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1980 eruption of Mount St. Helens

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The 1980 eruption of Mount St. Helens ( VEI = 5, 1.2 km³ of material
   erupted) is the most significant volcanic eruption to occur in the
   lower 48 U.S. states in recorded history, exceeding the destructive
   power and volume of material released by the 1915 eruption of
   California's Lassen Peak (the 1912 eruption of Novarupta in Alaska was
   the most powerful historic eruption in the U.S., although Alaska was
   not a U.S. state at the time). The eruption was preceded by a
   two-month-long series of earthquakes and steam-venting episodes, caused
   by an injection of magma at shallow depth below the mountain which
   created a huge bulge and a fracture system on Mount St. Helens' north
   slope. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire
   weakened north face to slide away, suddenly exposing the partly molten,
   gas- and steam-rich rock in the volcano to lower pressure. The rock
   responded by exploding into a super-heated mix of pulverized lava and
   older rock that sped toward Spirit Lake so fast that it quickly passed
   the avalanching north face.

   A volcanic ash column rose high into the atmosphere and deposited ash
   in eleven U.S. states. At the same time, snow, ice, and several entire
   glaciers on the mountain melted, forming a series of large lahars
   (volcanic mudslides) that reached as far as the Columbia River. Less
   severe outbursts continued into the next day only to be followed by
   other large but not as destructive eruptions later in 1980. By the time
   the ash settled, 57 people (including innkeeper Harry Truman and
   geologist David A. Johnston) and thousands of animals were dead,
   hundreds of square miles (mi²) reduced to wasteland, over a billion
   U.S. dollars in damage had occurred, and the once-graceful face of
   Mount St. Helens was scarred with a huge crater open to the north. The
   area was later preserved, as it was, in the Mount St. Helens National
   Volcanic Monument.
   Mount St Helens from Monitor Ridge showing the cone of devastation, the
   huge crater open to the north, and the post-eruption lava dome inside
   it. The small photo on the left was taken from Spirit Lake before the
   eruption and the small photo on the right was taken after the eruption
   from approximately the same place. Spirit Lake can also be seen in the
   larger image, as well as two other Cascade volcanos.
   Enlarge
   Mount St Helens from Monitor Ridge showing the cone of devastation, the
   huge crater open to the north, and the post-eruption lava dome inside
   it. The small photo on the left was taken from Spirit Lake before the
   eruption and the small photo on the right was taken after the eruption
   from approximately the same place. Spirit Lake can also be seen in the
   larger image, as well as two other Cascade volcanos.

Buildup to disaster

   Mount St. Helens, May 17, 1980.
   Enlarge
   Mount St. Helens, May 17, 1980.

   Several small earthquakes beginning as early as March 16, 1980,
   indicated that magma may have been moving below the volcano. Then on
   March 20 at 3:47 p.m. Pacific Standard Time (all times will be in PST),
   a shallow Richter magnitude 4.2 earthquake, centered below the
   mountain's north flank, definitely signaled the volcano's violent
   return from 123 years of hibernation. A gradually building earthquake
   swarm saturated area seismographs and started to climax at about noon
   on March 25, reaching peak levels in the next two days (a total of 174
   shocks of 2.6 or greater were recorded during those two days). Shocks
   of 3.2 or greater occurred at a slightly increasing rate through April
   and May with five earthquakes of magnitude 4 or above per day in early
   April, and 8 per day the week before May 18. Initially there was no
   direct sign of eruption, but small earthquake-induced avalanches of
   snow and ice were reported from aerial observations.

   Then at 12:36 p.m. on March 27, at least one but possibly two nearly
   simultaneous phreatic eruptions (exploding groundwater-derived steam)
   ejected smashed rock from within the old summit crater, excavating a
   new crater 250 feet (76 m) wide and sending an ash column about 6,000
   feet (1800 m) into the air. Also by this date, a 16,000-foot-long (4900
   m), east-trending fracture system had developed across the summit area.
   This was followed by more earthquake storms and a series of steam
   explosions that sent ash 10,000 to 11,000 feet (3050 to 3350 m) above
   their vent. Most of this ash fell within 3 to 12 miles (5 to 19 km)
   from its vent but some was carried as far as 150 miles (240 km) south
   to Bend, Oregon, and 285 miles (459 km) east to Spokane, Washington.

   A second, new crater and a blue flame were observed on March 29. The
   flame danced to and from the two craters and was probably created by
   burning gases. Static electricity generated from ash clouds rolling
   down the mountain sent out lightning bolts that were up to two miles (3
   km) long. Ninety-three separate outbursts were reported on March 30,
   and harmonic tremors were detected on April 3, alarming geologists and
   prompting the governor to declare a state of emergency.
   USGS photo showing a pre-avalanche eruption on April 10.
   Enlarge
   USGS photo showing a pre-avalanche eruption on April 10.

   By April 8 the two craters merged, eventually creating one that was
   1,700 to 850 feet (520 to 260 m) wide. A USGS team determined in the
   last week of April that a 1.5-mile-diameter (2.4 km) section of St.
   Helens' north face was displaced out at least 270 feet (82 m). For the
   rest of April and early May this bulge grew 5 to 6 feet per day (1.5 to
   1.8 m), and by mid-May it extended more than 400 feet (120 m) north. As
   the bulge moved northward, the summit area behind it progressively
   sank, forming a complex, down-dropped block called a graben. Geologists
   announced on April 30 that sliding of the bulge area was the greatest
   immediate danger and that such a landslide may spark an eruption. These
   changes in the volcano's shape were related to the overall deformation
   that increased the volume of the mountain by 0.03 cubic miles
   (125,000,000 m³) 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 lava dome exposed at the surface.
   Photo showing the bulge on April 27.
   Enlarge
   Photo showing the bulge on April 27.

   On May 7, eruptions similar to those in March and April resumed, and
   over the next several days the bulge grew to huge proportions. All
   activity to this point was confined to the 350-year-old summit dome and
   did not involve any new magma. A total of about 10,000 earthquakes were
   recorded prior to the May 18 event, with most concentrated in a small
   zone less than 1.6 miles (2.6 km) directly below the bulge. Visible
   eruptions ceased on May 16, reducing public interest and consequently
   the number of spectators in the area. Mounting public pressure then
   forced officials to allow a limited number of people to enter the
   danger zone on May 17 to gather whatever property they could carry.
   Another trip was scheduled for 10 a.m. the next morning. Since that was
   Sunday, more than 300 loggers would not be working in the area. By the
   time of the climactic eruption, an estimated 0.11 cubic kilometers of
   dacite magma had intruded into the volcano. The rising magma forced the
   volcano's north flank outward nearly 500 feet (150 m) and heated the
   volcano's groundwater system, causing many steam-driven explosions
   (phreatic eruptions).

North face slides away

   Sequence of events on May 18.
   Enlarge
   Sequence of events on May 18.

   At 7 a.m. on May 18, USGS volcanologist David A. Johnston, who had
   Saturday night duty at an observation post about 6 miles (10 km) north
   of the mountain, radioed in the results of some laser-beam measurements
   he had made moments earlier. Mount St. Helens' activity that day did
   not show any change from the pattern of the preceding month. The rate
   of bulge movement, sulfur-dioxide gas emission, and ground temperature
   readings did not reveal any unusual changes that might indicate a
   catastrophic eruption.
   North Fork Toutle River valley filled with landslide deposits.
   Enlarge
   North Fork Toutle River valley filled with landslide deposits.

   Without warning at 8:32 a.m., a magnitude 5.1 earthquake centered
   directly below the north slope triggered that part of the mountain to
   slide, approximately 7–20 seconds (about 10 seconds seems most
   reasonable) after the shock. One of the largest landslides in recorded
   history, the slide traveled at 110 to 155 miles per hour (175 to 250
   km/h) and moved across Spirit Lake's west arm; part of it hit a
   1,150-foot-high (350 m) ridge about 6 miles (9.5 km) north. Some of the
   slide spilled over the ridge, but most of it moved 13 miles (21 km)
   down the North Fork Toutle River, filling its valley up to 600 feet
   (180 m) deep with avalanche debris. An area of about 24 mile² (62 km²)
   was covered, and the total volume of the deposit was about 0.7 mile³
   (2.9 km³).

   Most of St. Helens' former north side became a rubble deposit 17 miles
   (27 km) long, averaging 150 feet (46 m) thick; the slide was thickest
   at one mile (1.6 km) below Spirit Lake and thinnest at its western
   margin. All the water in Spirit Lake was temporarily displaced by the
   landslide, sending 600-foot-high (180 m) waves crashing into a ridge
   north of the lake and adding 295 feet (90 m) of new avalanche debris
   above the old lakebed, raising its surface level by about 200 feet (60
   m). As the water moved back into its basin, it pulled thousands of
   trees felled by a super-heated wall of volcanic gas and searing ash and
   rock that overtook the landslide seconds before (see below).

Pyroclastic flows

Initial lateral blast

   Computer graphic showing the May 18 landslide (green) being overtaken
   by the initial pyroclastic flow (red).
   Enlarge
   Computer graphic showing the May 18 landslide (green) being overtaken
   by the initial pyroclastic flow (red).

   The landslide suddenly exposed the dacite magma in St. Helens' neck to
   much lower pressure, causing the gas-charged, partially molten rock and
   high-pressure steam above it to explode a few seconds after the slide
   started. Explosions burst through the trailing part of the landslide,
   blasting rock debris northward. The resulting blast laterally directed
   the pyroclastic flow of super-heated volcanic gases, ash and pumice
   from new lava, and pulverized old rock hugged the ground while
   initially moving at 220 mph (350 km/h) but quickly accelerating to 670
   mph (1080 km/h) (it may have briefly passed the speed of sound).

   Pyroclastic-flow material passed up the moving avalanche and spread
   outward, devastating a fan-shaped area 23 miles (37 km) across and 19
   miles (30 km) long. In all, about 230 square miles (600 km²) of forest
   were knocked down within an 8-mile (13 km) inner-fan area, and extreme
   heat killed trees miles beyond the blow-down zone. At its vent the
   lateral blast probably did not last longer than about 30 seconds, but
   the northward radiating and expanding blast cloud continued for about
   another minute.

   Superheated flow material flashed water in Spirit Lake and North Fork
   Toutle River to steam, creating a larger, secondary explosion that was
   heard as far away as British Columbia, Montana, Idaho, and Northern
   California. Yet many areas closer to the eruption ( Portland, Oregon,
   for example) did not hear the blast. This so-called "quiet zone"
   extended radially a few tens of miles from the volcano and was created
   due to the complex response of the eruption's sound waves to
   differences in temperature and air motion of the atmospheric layers
   and, to a lesser extent, local topography.

Lateral blast result

   Everyone in the quiet zone did see the resulting huge ash cloud that
   was sent skyward from St. Helens' northern foot. The near-supersonic
   lateral blast, loaded with volcanic debris, caused widespread
   devastation as far as 19 miles (30 km) from the volcano. The area
   affected by the blast can be subdivided into three roughly concentric
   zones:
    1. Direct blast zone, the innermost zone, averaged about 8 miles (13
       km) in radius, an area in which virtually everything, natural or
       artificial, was obliterated or carried away. For this reason, this
       zone also has been called the "tree-removal zone." The flow of the
       material carried by the blast was not deflected by topographic
       features in this zone.
    2. Channelized blast zone, an intermediate zone, extended out to
       distances as far as 19 miles (30 km) from the volcano, an area in
       which the flow flattened everything in its path and was channeled
       to some extent by topography. In this zone, the force and direction
       of the blast are strikingly demonstrated by the parallel alignment
       of toppled large trees, broken off at the base of the trunk as if
       they were blades of grass mown by a scythe. This zone was also
       known as the "tree-down zone."
    3. Seared zone, also called the "standing dead" zone, the outermost
       fringe of the impacted area, a zone in which trees remained
       standing but were singed brown by the hot gases of the blast. Later
       studies indicated that one-third of the 0.045 cubic miles
       (188,000,000 m³) of material in the flow was new lava, and the rest
       was fragmented, older rock.

   Photographer Reid Blackburn's car after the eruption.
   Enlarge
   Photographer Reid Blackburn's car after the eruption.

   By the time this pyroclastic flow hit its first human victims, it was
   still as much as 680°F (360°C) and filled with suffocating gas and
   flying angular material. Most of the 57 people known to have died in
   that day's eruption succumbed to suffocation while several died from
   burns. Lodge owner Harry Truman was buried under hundreds of feet (tens
   of metres) of avalanche material. Volcanologist David A. Johnston was
   one of those killed, as was Reid Blackburn, a National Geographic
   photographer.

Later flows

   Subsequent outpourings of pyroclastic material from the breach left by
   the landslide 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 mile³ (208,000,000 m³).

   The flow deposits were still about 570°F to 785°F (300°C to 420°C) two
   weeks after they erupted. Secondary steam-blast eruptions fed by this
   heat created 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 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.

Ash column grows

   Photo of the lower part of the May 18 ash column.
   Enlarge
   Photo of the lower part of the May 18 ash column.

   As the avalanche and initial pyroclastic flow were still advancing, a
   huge ash column grew to a height of 12 miles (19 km) above the
   expanding crater in less than 10 minutes and spewed tephra into the
   stratosphere for 10 straight hours. Near the volcano, the swirling ash
   particles in the atmosphere generated lightning, which in turn started
   many forest fires. During this time, parts of the now- mushroom-shaped
   ash-cloud column collapsed, sending additional pyroclastic flows
   speeding down St. Helens' flanks. Later, slower flows came directly
   from the new north-facing crater and consisted of glowing pumice bombs
   and very hot pumiceous ash. Some of these hot flows covered ice or
   water which flashed to steam, creating craters up to 65 feet (20 m) in
   diameter and sending ash as much as 6,500 feet (1980 m) into the air.
   Map of ash distribution.
   Enlarge
   Map of ash distribution.

   Strong high- altitude wind carried much of this material
   east-northeasterly from the volcano at an average speed of about 60 mph
   (100 km/h). By 9:45 a.m. it had reached Yakima, Washington, 90 miles
   (145 km) away, and by 11:45 a.m. it was over Spokane, Washington. A
   total of 4 to 5 inches (100 to 130 mm) of ash fell on Yakima, and areas
   as far east as Spokane were plunged into darkness by noon where
   visibility was reduced to 10 feet (3 m) and half an inch (10 mm) of ash
   fell. Continuing east, St. Helens' ash fell in the western part of
   Yellowstone National Park by 10:15 p.m. and was seen on the ground in
   Denver, Colorado, the next day. In time ashfall from this eruption was
   reported as far away as Minnesota and Oklahoma, and some of the ash
   drifted around the globe within about 2 weeks.

   During the 9 hours of vigorous eruptive activity, about 540 million
   tons of ash fell over an area of more than 22,000 mile² (60,000 km²).
   The total volume of the ash before its compaction by rainfall was about
   0.3 mile³ (1.3 km³). The volume of the uncompacted ash is equivalent to
   about 0.05 mile³ (208,000,000 m³) of solid rock, or only about 7% of
   the amount of material that slid off in the debris avalanche. By around
   5:30 p.m. on May 18, the vertical ash column declined in stature, but
   less severe outbursts continued through the night and the following
   several days.

Mudslides flow downstream

   Mudline next to Muddy River from the 1980 lahars.
   Enlarge
   Mudline next to Muddy River from the 1980 lahars.

   The hot, exploding material also broke apart and melted nearly all of
   the mountain's glaciers along with most of the overlying snow. As in
   many previous St. Helens' eruptions, this created huge lahars (volcanic
   mudflows) and muddy floods that affected 3 of the 4 stream drainage
   systems on the mountain and which started to move as early as 8:50 a.m.
   Lahars traveled as fast as 90 mph (145 km/h) while still high on the
   volcano but progressively slowed to about 3 mph (5 km/h) on the flatter
   and wider parts of rivers. Mudflows off the southern and eastern flanks
   had the consistency of wet concrete as they raced down Muddy River,
   Pine Creek, and Smith Creek to their confluence at the Lewis River.
   Bridges were taken out at the mouth of Pine Creek and the head of Swift
   Reservoir, which rose 2.6 feet (0.8 m) by noon to accommodate the
   nearly 18 million yard³ (13 million m³) of additional water, mud, and
   debris.

   Glacier and snow melt mixed with tephra on the volcano's northeast
   slope to create much larger lahars. These mudflows traveled down the
   north and south forks of the Toutle River and joined at the confluence
   of the Toutle forks and the Cowlitz River near Castle Rock, Washington,
   at 1:00 p.m. Ninety minutes after the eruption, the first mudflow had
   moved 27 river miles (43 km) upstream where observers at Weyerhaeuser's
   Camp saw a 12-foot-high (3.7 m) wall of muddy water and debris pass.
   Near the confluence of the Toutle's north and south forks at Silver
   Lake, a record flood stage of 23.5 feet (7.16 m) was recorded.

   A large but slower-moving mudflow with a mortar-like consistency was
   mobilized in early afternoon at the head of North Fork Toutle. By 2:30
   p.m. the massive mudflow had destroyed Weyerhauser's Camp Baker, and in
   the following hours seven bridges were carried away. Part of the flow
   backed up for 2.5 miles (4 km) soon after entering the Cowlitz River
   but most continued downstream. After traveling 17 miles (27 km) more,
   an estimated 3.9 million yard³ (2.98 million m³) of material were
   injected into the Columbia River, reducing the river's depth by 25 feet
   (7.6 m) for a four-mile (6 km) stretch. The resulting 13-foot (4 m)
   river depth temporarily closed the busy channel to ocean-going
   freighters, costing Portland, Oregon, an estimated five million US
   dollars. Ultimately more than 65 million yards³ (50 million m³) of
   sediment were dumped along the lower Cowlitz and Columbia Rivers.

Aftermath

Direct results

   Map showing 1980 eruption deposits.
   Enlarge
   Map showing 1980 eruption deposits.

   The May 18, 1980, event was the most deadly and economically
   destructive volcanic eruption in the history of the United States.
   Fifty-seven people were killed and 200 homes, 47 bridges, 15 miles (24
   km) of railways and 185 miles (300 km) of highway were destroyed. U.S.
   President Jimmy Carter surveyed the damage and stated it looked more
   desolate than a moonscape. A film crew 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 the next day (see below), but the crew survived and were
   rescued two days after that.

   In all, St. Helens released an amount of energy equivalent to 27,000
   Hiroshima-sized atomic bombs (approximately 350 megatons) and ejected
   more than 1 cubic mile (4 km³) of material. One-fourth of that volume
   was fresh lava in the form of ash, pumice, and volcanic bombs while the
   rest was fragmented, older rock. The removal of the north side of the
   mountain reduced St. Helens' height by about 1,313 feet (400 m) and
   left a crater 1 to 2 miles (2 to 3 km) wide and 2,100 feet (640 m) deep
   with its north end open in a huge breach.
   St. Helens in September 1980.
   Enlarge
   St. Helens in September 1980.

   More than 4 billion board feet (14.6 km³) of timber were damaged or
   destroyed, primarily by the lateral blast. At least 25% of the
   destroyed timber was salvaged after September 1980. Downwind of the
   volcano, in areas of thick ash accumulation, many agricultural crops,
   such as wheat, apples, potatoes, and alfalfa, were destroyed. As many
   as 1,500 elk and 5,000 deer were killed, and an estimated 12 million
   Chinook and Coho salmon fingerlings died when their hatcheries were
   destroyed. Another estimated 40,000 young salmon were lost when they
   swam through turbine blades of hydroelectric generators due to the need
   to have low reservoirs levels along the Lewis River to accommodate
   possible mudflows and flood waters.

Digging out

   The ash fall created some temporary but major problems with
   transportation, sewage disposal, and water treatment systems.
   Visibility was greatly decreased during the ash fall, closing many
   highways and roads. Interstate 90 from Seattle to Spokane was closed
   for a week and a half. Air transportation was disrupted for a few days
   to 2 weeks as several airports in eastern Washington shut down due to
   ash accumulation and poor visibility. Over a thousand commercial
   flights were cancelled following airport closures. 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.

   Removing and disposing of the ash was a monumental task for some
   eastern Washington communities. State and Federal agencies estimated
   that over 2.4 million yards³ (1.8 million m³) of ash—equivalent to
   about 900,000 tons in weight—were removed from highways and airports in
   Washington. 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.

Cost

   One of the 200 homes destroyed by the eruption.
   Enlarge
   One of the 200 homes destroyed by the eruption.

   Early estimates of the cost of the eruption ranged from US $2–3
   billion. A refined estimate of $1.1 billion was determined in a study
   by the International Trade Commission at the request of the United
   States 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 also indirect and intangible costs of the eruption.
   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. 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 reawakening, has regained its appeal for
   tourists. The National Forest Service and the State of Washington
   opened visitor centers and provided access for people to view the
   volcano's devastation.

Later eruptions

   St. Helens produced five more explosive eruptions between May and
   October of 1980. Through early 1990, a total of at least 21 periods of
   eruptive activity had occurred. The volcano remains active, with
   smaller, dome-building eruptions continuing into 2006.
   Eruption on July 22, 1980.
   Enlarge
   Eruption on July 22, 1980.

   An eruption occurred on May 25, 1980 at 2:30 a.m. that sent an ash
   column 9 miles (14 km) into the atmosphere. The eruption was preceded
   by a sudden increase in earthquake activity and occurred during a rain
   storm. Erratic wind from the storm carried ash from the eruption to the
   south and west, lightly dusting large parts of western Washington and
   Oregon. Pyroclastic flows exited the northern breach and covered
   avalanche debris, lahars, and other pyroclastic flows deposited by the
   May 18 eruption.

   At 7:05 p.m. on June 12, a plume of ash billowed 2.5 miles (4 km) above
   the volcano. At 9:09 p.m. a much stronger explosion sent an ash column
   about 10 miles (16 km) into the air. A dacite dome then oozed into
   existence on the crater floor, growing to a height of 200 feet (60 m)
   and a width of 1,200 feet (365 m) inside a week.

   A series of large explosions on July 22 broke more than a month of
   relative quiet. The July eruptive episode was preceded by several days
   of measurable expansion of the summit area, heightened earthquake
   activity, and changed emission rates of sulfur dioxide and carbon
   dioxide. The first hit at 5:14 p.m. as an ash column shot 10 miles (16
   km) and was followed by a faster blast at 6:25 p.m. that pushed the ash
   column above its previous maximum height in just 7.5 minutes. The final
   explosion started at 7:01 p.m. and continued for over two hours. When
   the relatively small amount of ash settled over eastern Washington, the
   dome built in June was gone.
   The growing third dome on October 24, 1980.
   Enlarge
   The growing third dome on October 24, 1980.

   Seismic activity and gas emission steadily increased in early August,
   and on August 7 at 4:26 p.m., an ash cloud slowly expanded 8 miles (13
   km) skyward. Small pyroclastic flows were sent through the northern
   breach and weaker outpouring of ash rose from the crater. This
   continued until 10:32 p.m. when a second large blast sent ash high into
   the air. A second dacite dome filled this vent a few days later.

   Two months of repose were ended by an eruption lasting from October 16
   to October 18. This event obliterated the second dome, sent ash 10
   miles (16 km) in the air and created small, red-hot pyroclastic flows.
   A third dome began to form within 30 minutes after the final explosion
   on October 18, and within a few days, it was about 900 feet (275 m)
   wide and 130 feet (40 m) high.

   All of the post-1980 eruptions were quiet dome-building events,
   beginning with the December 27, 1980, to January 3, 1981, episode. By
   1987 the third dome had grown to be more than 3,000 feet (900 m) wide
   and 800 feet (240 m) high. At this rate and assuming additional
   destructive eruptions do not occur, St. Helens' summit should be
   restored sometime in the mid to late 22nd century. See the main Mount
   St. Helens article for the most recent updates.

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