 2005 and into 2006. Several
transient features were observed, such as the "whaleback," which comprised long shafts of solidified magma
being extruded by the pressure of magma beneath. These features were fragile and broke down soon after they
were formed. On July 2, 2005, the tip of the whaleback broke off, causing a rockfall that sent ash and dust several
hundred meters into the air.
Mount St. Helens showed significant activity on March 8, 2005, when a plume of steam and ash emerged—visible
from Seattle. This relatively minor eruption was a release of pressure consistent with ongoing dome building. The
release was accompanied by a magnitude 2.5 earthquake.
Another feature to emerge from the dome was called the "fin" or "slab." Approximately half the size of a football
field, the large, cooled volcanic rock was being forced upward as quickly as per day. In mid-June 2006, the slab
was crumbling in frequent rockfalls, although it was still being extruded. The height of the dome was, still below
the height reached in July 2005 when the whaleback collapsed.
On October 22, 2006, at 3:13 p.m. PST, a magnitude 3.5 earthquake broke loose Spine 7. The collapse and
avalanche of the lava dome sent an ash plume over the western rim of the crater; the ash plume then rapidly
dissipated.
On December 19, 2006, a large white plume of condensing steam was observed, leading some media people to
assume there'd been a small eruption. However, the Cascades Volcano Observatory of the USGS didn't mention
any significant ash plume. The volcano was in continuous eruption from October 2004, but this eruption in large
part, consisted of a gradual extrusion of lava forming a dome in the crater.
On January 16, 2008, steam began seeping from a fracture on top of the lava dome. Associated seismic activity
was the most noteworthy since 2004. Scientists suspended activities in the crater and the mountain flanks, but
the risk of a major eruption was deemed low. By the end of January, the eruption paused; no more lava was being
extruded from the lava dome. On July 10, 2008 it was determined that the eruption had ended after more than six
months of no volcanic activity.
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Mt St Helens
External Links & Resources
Credit & Thanks to the US Geological Survey for the use of their Public Domain Resources
Credit & Thanks Also to Totally Explained
Mount St. Helens' an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest
region of the United States. It's south of Seattle and northeast of Portland, Oregon. Mount St. Helens takes its
English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey
of the area in the late 18th century. The volcano's located in the Cascade Range and is part of the Cascade
Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano's well
known for its ash explosions and pyroclastic flows.
Mount St. Helens' most famous for its catastrophic eruption on May 18, 1980, at 8:32am PDT which was the
deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people
were killed; 250 homes, 47 bridges, of railways, and of highway were destroyed. The eruption caused a massive
debris avalanche, reducing the elevation of the mountain's summit from to and replacing it with a wide horseshoe-
shaped crater. The debris avalanche was up to in volume. The Mount St. Helens National Volcanic Monument was
created to preserve the volcano and allow for its aftermath to be scientifically studied.
As with most other volcanoes in the Cascade Range, Mount St. Helens' a large eruptive cone consisting of lava
rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite
through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous
summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.
Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain
collapsed in a massive rock debris avalanche. Nearly of forest was blown down or buried beneath volcanic
deposits. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted
downwind, turning day into night as dark, Grey ash fell over eastern Washington and beyond. The eruption lasted
9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.
Geographic setting and description
General
Mount St. Helens' west of Mount Adams, in the western part of the Cascade Range. These "sister and brother"
volcanic mountains are approximately from Mount Rainier, the highest of Cascade volcanoes. Mount Hood, the
nearest major volcanic peak in Oregon,'s southeast of Mount St. Helens.
Mount St. Helens' geologically young compared to the other major Cascade volcanoes. It formed only within the
past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago. The volcano's considered
the most active in the Cascades within the Holocene epoch (the last 10,000 or so years).
Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington. It stood out prominently
from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone,
earning it the nickname "Fuji-san of America" ("Mount Fuji of America"). The peak rose more than above its base,
where the lower flanks merge with adjacent ridges. The mountain's across at its base, which is at an altitude of
on the northeastern side and elsewhere. At the pre-eruption tree line, the width of the cone was. Streams that
originate on the volcano enter three main river systems: the Toutle River on the north and northwest, the Kalama
River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow.
The average annual rainfall's, and the snow pack on the mountain's upper slopes can reach .
With the recent volcanic activity starting in 2004, the glacier lobes were pushed aside and upward by the growth of
new volcanic domes. The surface of the glacier, once mostly without crevasses, turned into a chaotic jumble of
icefalls heavily criss-crossed with crevasses and seracs caused by movement of the crater floor. The new domes
have almost separated the Crater Glacier into an eastern and western lobe. Despite the volcanic activity, the
termini of the glacier have still advanced, with a slight advance on the western lobe and a more considerable
advance on the more shaded eastern lobe. Due to the advance, two lobes of the glacier joined together in late-
May 2008 and thus the glacier completely surrounds the lava domes. In addition, since 2004, new glaciers have
formed on the crater wall above Crater Glacier feeding rock and ice onto its surface below; there are two rock
glaciers to the north of the eastern lobe of Crater Glacier.
Human history
Importance to Native Americans
Traces of ancient campsites have been found in the Gifford Pinchot National Forest, which surrounds the
monument. Dating of these sites reveals that people have lived in this area for at least 6,500 years. Throughout
human history, Mount St. Helens eruptions have had a dramatic effect on the lives of local inhabitants. Work by
archaeologists has shown that a massive eruption 3,500 years ago buried native settlements with a thick layer of
pumice. As a result, people abandoned the area for nearly 2,000 years.
They came upon an area that's now called The Dalles and thought they'd never seen a land so beautiful. The
sons quarrelled over the land, so to solve the dispute their father shot two arrows from his mighty bow — one to
the north and the other to the south. Pahto followed the arrow to the north and settled there while Wy'east did the
same for the arrow to the south. Saghalie then built Tanmahawis, the Bridge of the Gods, so his family could meet
periodically.
For punishment, Saghalie struck down each of the lovers and transformed them into great mountains where they
fell. Wy'east, with his head lifted in pride, became the volcano known today as Mount Hood. Pahto, with his head
bent toward his fallen love, was turned into Mount Adams. The fair Loowit became Mount St. Helens, known to the
Klickitats as Louwala-Clough, which means "smoking or fire mountain" in their language (the Sahaptin called the
mountain Loowit).
Exploration by Europeans
Royal Navy Commander George Vancouver and the officers of HMS Discovery made the Europeans' first recorded
sighting of Mount St. Helens on May 19, 1792, while surveying the northern Pacific Ocean coast. Vancouver named
the mountain for British diplomat Alleyne Fitzherbert, 1st Baron St Helens on October 20, 1792, They did however
report the presence of quicksand and clogged channel conditions at the mouth of the Sandy River near Portland,
suggesting an eruption by Mount Hood sometime in the previous decades.
In 1829 Hall J. Kelley led a campaign to rename the Cascade Range as the President's Range and also to
rename each major Cascade mountain after a former President of the United States. In his scheme Mount St.
Helens was to be renamed Mount Washington.
European settlement and use of the area
The area's first non-aboriginal inhabitants were European fur traders and trappers. Most of these men worked for
the fur trading enterprise of the British-owned Hudson's Bay Company. In the early 1890s, Ole' Peterson set up
housekeeping at Cougar Flats, on the Upper Lewis River. He sent an account to the Edinburgh New
Philosophical Journal, which published his letter in January 1836. James Dwight Dana of Yale University, while
sailing with the United States Exploring Expedition, saw the quiescent peak from off the mouth of the Columbia
River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base.
In late fall or early winter of 1842, nearby settlers and missionaries witnessed the so-called "Great Eruption." This
small-volume outburst created large ash clouds, and mild explosions followed for 15 years. The eruptions of this
period were likely phreatic (steam explosions). Josiah Parrish in Champoeg, Oregon witnessed Mount St. Helens
in eruption on November 22, 1842. Ash from this eruption may have reached The Dalles, Oregon, 48 miles (80
km) southeast of the volcano. British lieutenant Henry J. Warre sketched the eruption in 1845, and two years later
Canadian painter Paul Kane created watercolours of the gently smoking mountain. Warre's work showed erupting
material from a vent about a third of the way down from the summit on the mountain's west or northwest side
(possibly at Goat Rocks), and one of Kane's field sketches shows smoke emanating from about the same
location.
On April 17, 1857, the Republican, a Steilacoom, Washington newspaper, reported that "Mount St. Helens, or
some other mount to the southward,'s seen ... to be in a state of eruption". The lack of a significant ash layer
associated with this event indicates that it was a small eruption. This was the first reported volcanic activity since
1854. The newly exposed hot and pressurised rock in the volcano responded by producing the largest historic
volcanic eruption in the 48 contiguous U.S. states.
Among the victims of the 1980 eruption was 30-year-old volcanologist David A. Johnston, who was stationed on
the nearby Coldwater Ridge. Moments before his position was hit by the hot ash cloud, Johnston uttered his
famous last words: "Vancouver! Vancouver! This is it!" Johnston's body was never found. U.S. President Jimmy
Carter surveyed the damage and said "Someone said this area looked like a moonscape. But the moon looks
more like a golf course compared to what's up there." A film crew, led by Seattle filmmaker Otto Seiber, was
dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, spun in
circles and they quickly became lost. A second eruption occurred on May 25, but the crew survived and was
rescued two days later by National Guard helicopter pilots. Their film, The Eruption of Mount St. Helens, later
became a popular documentary.
Protection and later history
In 1982, President Ronald Reagan and the U.S. Congress established the Mount St. Helens National Volcanic
Monument, a area around the mountain and within the Gifford Pinchot National Forest.
Following the 1980 eruption, the area was left to gradually return to its natural state. In 1987, the U.S. Forest
Service reopened the mountain to climbing. It remained open until 2004 when renewed activity caused the closure
of the area around the mountain (see Geology section for more detail).
Most notable was the closure of the Monitor Ridge trail, which previously let up to 100 permitted hikers per day
climb to the summit. However, on July 21, 2006, the mountain was again opened to climbers.
Geologic history
Ancestral stages of eruptive activity
The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage" (around 40–35,000 years
ago), the "Cougar Stage" (ca. 20–18,000 years ago), and the "Swift Creek Stage" (roughly 13–8,000 years ago).
The modern period, since about 2500 BCE,'s called the "Spirit Lake Stage." Collectively, the pre-Spirit Lake stages
are known as the "ancestral stages." The ancestral and modern stages differ primarily in the composition of the
erupted lavas; ancestral lavas consisted of a characteristic mixture of dacite and andesite, while modern lava's
very diverse (ranging from olivine basalt to andesite and dacite).
St. Helens started its growth in the Pleistocene 37,600 years ago, during the Ape Canyon stage, with dacite and
andesite eruptions of hot pumice and ash. All told there may have been up to of material ejected in this cycle. The
pre-1980 summit cone started to form during the Castle Creek period. Significant lava flows in addition to the
previously much more common fragmented and pulverised lavas and rocks (tephra) distinguished this period.
Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 BCE
that travelled all the way into the Lewis and Kalama river valleys. In 1482, another large eruption rivalling the 1980
eruption in volume's known to have occurred. Large parts of the dome's sides broke away and mantled parts of
the volcano's cone with talus. Lateral explosions excavated a notch in the southeast crater wall. St. Helens
reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended,
about 1647. By the end of April, the north side of the mountain started to bulge. With little warning, a second
earthquake of magnitude 5.1 May 18 triggered a massive collapse of the north face of the mountain. It was the
largest known debris avalanche in recorded history. The magma inside of St. Helens burst forth into a large-scale
pyroclastic flow that flattened vegetation and buildings over . Over 1.5 million metric tons of sulphur dioxide were
released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a five (a Plinian
eruption).
The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic
mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber
camps. A total of material was transported south into the Columbia River by the mudflows.
For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27 km)
above sea level. The plume moved eastward at an average speed of with ash reaching Idaho by noon. Ashes
from the eruption were found collecting on top of cars and roofs next morning, as far as the city of Edmonton in
Alberta, Canada. By about 5:30 p.m. on May 18, the vertical ash column declined in stature, and less severe
outbursts continued through the night and for the next several days. The St. Helens May 18 eruption released 24
megatons of thermal energy; it ejected more than of material.
2004 to 2008 activity
Magma reached the surface of the volcano about October 11, 2004, resulting in the building of a new lava dome
on the existing dome's south side. This new dome continued to grow throughout 2005 and into 2006. Several
transient features were observed, such as the "whaleback," which comprised long shafts of solidified magma
being extruded by the pressure of magma beneath. These features were fragile and broke down soon after they
were formed. On July 2, 2005, the tip of the whaleback broke off, causing a rockfall that sent ash and dust several
hundred meters into the air.
Mount St. Helens showed significant activity on March 8, 2005, when a plume of steam and ash emerged—visible
from Seattle. This relatively minor eruption was a release of pressure consistent with ongoing dome building. The
release was accompanied by a magnitude 2.5 earthquake.
Another feature to emerge from the dome was called the "fin" or "slab." Approximately half the size of a football
field, the large, cooled volcanic rock was being forced upward as quickly as per day. In mid-June 2006, the slab
was crumbling in frequent rockfalls, although it was still being extruded. The height of the dome was, still below
the height reached in July 2005 when the whaleback collapsed.
On October 22, 2006, at 3:13 p.m. PST, a magnitude 3.5 earthquake broke loose Spine 7. The collapse and
avalanche of the lava dome sent an ash plume over the western rim of the crater; the ash plume then rapidly
dissipated.
On December 19, 2006, a large white plume of condensing steam was observed, leading some media people to
assume there'd been a small eruption. However, the Cascades Volcano Observatory of the USGS didn't mention
any significant ash plume. The volcano was in continuous eruption from October 2004, but this eruption in large
part, consisted of a gradual extrusion of lava forming a dome in the crater.
On January 16, 2008, steam began seeping from a fracture on top of the lava dome. Associated seismic activity
was the most noteworthy since 2004. Scientists suspended activities in the crater and the mountain flanks, but
the risk of a major eruption was deemed low. By the end of January, the eruption paused; no more lava was being
extruded from the lava dome. On July 10, 2008 it was determined that the eruption had ended after more than six
months of no volcanic activity.
Student Activities & Crafts
Volcano Hazards
Volcanoes Main
Volcano Printables
Mt St Helens
External Links & Resources
Credit & Thanks to the US Geological Survey for the use of their Public Domain Resources
Credit & Thanks Also to Totally Explained
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