1. Mt. St. Helens, pre-1980

2. Cascade Range
subduction zone volcanoes

4. March 27, 1980
April 3, 1980

5. The Bulge:
north side of mountain expands;
300 feet

6. The bulge is highly fractured; the north face starts slowly sliding off
~40 feet in 11 days

7. May 18

8. Earthquake triggers massive collapse of north face, exposing magma chamber
First major blast heads to the north, not straight up.

9. Mt. St. Helens
Force of blast stripped and knocked down trees for miles

10. Lahar - flow of water saturated with volcanic ash and debris
(mud flow)

11. predicted
actual

13. Ash and coarser debris composed of rock fragments

14. Mud Flows , Debris Flows Heat from eruption can instantly melt snow
Up to 60 mph floods with the density of wet concrete
Piles of newly fallen ash may be unstable in rain
Hazards to Tacoma, WA, from Mt. Rainier, based on mud deposited by ancient eruptions
Scott, K.M., Wolfe, E.W., and Driedger, C.L., Hawaiian Volcano Observatory, U.S. Geological Survey

15. Rabaul, New Guinea ~600 A.D. 1878 1937 1994 -present
City of Rabaul is a natural harbor that has lived with occaisional volcanic eruptions throughout its history. Mostly destroyed in 1937 and again in 1994, by eruptions on both sides of bay. Actually, these volcanoes are just small vents- the entire bay is the volcano: a caldera (collapsed volcano), ~10 km across formed by a huge eruption in ~600 AD.

16. Yellowstone Caldera A hotspot (like Hawaii) sits beneath North America
Montana
Wyoming
A hotspot (like Hawaii) sits beneath North America
Eruptions are very infrequent, but can be huge
Idaho

17. Supervolcanoes; volume erupted 2.1 Mya Yellowstone 0.62 Mya 1.3 Mya
Krakatoa, 1883
Mt. St. Helens, 1980