Mt. St. Helens

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Mt. St. Helens

• 1980 Eruption
• Before, During and After

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Where is Mt. St. Helens

• The Cascade Range is a volcanic chain stretching
  from Northern California to British Columbia
• Mt. St. Helens is part of this range

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Mt. St. Helens, the smallest of five major
volcanic peaks in Washington state had an
elevation of 9677 feet before the eruption of May
1980.
Mt. Adams (12,286) in the distance
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First Evidence of Activity

• Initial ice fractures on March 27, 1980
• 3/28- First phreatic (steam) eruption
• The vent 250 ft in diameter

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March 29, 1980 Second Crater

• Crater appeared on the NE side of the first
  crater
• The new vent was 75ft. Wide and 30-60ft deep
• 3/31 the two craters joined together

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March 30, 1980 Ash Darkens Slopes

• Ash came from rock pulverized by the
  explosively-expanding, high-temperature steam and
  other gases- No new rock produced
• Mt. Rainier in the background

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Steam Explosion -4/10/80
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Bulge Forms on N-NE Flank

• Bulge first detected on 4-23
• End of April the bulging area was one mile long
  and 0.6 mile wide
• Maximum uplift 320 feet
• Some areas of uplift - 5 feet a day!

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Another View of the Bulge April 27,1980
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Steam Eruption Prior to 5-18-80

• Typical phreatic (steam) eruption in early May
• Eruptions reached 13,000 feet in elevation
• This activity was followed a two week quiet period

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On may 18th 1980 at 832

• Geologist David Johnston was taking
  measurements on a ridge just 5 miles NW of the
  summit when he radioed Vancouver! Vancouver!
  This is it! Moments later he was vaporized by
  the cloud of hot gas, ash, rock, ice and trees
  that hit him

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For more than nine hours, the eruption continued.
People that escaped had to drive 80mph on old
logging roads to escape
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May 18th 1980

• 832 AM PDT a magnitude 5.1 shook Mt. St. Helens
• The bulge and surrounding material slid away in
  a gigantic landslide and debris avalanche

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Computer Version of Landslide

• The initial earthquake and eruption triggered one
  of the largest landslides in recorded history.
• The slide traveled 110-155mph
• Hit a 1150ft high ridge about 6 miles north.
  Some of the slide spilled over the ridge
• Most of the slide moved 13 miles down the North
  Fork of the Toutle River filling its valley up to
  600 ft deep

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The initial May 18th Eruption View From Less
Than Three Miles

• The cloud rose to more than 12 miles in 10
  minutes
• The cloud generated its own lightning
• Ash rained down initially in an area that
  extended 25-30 miles to the north and east
• Later, ash covered parts of eleven states

From an airplane that barely made it back!
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• For over nine hours, a vigorous plume of ash
  erupted reaching 12-15 miles above sea level
• The plume moved eastward at an average speed of
  60 mph with ash reaching Idaho by noon
• Many cities had to use snowplows to remove the
  several inches of ash accumulation

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Eleven States Had Ash Fallout
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View from the NE on June 19, 1980
This area was completely forested prior
to the May 18, 1980 eruption
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• Helicopters were deployed to try and save
  survivors. Few were found
• Some people were found with their arms calmly
  folded or a camera held to their eye. The hot
  gasses killed them as they stood. It is believed
  that some people and vehicles were instantly
  incinerated

The total volume of ash as approximately .26
cubic mile or enough ash to cover a football
field to a depth of 150 miles
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The initial blast flattened over four billion
board feet - Enough to build over 150,000 homes.
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Nearly 135 miles of river channels surrounding
the volcano were affected by the lahars.
Note the mud line on the trees and the person
in the picture
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The remains of Spirit Lake, once surrounded by
lush forest
Mt. Rainier, 14,410ft in the distance
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• After the May 18,1980 eruption, Mt.St. Helens
  elevation was only 8364 feet and the volcano had
  a one-mile wide horseshoe-shaped crater

Craters were formed when water and hot ash came
together making large steam explosions
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Reid Blackburn, a National Geographic
Photographers car
Ried Blackburn along with Volcanologist David A.
Johnston were never found
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The U shaped basin formed by the landslide and
5/18/80 eruption
June 4th 1980 - Mudflows are visible in the
picture
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• Mt. St. Helens erupted often between 1980 and
  1986
• An explosive eruption on 3/19/82 sent pumice and
  ash 9 miles into the air and resulted in a lahar

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Pyroclastic flow from August 7,1980 eruption

• During the 5/18/80 eruption, at least 17 separate
  pyroclastic flows descended the flanks of the
  mountain
• Pyroclastic flows typically move at speeds of
  over 60 mph and reach temperatures of over 800
  degrees

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October 24, 1980 a new dome started growing.
This October dome was 112ft high and 985 feet
wide making it taller than a nine-story building
and wider than the length of three football fields
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Mt. St. Helens soon after the 5/18/80 eruption
from Johnstons Ridge
Taken September 10, 1980
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USGS Scientist Explore Pumice Blocks from a
Pyroclastc flow
October 17, 1980
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Geologists install a seismic station near the
dome.
An increase in seismicity is often the first
precursor to an approaching eruption
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5/19/82- Plume from Harry's Ridge
Plumes of steam, gas, and ash often occurred at
Mt. St. Helens in the early 1980s. On clear
days, they could be seen from Portland Oregon, 50
miles to the south
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Harry's Ridge Was Named After Harry Truman of
Spirit Lake
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Measuring magnetic field in crater 6/18/84
Geologist measured the strength of the magnetic
field surrounding the dome. The strength of the
field increased as the dome cooled and magnetic
minerals formed. During the eruptions, the
strength usually changed rapidly as magma heated
and deformed the dome
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USGS scientists survey the dome 5/26-84
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April 18, 1988 USGS Deformation Monitoring on the
Dome
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Mt. St. Helens, Sept. 1994
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Aerial View of Mt. St. Helens 2002
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Mt.St Helens night shot of Dome 10-22-86
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Mt.St Helens Spirit Lake before The May1980
Eruption
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Mt.St. Helens Lake Reflection- After the Eruption
Picture taken May 19, 1982
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2005 Saw An Increase In Activity
Picture Taken May 7, 2005