FALL 2005

GEOLOGY 285:INTRO. PETROLOGY

Dr. Helen LangDept. of Geology & GeographyWest Virginia University

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

Gary Rosenquistphotos made reconstruction of the eruption sequence possible

Photos taken from a ridge about 10 miles NE of the summit

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, specialMt. 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

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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 - 1983The dome - 1986

Before and After May 18, 1980

National Geographic

We think Mount St. Helens 1980 was a big deal

By comparison to other eruptions of subduction-related volcanoes, it was tiny!

1816 “the year without a summer” Pinatubo 1991

~5 cubic km

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

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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