Dr. Helen Lang Dept. of Geology & Geography West Virginia University FALL 2005 GEOLOGY 285: INTRO. PETROLOGY
Mount St. Helens 1980 Eruption Small earthquakes Small steam and ash eruptions in March and April 1980 USGS monitoring station set up north of the mountain on Coldwater Ridge May 1980 - North side of mountain began to bulge, many micro-earthquakes, eruption was imminent; red-zone closed See USGS Professional Paper 1250 (1981) for much more information and pictures
Vancouver! Vancouver! This is it... David Johnston Photo of Dave at Coldwater II, May 17
8:32 AM 5/18/80 Magnitude 5.1 earthquake triggered the Big Eruption The unstable bulge on the North side collapsed and exposed magma in a shallow chamber to air Gas bubbles formed instantaneously in the magma causing expansion, a shock wave and a big ash eruption Most of the force of the eruption was directed horizontally to the North Only 57 people died in the eruption
Eruption viewed from Mount Adams
The 1980 Eruptions of Mt. St. Helens, Washington USGS Professional Paper 1250, 1981, P.W. Lipman and D.R. Mullineaux, editors Photos, eye-witness accounts and complete descriptions of all aspects of the eruptions Much of the content of this presentation comes from this publication
Photos taken from a ridge about 10 miles NE of the summit Gary Rosenquist photos made reconstruction of the eruption sequence possible
Rosenquist 4 and 5
Rosenquist 6 & 8
Rosenquist 10
Reconstructed Eruption Sequence
After the first few minutes, the ash erupted upward and drifted ENE with the prevailing winds
View from the South on May 19
Effects of May 18, 1980 eruption National Geographic, January 1981, special Mt. St. Helens issue
Summit collapsed to form debris flows to north and west
Debris flow deposits to North
Toutle River Mud Flow
Directed blast vaporized vegetation near the volcano, stripped and flattened trees further out
Direction of trees indicates blast dynamics Fig. 224 USGS PP 1250 - Kieffer
Ashflows Aug. 7, 1980 ash flow (USGS) Aug. 7, 1980 deposit on Aug. 8 (HL)
Ashflow on Snow
Dome growth over next few years: Will it fill the crater? Baby dome - Aug. 8, 1980 The dome - 1983 The dome - 1986
Before and After May 18, 1980 National Geographic
We think Mount St. Helens 1980 was a big deal 1816 the year without a summer Pinatubo 1991 ~5 cubic km By comparison to other eruptions of subduction-related volcanoes, it was tiny! National Geographic
Mt. St. Helen s 1998
Recovery - 20th Anniversary MUDFLOW National Geographic, 2000
Johnston Ridge Observatory MUDFLOW THE CRATER National Geographic
Recent Images Landsat 2000 Space Shuttle 1994
Mount St. Helens 2004 October 1, 2004. A small explosive eruption of Mount St. Helens on October 1st; the first in more than a decade followed a week of increasing earthquake activity beneath the volcano and deformation of the lava dome. This eruption sent a steam and minor ash plume to an altitude of about 10,000 feet above sea level.
Mount St. Helens 2004 October 13, 2004. Almost vertical look at Mount St. Helens dome, old and new. The "old dome" (1980-86) is in the lower half of the image and the new uplift and growth is in the upper half of the image. The image shows part of the crater rim and south wall of the crater. (USGS photo, Kathy Cashman)
MSH new dome Sept. 21, 2005
Fun at Spirit Lake in the 1940s MUDFLOW
Volcanic Hazards Directed blast Hot ash flows, lava flows Airfall ash (threatens airplanes) Mudflows = lahars Mixture of melted ice, debris, water, and ash Threaten people, cities and towns far away Lahars from Mt. Rainier could threaten Seattle and/or Tacoma
Lahars=Mudflows are the main Volcanic Hazards at Mt. Rainier
USGS-USAID - Volcano Disaster Assistance Program (VDAP) http://volcanoes.usgs.gov/about/where/vdap/main.html Established after volcanic disaster in Columbia (Armero) in 1985 VDAP is a mobile, well equipped team of experienced volcanologists who can respond whenever a volcano crisis threatens anywhere in the world VDAP monitoring and successful prediction of 1991 eruption at Pinatubo saved thousands of lives
Recent Eruptions Nevado del Ruiz, Columbia Nov. 13, 1985 eruption Armero mudflow >23,000 killed Pinatubo, Philipines Erupted June 15, 1991 Ejected ~5 cubic km of magma (~10x St. Helens) threatened 1,000,000 people, only a few hundred perished
There s a Great Book about Volcanologists studying explosive Volcanoes in the post-st. Helens era Volcano Cowboys by Dick Thompson, 2000