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Volcanoes Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Mount St. Helens

In March of 1980, Mount St. Helens was considered one of the most beautiful mountains in the Pacific Northwest of the United States. Campers, hikers, and fishermen didn’t think about the fact that the striking mountain in central Oregon sits on a plate boundary and had erupted as recently as 400 years earlier. Then, after a series of over 170 increasingly stronger earthquakes warned scientists of the danger, the top of the mountain was blown off in a tremendous explosion sending a column of volcanic ash and steam into the sky.

Volcanic ash ejected during a volcanic eruption is made up of rock particles less than 4 millimeters in diameter. Coarse ash is sized from Volcanoes mm, with fine ash (dust) measuring < Volcanoes mm in grain size.

Two months later, after nearly constant tremors, a large bulge on the mountain’s side appeared. Then, on the morning of May 18, 1980, an earthquake a mile underground lowered the internal magma pressure within the volcano and caused the bulge to collapse, followed by a huge landslide moving at 241 kilometers per hour. The removal of the overlying weight allowed the pressurized magma below to cut loose with a violence calculated at over 500 times the force of the Hiroshima bomb or 10 million tons of TNT. The blast from the volcanic vent sent ash into the air to an estimated height of 19 km. The amount of lumber flattened in the 373km area affected by the blast could build more than 250,000 homes.

Since Mount St. Helens’ eruption happened early on a Sunday morning, many of the visitors and lumber people usually near the mountain had not yet arrived. Sixty people, including a number of volcanologists, died from the blast and hot choking ash. Of the local animal population, 5000 black-tailed deer, 200 black bear, and 1500 elk died in the disaster, along with nearly all the birds and small mammals within the blast area.

Spokane, Washington, over 320 kilometers away, was in total darkness by mid-afternoon from falling ash. The wind-driven ash traveling at nearly 100km/h caused power failures and clogged automobile and emergency vehicle air filters.

It is estimated that over 540 million tons of ash was spewed from Mount St. Helens over a 35,410km 2 area, with most of the ash dropped on Oregon, Washington, and Idaho. In the out-lying cities hardest hit, as much as three inches of ash coated the countryside.

Mount St. Helens, along with Mt. Rainier, Mt. Hood, and others are part of the Cascade mountain range, formed where the western edge of the North American plate overrides the descending San Juan de Fuca plate. This fairly young mountain range, which lies along the eastern edge of the Pacific Ring of Fire, began forming between 3 and 7 million years ago when magma began rising through conduits to the surface. Table 11-2 lists the volcanic peaks that lie along the Cascade mountain range.

Table 11-2 The Cascade Mountain range extends across 3 states and Canada.

Mountain or crater

Elevation (meters)

Location

Mt. Garibaldi

2678

British Columbia

Mt. Baker

3285

Washington

Glacier Peak

3213

Washington

Mt. Rainier

4394

Washington

Mt. St. Helens

2950 (before eruption) 2549 (after)

Washington

Mt. Adams

3742

Washington

Crater Lake (Mt. Mazama)

4394

Oregon

Mt. Hood

3426

Oregon

Mt. Je.erson

3199

Oregon

The Three Sisters (Paulina Pk.)

3157

Oregon

Sacajawea Peak

2999

Oregon

Mt. Moloughlin

2894

Oregon

Mt. Thielsen

2799

Oregon

Newberry Crater

2434

Oregon

Mt. Shasta

4401

California

Lassen Peak

3187

California

 

Practice problems of this concept can be found at: Volcano Practice Test

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