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Mount St. Helens Glaciers and Glaciations

USGS glaciologist Melinda Brugman reports that about 70 percent, or 170 million cubic yards, of Mount St. Helens' glacial ice mass was lost during the May 18 eruption. All of Leschi and Loowit Glaciers, most of Wishbone Glacier, and the upper parts of Forsyth, Nelson, Ape, and Shoestring Glaciers disintegrated on May 18. Toutle and Talus Glaciers now appear to be significantly thinner than they were before; large amounts of snow were removed from the surfaces of these two glaciers and Shoestring Glacier by both the heat of tephra and scouring. Only Swift and Dryer Glaciers appear largely unchanged. Surprisingly, the melting rate of the surviving glaciers at their surfaces of contact with the mountain's rock flanks does not seem to have increased. Increased melting probably would cause accelerated downslope ice movement and (or) increased flows in streams still draining Mount St. Helens' glaciers, but neither effect has been seen so far.

Much of the Swift Creek assemblage accumulated during the last major glacial episode (Fraser Glaciation) in western Washington which occurred between about 25,000 and 10,000 years ago, and glacial drift is interbedded with deposits of volcanic origin in some areas. ...

The glacial drift that is interbedded with the Swift Creek assemblage at Mount St. Helens probably was deposited during the Evans Creek Stade of the Fraser Glaciation. This correlation is based on a comparison of such relative weathering features as depth of oxidation and thickness of weathered rinds on stones in glacial drift in this area with those described in other areas of western Washington. The Evans Creek Stade was originally thought to have occurred between about 25,000 and 15,000 years ago. Subsequent work has suggested that alpine glaciers in southwestern British Columbia began to advance after 20,000 years ago, and alpine glaciers on the west side of the Olympic Peninsula evidently reached their maximum extents before about 18,800 years ago. ...

Exposures of glacial drift in the vicinity of Mount St. Helens and in the Lewis River valley west of the volcano show that the area was glaciated at least three times prior to the Fraser Glaciation and prior to the formation of the Swift Creek assemblage. These earlier alpine glaciers extended at least 50 kilometers down the Lewis River valley than did the glacier of Fraser age. During these earlier glaciations all the area at, and adjacent to, the present site of Mount St. Helens, except perhaps the highest peaks, may have been covered by glaciers.

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The conspicuous notch in the east rim of Mount St. Helens is the truncated valley cut by the Shoestring Glacier. This glacier formerly was fed by an ice and snow field at the summit of Mount St. Helens. The eruption of May 18, 1980, decapitated the glacier and removed approximately three-quarters of its original volume.

The May 18, 1980, pyroclastic flow scoured the surface of Shoestring Glacier and incorporated its water, snow, and ice to form the lahars that swept down across Muddy fan. Shoestring Glacier was beheaded, and most of the glacier's snow and ice accumulation zone was removed when the upper parts of the mountain collapsed during the early moments of the eruption.

In the unique laboratory of Mount St. Helens, scientists that study glaciers and glacier caves are observing and documenting a newly formed glacier. Over the last 21 years, snow, ice and rock debris have accumulated behind the Lava Dome to an average depth of 100 meters (325 feet) thick. The snow has been stacking higher each year compressing the past years' snow into a dense crystalline ice body, as deep as 190 meters (600 feet). Giant cracks in the ice, called crevasses, and other flow features, indicate that the ice body is transforming into a glacier. Scientists, known as Glaciospeleologists, have been studying the movement and growth of the glacier as it creeps around both sides of the Lava Dome, flowing north.

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