DESCRIPTION:
Glacier Peak Volcano, Washington

Location: Washington

Latitude: 48.112 N

Longitude: 121.113 W

Height: 3,213 Meters (10,541 Feet)

Type: Stratovolcano

Number of eruptions in past 200 years: More than 1 (?) (Before 1800) ²

Latest Eruptions: About 200-300 years ago; 1,000-1,800 years ago; 1,800-2,800 years ago ³.

Present thermal activity: Two hot springs on the volcano's lower flanks.

Remarks: Eruptions have characteristically produced large volumes of volcanic ash and airborne pumice that could endanger closest centers of population ³.

Glacier Peak is not prominently visible from any major metropolitan centers, and so its attractions, as well as its hazards, tend to be overlooked. Yet, Glacier Peak has produced larger and more explosive eruptions than any other Washington volcano except Mount St. Helens. In the past 14,000 years, Glacier Peak has erupted at least a dozen times, most recently around the eighteenth century.

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 over-looked. 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 lies 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. These pages describe some of the effects of past eruptions and possible consequences of future activity.

Glacier Peak (3,213 meters) is a small Cascade Range stratovolcano. Although its summit reaches greater then 3,000 meters above the surrounding valleys, the main cone of Glacier Peak is perched on a high ridge, and the volcanic pile is no more than 500-1,000 meters thick. More than a dozen glaciers occur on the flanks of the volcano, and unconsolidated pyroclastic deposits over 12,000 years old have been largely removed by glaciation. Lava flows locally cap ridges to the northeast of the volcano, indicating a topographic reversal, and glacial and fluvial downcutting of more than 2,000 meters has occurred since the earliest cone-building eruptions. While small basaltic flows and cones are found at several points around the flanks of Glacier Peak, the main edifice is largely dacite and andesite. Lava flows extend no more than a few kilometers from the summit.

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.

Indian legends and a thin tephra fall preserved east of the volcano may record a recent eruption in the 18th century, although no eruptive activity has occurred during at least the last 150 years.

For thousands of years Native Americans traveled through this area to the meadows and peaks of the North Cascades in search of plants and game. The first white man to record seeing Glacier Peak was Daniel Linsley, surveying in 1870 for a possible railroad route for the Northern Pacific Railroad. Mining claims were filed in the Glacier Peak area during the North Cascades "gold rush" of the 1880s and 1890s. Except for the Holden Mine, just east of the Wilderness, all that remains of early day mining and trapping activities are abandoned mine shafts, rusting equipment, and a few rotting cabins.

Glacier Peak is probably best known as the source of voluminous tephra eruptions dated to 11,250 years B.P. Two tephra layers produced at this time have been identified as far as 800-1,000 kilometers to the east, and are widely used by geologists, anthropologists, and paleoecologists to date late Pleistocene sediments. Also at this time, an extensive valley fill of pumiceous lahars and alluvium was deposited downriver to the west, blocking valleys and affecting drainages as far as 80 kilometers from the volcano.

After these major eruptions, Glacier Peak apparently was dormant for 6,000 years. The volcano rewoke 5,500-5,100 years B.P. and intermittent eruptions of pyroclastic flows and tephra have occurred since that time. perhaps the most dramatic geologic features at Glacier Peak are enormous and relatively undissected late Pleistocene and Holocene pyroclastic fans which almost completely fill valleys on the eastern and western flanks of the volcano.

Indian legends and a thin tephra fall preserved east of the volcano may record a recent eruption in the 18th century, although no eruptive activity has occurred during at least the last 150 years. Three hot springs surround the volcano, and warm ground and snow-free areas occur near fresh-appearing dacite domes which form subsidiary summits both north and south of the ice-covered main summit.

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 are called 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.

Glacier Peak, geographically the most remote of the Cascade volcanoes, is a Pleistocene and Holocene composite volcano composed chiefly of dacite, with a minor amount of basalt erupted from satellitic vents (Tabor and Crowder, 1969; Beget, 1982, 1983). Large explosive eruptions about 11,000- 12,000 years ago produced:

1. two tephra-fall deposits of large (>1 cubic kilometer, dense-rock equivalent) volume, which are widely distributed east of the volcano (Lemke and others, 1975; Porter, 1978; Sarna-Wojcicki and others, 1983; Mehringer and others, 1984),
2. seven tephra falls of small (0.01-0.1 cubic kilometers) volume (Porter, 1978), and
3. many pyroclastic-flow deposits and lahars that form thick (locally >100 meters) fills in the valleys that head on the volcano (Tabor and Crowder, 1969; Beget, 1982, 1983).

The two large tephra eruptions were separated in time by probably no more than a few centuries (Mehringer and others, 1984). Tephra of each eruption is about 1 meter thick at a distance of 50 kilometers downwind from the volcano, and about 0.5 meters thick at a distance of 70 kilometers (Porter, 1978). These deposits represent two of the largest Cascade tephra eruptions of postglacial time, although they are less voluminous than the tephra fall that accompanied the climactic eruption of Mount Mazama (about 34 cubic kilometers, dense-rock equivalent).

Pyroclastic flows associated with the eruptive period of 11,000-12,000 years ago traveled as far as 15 kilometers from the volcano, and lahars reached areas along the Stillaguamish and Skagit Rivers more than 100 kilometers from the volcano (Beget, 1982, 1983).

Beget (1982, 1983) also describes Holocene eruptions, associated with dome extrusion near the summit, which produced lahars, pyroclastic flows, and minor tephra. The tephra and pyroclastic flows were less extensive than those of the eruptive period of 11,000-12,000 years ago. Several Holocene lahars extended tens of kilometers downvalley, and two reached distances of more than 100 kilometers.

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