Mount St Helens by Alice Graham

1
Mount St Helensby Alice Graham
2
Location of the volcano and background history

• A volcanic peak, 2,549 m (8,363 ft) high
• The mountain is 6 miles (9.5 km) across at its
  base
• Location South West Washington, USA
• The mountain is part of the Cascade volcanoes and
  the Cascade Range
• Type Composite
• Mount St. Helens is a part of the Pacific Ring of
  Fire that includes over 160 active volcanoes.
• It only formed within the last 40,000 years, and
  the pre-1980 summit cone began rising around
  2,200 years ago.

3
What were the main causes of the volcano erupting
The Mt St Helens volcano results from subduction
zone created between the Juan de Fuca plate and
the North American plate. The oceanic Juan de
Fuca plate is denser so subducts below the North
American plate.
4
What were the main events and times of the
eruptions

• March 20 1980 Earthquake measuring 4.2 magnitude
• March 27 Steaming vent
• April A bulge on the north side of Mountain
• May18 A second earthquake- Massive collapse of
  North face of Mountain (Little warning).
• The magma burst out causing large pyroclastic
  flow flattening vegetation and buildings for over
  230 square miles ( 600 km²).
• Plinian eruption.
• Volcanic Mudflows- Due to collapse of northern
  side mixed with ice, snow and water. These flowed
  many miles down the Toutle and Cowlitz Rivers.
• 3million m³ of material transported 7 miles (27
  km) south into Columbia river by mudflows
• For 9 hours ash erupted
• By 539 became less severe outbursts continued
  through the night.

5
What were the main short term impacts of the
eruption?

• The eruption killed 57 people
• Nearly 7,000 big game animals (deer, elk and
  bear)
• An estimated 12 million fish from a hatchery.
• Many tens of thousands of acres of prime forest
  destroyed
• It destroyed or extensively damaged over 200
  homes
• Destroyed/ damaged 185 miles (300 km) of road
• Destroyed/ damaged 15 miles of railways.
• Airports closed (from 2days ? 2 weeks). Over a
  thousand commercial flights were cancelled
  following airport closures.
• Unemployment rose rapidly after eruption. However
  returned to normal when the timber salvaging and
  ash-cleanup began.

6
What were the main long term impacts of the
eruptions?

• Economic Trees amounting to more than 4 billion
  board feet of salable timber were damaged or
  destroyed, primarily by the lateral blast
• Economic Many agricultural crops, such as wheat,
  apples, potatoes, and alfalfa, were destroyed
• Economic Ash removal cost 2.2 million and took
  10 weeks in Yakima
• Economic 1.1 billion cost overall
• Social Several months after May 18, a few
  residents reported suffering stress and emotional
  problems, even though they had coped successfully
  during the crisis. The counties in the region
  requested funding for mental health programs to
  assist such people.
• Social Homelessness- More than 200 houses and
  cabins were destroyed and many more were damaged
  in Skamania and Cowlitz Counties.

7
In what way was the volcano being monitored was
the eruption predicted

• The volcano was being monitored to detect early
  signs of an impending eruption.
• No one knew what types of monitoring techniques
  would prove most reliable because knowledge about
  the behaviour of Cascade volcanoes in general was
  limited and because Mount St. Helens had not been
  active recently.
• University and USGS studies in the 1970's had
  monitored the volcano intermittently using three
  approaches
• 1. seismometers that could detect earthquakes
  caused by the movement of molten rock
• 2. precise ground-surface measurements that could
  detect swelling of the volcano
• 3. aerial infrared surveys and surface
  temperature measurements of the two "hot spots"
  high on the mountain to detect any changes in
  heat emission.
• The volcano was also photographed routinely from
  the air to detect changes in snow and ice as part
  of a USGS glaciology research project.
• At the beginning of March 1980 the only
  instrument directly monitoring Mount St. Helens
  was a seismometer on the western flank of the
  volcano. This station (SHW) automatically
  transmitted earthquake data by radio
  (telemetered) to seismic recorders at the
  University of Washington in Seattle.

8
How did people respond to the eruption?

• The first response to the May 18, 1980 eruption
  of Mount St. Helens was to make sure that people
  and communities were safe.
• People then needed to restore the area
• Debris such as saleable logs were retrieved so as
  to try to restore the economy of the area

9
Has there been any activity since the main
volcanic eruption

• Five smaller explosive episodes occurred during
  the summer and fall of 1980.
• Beginning with the October 1980 eruption, 17
  eruptive episodes built a new lava dome that
  reached 876 feet above the crater floor
• During a 12-month-long episode beginning in 1983
  magma moved into the dome's molten interior,
  pushing its east side outward by at least 250
  feet.
• Since late 1986 several periods of increased
  seismicity have occurred. 
• Between 1989 and 1991 there were about 30 bursts
  of brief but intense seismic activity lasting
  minutes to hours some also had small explosions
  from the dome
• The explosions formed a new vent on the north
  side of the dome and produced small eruption
  columns that rose a few miles above the volcano.
• A few explosions also hurled hot rocks three feet
  in diameter at least 1/2 mile northward from the
  dome, generated small pyroclastic flows in the
  crater, and formed small lahars.
• During 1995 and 1998 seismicity increased for
  several months, but there were no accompanying
  explosions.