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Mount St. Helens Glaciers and Glaciations

Mount St. Helens Glaciers
Mount St. Helens Glaciations and Ice Sheets
Shoestring Glacier
The Newest Glacier

Mount St. Helens Glaciers

From: Foxworthy and Hill, 1982, Volcanic Eruptions of 1980 at Mount St. Helens: The First 100 Days: USGS Professional Paper 1249, 125p.

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.

Mount St. Helens Glaciations and Ice Sheets

From: Hyde, 1975, Upper Pleistocene Pyroclastic-Flow Deposits and Lahars South of Mount St. Helens Volcano, Washington: USGS Bulletin 1383-B

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.

Glacier Extents During Swift Creek Time -- Hyde, 1975

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Shoestring Glacier

From: Doukas, 1990, Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington: U. S. Geological Survey Bulletin 1859, 53p.

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.

From: Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity: Washington Department of Natural Resources, Division of Geology and Earth Resources Information Circular 88.

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.

The Newest Glacier

Mount St. Helens 2002 Crater Map

From: USFS Volcano Review, Summer 2002, contribution by Charlie Anderson, Director of the International Glaciospeleological Survey

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.

August 31: Is there a New Glacier at Mount St. Helens? Mapping the Growing Snow and Ice Mass in the Crater
Steve Schilling, USGS, Cascades Volcano Observatory, Vancouver, WA

Over the last 20 years snow, ice and rock debris in excess of 300 feet thick have accumulated behind the lava dome. A new glacier may be forming in the crater of Mount St. Helens. Accumulated snow and ice poses a significant flood hazard in the event of a future eruption. A small eruption in 1982 melted snow within the crater and formed a large, debris flow that swept down the Toutle River valley. A similar eruption today could generate a comparably sized, or perhaps larger, flood and debris flow. Learn how current efforts to map the growing ice mass are contributing to our understanding of the potential hazards of future eruptions.

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Mount St. Helens Snowpack and Ice Accumulation

Snow and Ice

From: Meyer, 1987, Post-Eruption Changes in Channel Geometry of Streams in the Toutle River Drainage Basin, 1983-85, Mount St. Helens, Washington: USGS Open-File Report 87-549

Mount St. Helens is located on the west side of the Cascade Range in southern Washington. Average annual precipitation in the Toutle River drainage basin varies from about 1,100 millimeters (45 inches) in the valleys near sea level, to more than 3,500 millimeters (140 inches) above 1,000 meters (3,300 feet). Most precipitation occurs during a well-defined winter rainy season which begins about October, peaks in December or January, and declines into the spring. During this period, precipitation usually occurs as light to moderate intensity rain, rather than as heavy rains. Freezing level may vary from sea level to above 1,000 meters (3,300 feet) throughout the winter. Midwinter snow line typically lies between about 750 - 1,200 meters (2,500 - 4,000 feet) above sea level. Snow accumulation varies from zero at lower elevations to more than 7.6 meters (25 feet) above 1,500 meters (5,000 feet). Density of the snowpack increases from about 25 to 45 percent water equivalent between early winter and April (Pacific Northwest River Basin Commission, 1970). The largest flows (both pre-and post eruption) occur when relatively warm, heavy rains fall on snow, and snow melt supplements the rainfall runoff.

A large volume of snow and ice is presently accumulating in the Mount St. Helens crater, protected by the shade of the high, steep crater walls. This accumulation provides a growing potential water source for lahars in the North Fork Toutle River valley. It is already mixed with rock debris eroded from the crater walls, and this debris would augment the formation of a lahar. It is possible that a large eruption could melt most or all of this snow and ice in a matter of tens of minutes. A very small eruption in 1982 rapidly melted enough snow and ice in the crater to trigger a 4 million cubic meters (5.2 million cubic yards) flood that transformed into a lahar and flowed all the way to the Cowlitz River. At the present time (1995), about 53 million cubic meters (70 million cubic yards) of snow and ice has accumulated. If completely melted, this would produce about 38 million cubic meters (50 million cubic yards) of water. At the present rate of accumulation, the volume of snow and ice will double in about 15 years.

Permanent and seasonal snow and ice also blanket the outer flanks of Mount St. Helens. A sufficient volume exists there in winter or spring to produce flank lahars similar in magnitude to those of May 18, 1980, if another large eruption were to occur. Lahars formed on the outer flanks can be expected to be substantially smaller than flows generated in the crater.

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