Tuesday, May 17, 2005

Mount St.Helen's volcanic history

Here is a modern-day timeline of Mount St. Helens, the youngest and most active volcano in the nation:

1847-57: Explorers and early settlers report several eruptions of Mount St. Helens over this 10-year span.

1979: The mountain is a major tourism and outdoor recreation draw. Some 500,000 people a year visit the Spirit Lake area below the cone-shaped, 9,677-foot summit.

March 20, 1980: After a 123-year slumber, the mountain is awakened by a shallow, 4.1 magnitude earthquake beneath it, followed by numerous aftershocks. Scientists step up seismographic monitoring of the volcano.

March 25: The earthquakes continue, including five larger than 4.0 magnitude within a one-hour period, causing officials to close the upper reaches of the mountain to visitors and logging.
March 26: U.S. Geological Survey geologist Don Mullineaux, one of the foremost researchers on the hazards and volcanic history of Mount St. Helens, and other scientists monitoring the mountain tell federal, state and county emergency management officials that they have an active volcano on their hands.

March 27: A large boom about 12:30 p.m. signals the first eruption of Mount St. Helens since 1857. A black plume of ash and smoke rises 7,000 feet above the mountain and a small crater about 200 feet to 250 feet wide opens at the summit. Hundreds of people are evacuated from a 15-mile radius around the volcano, but Harry R. Truman, 84, owner of Mount St. Helens Lodge at Spirit Lake, refuses to leave.

April 1: A harmonic tremor, which signals the movement of magma deep within the bowels of the volcano, is recorded early that evening.

April 2: At least 12 plumes of steam and ash are observed above the volcano, one reaching an altitude of 20,000 feet.

April 3: Gov. Dixy Lee Ray declares a state of emergency and forms a Mount St. Helens watch group.

April 7: Aerial photographs taken that day show that the north flank of the volcano was beginning to crack and bulge out more than 300 feet. But the data isn't available to scientists until April 23.
April 30: Earthquake activity at the volcano is leveling off, but the growing bulge prompts officials to create a controversial red zone around the volcano off limits to everyone but scientists, law enforcement and rescue personnel. Debate begins over whether the restricted zone is too large, or too small.

May 1-17: Small explosions, releases of steam and ash and earthquakes continue. And the bulge on the north flank keeps growing at a rate of about 5 feet a day.

8:32 a.m., May 18: Within seconds after a 5.1 magnitude earthquake, the volcano's summit and bulge slide away in the largest landslide ever recorded, taking 1,300 feet of the mountain top.
The landslide depressurizes the volcano's magma system, triggering powerful blasts of rocks, ash and superheated gas and steam.

The eruption unleashes several lahars, or mudflows, that pour with destructive force into nearby river valleys and flows of hot lava that reach five miles north of the crater.

Throughout the day, 520 million tons of ash spew from Mount St. Helens eastward across Eastern Washington and the rest of the nation, causing complete darkness in Spokane, 250 miles away.
The blast flattens 160 square miles of forestland, kills 57 people and countless fish and wildlife, including 7,000 big-game animals and 12 million salmon fingerlings.

Property damage, including 200 river valley homes destroyed by mudflows the first day, nears $1 billion.

1980-86: Repeated, smaller eruptions create a new lava dome in the crater floor about 876 feet tall.

1982: Congress and President Ronald Reagan create the 110,000-acre Mount St. Helens National Volcanic Monument for research, recreation and education.

September 1985: King County Superior Court Judge James McCutcheon dismisses the state of Washington from a lawsuit brought by relatives of 10 people who died in the eruption. Plaintiffs claim the state and Weyerhaeuser were negligent for allowing people so close to the volcano. Case against Weyerhaeuser Co. allowed to proceed.

September 1986: Weyerhaeuser settles with plaintiffs for $240,000.

1990-present: Most of the plant, tree and animal species found around Mount St. Helens before the eruption have returned to the disturbed landscape, albeit, in most cases, in much smaller numbers.

1998: Seismic activity returns to the mountain for the first time since 1986. Scientists say earthquakes push magma to within a mile of the dome.

Sept. 23, 2004: A swarm of small earthquakes is recorded at Mount St. Helens.

Sept. 25: Crater and areas around the volcano are closed to hikers and climbers.

Sept. 29: Seismic activity increases, causing geologists to raise the eruption-warning level and expand the restricted zone.

Oct. 1: A large cloud of steam and ash erupts from the volcano, rising to 10,000 feet. More eruptions and lava flows are predicted, but nothing to rival the May 18, 1980, event.

Oct. 5: A steam and ash eruption climbs to 12,000 feet and lightly dusts the communities of Morton, Randle and Packwood.

Oct. 11 to present: New lava dome consisting of old magma from the 1980 eruption continues to grow and change.

February 2005: Weyerhaeuser begins first commercial thinning harvest of trees planted after the 1980 eruption.

March 8: Mount St. Helens experiences a small explosive eruption with steam and ash reaching 36,000 feet above sea level.

May 6: U.S. Forest Service reopens to the public Johnston Ridge Observatory, which is 5.5 miles from the volcano crater, and the Coldwater Ridge Visitor Center, which is 8.5 miles from the crater.

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