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