Tuesday: bring calculator
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Transcript of Tuesday: bring calculator
Tuesday: bring calculator
Mt. St. Helens: A Case Study from 1980
Photo pre-eruption
Mt. St. Helens: Volcanic Precursors
Tremors around the mountain
Mt. St. Helens: Volcanic Precursors
Harmonic tremors are different from tectonic movements
Mt. St. Helens: March 20
Quakes triggered snow avalanches
Mt. St. Helens: March 21-26
Earthquake activity dramatically increases
Mt. St. Helens: March 27
Dark ash blankets the top of the mountain: Phreatic eruption
Mt. St. Helens: March 27
Phreatic eruption = steam related, with ash
Magma+ groundwater = explosion
Mt. St. Helens: March 27
Crater is formed
Mt. St. Helens: March 28
12 explosions occur
Ash landslides
Mt. St. Helens: March 31
Harry Truman refuses to leave
Mt. St. Helens: April 1
Huge crater develops: 200 feet deep and growing
4/3/80: 1500’ wide, 300’ deep
Mt. St. Helens: April 7-April 11
Now a bulge develops on the side of the volcano
Mt. St. Helens: April 7-April 11
And yet the crater keeps growing: 500’ deep
Mt. St. Helens: April 11-26
Bulge is still there, but explosions cease on 4/26…is it over, or is the pressure building?
4/29: nearly 1 mile across
The Bulge: March 29 through May 2
May 7: Explosions resume, end May 15
May 17…
Eruption Warnings: Volcanic Precursors
• Small changes in summit elevations or slope steepness can be observed by lasers of tiltmeters
Mt. St. Helens Case Study
• Last eruption: 1857• Activity begins in March• Eruption occurred in May
Mt. St. Helens
5/18/80
Volcanic Precursors Harmonic tremors
Ground swelling or tilting
Emitting steam and gases
Increased ground temperatures
Small phreatic eruptions (water)
Ground swelling