Glacier Peak

From Academic Kids

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Glacier Peak is the most remote of the five active volcanoes in Washington State. It is not prominently visible from any major population center, and so its attractions, as well as its hazards, tend to be overlooked. Yet since the end of the last ice age, Glacier Peak has produced some of the largest and most explosive eruptions in the state. During this time period, Glacier Peak has erupted multiple times during at least six separate episodes, most recently about 300 years ago.

The stunning snow-capped volcanoes of Washington State have long been recognized by Native Americans in their language and legends, and they immediately caught the eyes of U.S. and European explorers in the late 18th and early 19th centuries. By the 1790s, Mounts Baker, Rainier, and St. Helens were noted and named in the first written descriptions of the Columbia River and Puget Sound regions. In 1805, Lewis and Clark noted Mount Adams. By the mid-19th century, each of these four volcanoes had their place on a published map.

Glacier Peak wasn't known by settlers to be a volcano until the 1850s, when Native Americans mentioned to naturalist George Gibbs that "another smaller peak to the north of Mount Rainier once smoked." Not until 1898 did Glacier Peak appear on a published map under its current name.

Glacier Peak is situated in Snohomish County, only 70 miles northeast of Seattle closer to that city than any volcano except Mount Rainier. But unlike Mount Rainier, it rises only a few thousand feet above neighboring peaks, and from coastal communities it appears merely as a high point along a snowy saw-toothed skyline. Yet Glacier Peak has been one of the most active and explosive of Washington's volcanoes.

Since the continental ice sheets receded from the region, Glacier Peak has erupted repeatedly during at least six episodes. Two of these eruptions were among the largest in Washington during the past 15,000 years.

Eruptive history

Glacier Peak and Mount St. Helens are the only volcanoes in Washington State that have generated large, explosive eruptions in the past 15,000 years. Their violent behavior results from the type of molten rock (magma) they produce. Dacite, the typical magma type of Mount St. Helens and Glacier Peak, is too viscous to flow easily out of the eruptive vent; it must be pressed out under high pressure. As it approaches the surface, expanding gas bubbles within the magma burst and break it into countless fragments. These fragments are collectively known as tephra; the smallest make up what is called volcanic ash.

About 13,100 years ago, Glacier Peak generated a sequence of nine tephra eruptions within a period of less than a few hundred years. The largest ejected more than five times as much tephra as the May 18, 1980 eruption of Mount St. Helens and was one of the largest in the Cascade Range since the end of the last ice age.

Some of the tephra from these eruptions fell back onto the volcano and avalanched down its flanks. Much of the rest rose high into the atmosphere and drifted hundreds to thousands of miles downwind. Deposits from these eruptions are more than a foot thick near Chelan, Washington, and an inch thick in western Montana.

Since these events, Glacier Peak has produced several tephra eruptions, all of much smaller volume.

During most of Glacier Peak's eruptive episodes, lava domes have extruded onto the volcano's summit or steep flanks. Parts of these domes collapsed repeatedly to produce pyroclastic flows and ash clouds. The remnants of prehistoric lava domes make up Glacier Peak's main summit as well as its "false summit" known as Disappointment Peak. Pyroclastic-flow deposits cover the valley floors east and west of the volcano. Ridges east of the summit are mantled by deposits from ash clouds.

Past eruptions have severely affected river valleys that head on Glacier Peak. Pyroclastic flows mixed with melted snow and glacial ice to form rapidly flowing slurries of rock and mud known as lahars.

About 13,100 years ago, dozens of eruption-generated lahars churned down the White Chuck, Suiattle, and Sauk Rivers, inundating valley floors. Lahars then flowed down both the North Fork Stillaguamish (then an outlet of the upper Sauk River) and Skagit Rivers to the sea. In the Stillaguamish River valley at Arlington, more than 60 miles downstream from Glacier Peak, lahars deposited more than seven feet of sediment. Shortly after the eruptions ended, the upper Sauk's course via the Stillaguamish was abandoned and the Sauk River began to drain only into the Skagit River, as it does today.

About 5,900 years ago and 1,800 years ago, dome-building eruptions generated lahars that extended once again to the sea, this time only along the Skagit River. In small eruptions since 1,800 years ago, lahars have extended the entire length of the White Chuck River and part way down the Suiattle.

Lahars can also be generated by landslides (also called flank collapses) on volcanoes, as has happened repeatedly at Glacier Peak's neighbor to the north, Mount Baker. At Mount Baker, lahars from numerous landslides, some without accompanying eruptive activity, have affected valley floors near the volcano. A few much larger landslides during eruptive periods generated lahars that flowed hundreds of feet deep through upper valleys and reached the sea. At Glacier Peak landslide-generated lahars have occurred less frequently than at Mount Baker.

See also

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