Mt. St. Helens, pre-1980

Cascade Range subduction zone volcanoes

March 27, 1980 April 3, 1980

The Bulge: north side of mountain expands; 300 feet

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

May 18

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

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

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

predicted actual

Ash and coarser debris composed of rock fragments

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

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.

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

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