Mt. St. Helens 1980s

Question 1

Eruption History of Mt. St. Helen’s

Answer 1

50,000 years of volcanic activity with little eruptions and minor steam eruptions.

Symmetric Cone (Fuji of America)

Question 2

Mt. St. Helen’s as a Cascade Volcano

Answer 2

Youngest Cascade Volcano yet the most active

Question 3

1980 Sequence of Events

Answer 3

1. March 22nd-28th: Earthquakes and general unrest increase (Phreatic Explosions begin) including steam eruption.
2. March 29th-April 11th: Cone starts to bulge and grow due to magma rising near surface.
3. May 18th: Earthquake triggers Debris Avalanche as bulge begins to slide down (flushed out North Fork Toutle)
4. May 18th: Pressure on flank triggers Lateral Blast eruption !
5. Plinian Eruption continues to release pressure in Magma reservoir. Lahars occur.

Question 4

Lateral Blast

Answer 4

Short Eruption (30 seconds) but very extreme.
Ground-hugging pyroclastic Surge

Question 5

What did Plinian Eruption cause?

Answer 5

Removal of upper 1312 ft. Cone; Vertical Column generated dense ash clouds; 9 hour eruption

Question 6

Post-Climatic Eruptive Activity & Behavior

Answer 6

August 7th & October 17th-18th: Explosive Episode

October 18th: Dome of Gas

Minor lava flows, dome growth, continued mudflow