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Glaciers and Glacier Hazards
Glaciers and Ice Sheets and Volcanic Eruptions

Glaciers exist where, over a period of years, snow remains after summer's end. They exist in environments of high and low precipitation and in many temperature regimes; they are found on all the continents except Australia and they span the globe from high altitudes in equatorial regions to the polar ice caps. There is a delicate balance between climatic factors that allows snow to remain beyond its season. ...

Scientists and skiers alike can note that within a few days of falling, snowflakes have noticeably begun to change. ... The snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is firn -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Several years are usually required for the snow to settle and to season into the substance we call glacier ice. ...

We can best determine the health of a glacier by looking at its mass balance. Each year glaciers yield either a net profit of new snow, a net loss of snow and ice, or their mass may remain in equilibrium. Scientists divide each glacier into upper and lower sections termed the accumulation area, where snowfall exceeds melting during a year; and the ablation area, where melting exceeds snowfall. An equilibrium line, where mass accumulation equals mass loss, separates these areas. You can see it as the boundary between the winter's snow and the older snow or ice surface. Its altitude changes annually with the glacier's mass balance. To find mass balance, scientists measure the area of each region and observe amounts of accumulation and ablation relative to preset stakes. After density measurements are made they may calculate how much water has been added or lost to the glacier. ...

After a series of positive mass balance years, the glacier may respond to the increased thickness by making a glacial advance downvalley. A series of negative years may cause a glacial retreat, meaning that the terminus is melting faster than the ice is moving downvalley. ...

Glaciers have been likened to mighty rivers of ice. Although they move many times more slowly, glaciers have equivalent changes in flow rate and often form falls of fast-moving ice above slow-moving ice pools. Glaciers flow faster down their centers than at ice margins, and more quickly at the surface than at the bed. ...

How fast a glacier moves is mostly dependent on the thickness of the ice, and on the angle of its surface slope. Glacier speeds vary when changes are made in this geometry. They respond to excessively high seasonal snow accumulations by generating bulges of thicker ice that may move downvalley many times faster than the glacier's normal velocity. ...

MORE Glacier and Related Terminology

Among the more prominent theories of events that have triggered global climatic changes and lead to repeated glaciation are: (1) known astronomical variations in the orbital elements of the Earth (the so-called Milankovitch theory); (2) changes in energy output from the Sun; and (3) increases in volcanism that could have thrown more airborne volcanic material into the stratosphere, thereby creating a dust veil and lowered temperatures.

The years 1980, 1981, and 1982, for example, saw several major volcanic eruptions adding large quantities of particulate volcanic material and volatiles to the stratosphere, including the catastrophic eruption of Mount St. Helens, Washington, on May 18, 1980, and a large eruption of Mount Hekla, Iceland, on August 17, 1980. The 1982 series of eruptions from El Chichón volcano, Mexico, caused death and destruction in the populated area around the volcano, but a further reaching impact may result from the effect on Earth's climate because of the enormous ejection of volcanic material into the stratosphere.

The potential climatic effect of the Laki volcanic eruption in Iceland in 1783, the largest effusive (lava) volcanic eruption in historic time, was noted by the diplomat-scientist Benjamin Franklin in 1784, during one of his many sojourns in Paris. Franklin concluded that the introduction of large quantities of volcanic particles into the Earth's upper atmosphere could cause a reduction in surface temperature, because the particles would lessen the amount of solar energy reaching the Earth's surface. The catastrophic eruption of the Tambora volcano, Indonesia, in 1815 was followed by a so-called "year-without-a-summer." In New England, for example, frost occurred during each of the summer months in 1816.

MORE about Volcanoes and Weather

Rockfalls occur on unstable slopes high upon the mountain and in valleys near glacier termini. Sometimes the rising dust of rockfalls startles local residents by mimicking steam vents and volcanic eruptions. Glaciers undercut headwalls and valley walls, making slopes steep and unstable. Some moraines and debris-covered slopes have cores of slowly melting ice. Rocks often tumble from these unstable slopes. ...

Some smaller lahars may be triggered by the sudden release of water from cavities within or beneath the ice. We call these events glacial outburst floods, or jökulhlaups (an Icelandic term pronounced Yo-kul-hloips). They have occurred an numerous glaciers on the mountain (Mount Rainier). Jökulhlaups often become lahars when they incorporate the rock debris that lies within their path. ...

Because outburst floods are unpredictable, you should be alert when visiting valleys with glacier-fed streams, particularly on unusually hot or rainy days. If you are near a stream and hear a roaring sound coming from upvalley, or note a rapid rise in water level, move quickly up the stream embankment, away from the stream channel and to higher ground. Do not try to escape by moving downstream; debris flows move faster than you can run. ...

Non-magmatic debris avalanches are especially dangerous, because they can occur spontaneously, without any warning. Earthquakes, steam explosions, and intense rainstorms can trigger debris avalanches from parts of a volcano that have already been weakened by glacial erosion or hydrothermal activity. ...

... debris avalanches commonly contain enough water or incorporate enough water, snow, or ice to transform into debris flows. Debris flows are slurries of water and sediment (60 percent or more by volume) that look and behave much like flowing concrete. ... During the past 10,000 years, at least 60 debris flows of various sizes have moved down valleys that head at Mount Rainier. All these can be grouped into two categories, called cohesive and non-cohesive debris flows. Cohesive debris flows form when debris avalanches originate from water-rich, hydrothermally altered parts of the volcano. They are cohesive because they contain relatively large amounts of clay derived from chemically altered rocks. Non-cohesive debris flows, in contrast, contain relatively little clay. Mount Rainier's non-cohesive debris flows are triggered whenever water mixes with loose rock debris, such as the mixing of pyroclastic flows or pyroclastic surges with snow or ice; relatively small debris avalanches; unusually heavy rain; or abrupt release of water stored within glaciers. ...

Tuya: A volcano that erupted under a glacier.

Because volcanic activity in western Canada was contemporaneous with the ebb and flow of Cordilleran glaciations, many of the volcanoes display ice contact features. Mount Garibaldi itself is a supraglacial volcano which erupted onto a regional ice sheet. Others, such as Hoodoo Mountain, were contained within basins thawed in the ice and assumed the flat-topped form of tuyas. Still others, such as the subglacial mounds of the Clearwater Field, were erupted under glacial ice to form piles of pillow lava and hyaloclastite.

The Vatnajökull glacier in Europe is a temperate glacier covering about 8,300 square kilometers in the SE part of Iceland. Volcanic fissure systems of the Mid-Atlantic Ridge plate boundary are partly covered by the western part of the ice sheet. Two major volcanic centers lie beneath the ice, the Bardarbunga volcanic centre and the Grimsvötn volcanic centre both with large subglacial caldera depressions.

Grimsvötn, together with Mardarbunga and Kverkfjöll, lie beneath the vast Vatnajökull icecap in east-central Iceland. Due to its subglacial setting and remoteness, the geology of Grimsvötn is poorly known. The volcano has a composite central caldera, of which the southern rim is the only exposed part of the volcano. Only basaltic material is known to have erupted from Grimsvötn. ... A large geothermal area in the caldera, with an estimated heat output of 5000 megawatts (Bjornsson, 1974, 1983), continually supplies meltwater to an ice-covered caldera lake. Approximately every 4-10 years the accumulated water is released in glacial outburst floods (jökulhlaups) when the water level is high enough to lift its ice dam (Thorarinsson, 1953; Bjornsson, 1974, 1975; Bjornsson and Kristmannsdottir, 1984). Some jökulhlaups are associated with eruptions. In some instances it can be argued that the extra meltwater generated by a subglacial eruption triggered the draining of the caldera lake (for example, during the 1938 eruption). In other cases the sudden pressure release associated with draining may trigger an eruption (Thorarinsson, 1974).

Iceland Volcanoes and Volcanics Menu

Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jokulhlaups (glacier-outburst floods). ...

Glaciations and Ice Sheets Menu

( At Mount Hood) ... Jökulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers. In 1922, a dark debris flow issued from a crevasse high on Zigzag Glacier and moved 650 meters over the ice before entering another crevasse; this event initiated a scare that Mount Hood was erupting. The Ladd Glacier jokulhlaup in 1961 destroyed sections of the road around the west side of the mountain and partly undermined a tower of a major powerline. The Coe Glacier outburst occurred around 1963, causing a section of trail to be abandoned and the "round-the-mountain" trail to be rerouted farther from the glacier. Jökulhlaups from White River Glacier were reported in 1926, 1931, 1946, 1949, 1959, and 1968; the Highway 35 bridge over the White River was destroyed during each episode. The more frequent outbursts from White River Glacier may be due in part to an increase in size of the fumarole field at the head of the glacier at Crater Rock.

Outburst floods from White River Glacier (Mount Hood) have taken out numerous, lesser versions of the highway bridge. The aggrading valley floor downstream displays several surfaces formed during this century that can be differentiated by the size (age) of the trees growing on them. The sediment sources for the aggradation are White River Glacier and the canyons that are being cut into diamicts of Polallie and Old Maid age downstream from White River Glacier

Mount Hood Glaciers and Glaciations Menu

Because volcanic activity in western Canada was contemporaneous with the ebb and flow of Cordilleran glaciations, many of the volcanoes display ice contact features. Mount Garibaldi itself is a supraglacial volcano which erupted onto a regional ice sheet. Others, such as Hoodoo Mountain, were contained within basins thawed in the ice and assumed the flat-topped form of tuyas. Still others, such as the subglacial mounds of the Clearwater Field, were erupted under glacial ice to form piles of pillow lava and hyaloclastite.

Hoodoo Mountain lies west of the main axis of the Stikine Volcanic Belt. It consists of a symmetrical lava dome, approximately 6 kilometers in diameter, surrounded on three sides by alpine glaciers. Only its southern slope, which extends down to the floodplain of Iskut River is ice free. Hoodoo's steep sides and nearly flat 900-meter summit suggest it formed as a subglacial tuya when regional ice sheets covered all but the highest peaks of the northern Coast Mountains. Subaerial lava flows which rest on glacial till along Iskut River indicate that volcanic activity continued after retreat of the ice. Radiometric dates of 0.11 and 0.09 million years are consistent with the age of other ice-contact features in the Stikine Volcanic Belt.

Canada Volcanoes and Volcanics Menu

Glacial outburst floods at Mount Rainier result from sudden release of water stored within or at the base of glaciers. Outburst floods and the debris flows they often trigger pose a serious hazard in river valleys on the volcano. The peak discharge of an outburst flood may be greater than that of an extreme meteorological flood (such as the 100-year flood commonly considered in engineering practice) for any given stream valley. At least three dozen outburst floods have occurred during the 20th century. Bridges, roads, and National Park visitor facilities have been destroyed or damaged on about ten occasions since 1926. However, the effects of outburst floods are rarely noticeable outside the boundaries of Mount Rainier National Park. Because they commonly transform downvalley to debris flows, outburst floods are included with debris flows for purposes of hazard zonation. ... Glacial outburst floods at Mount Rainier are unrelated to volcanic activity. The best-studied outbursts those from South Tahoma Glacier are correlated with periods of unusually high temperatures or unusually heavy rain in summer or early autumn. The exact timing of outbursts is unpredictable, however.

The smallest, but most frequent, debris flows at Mount Rainier begin as glacial outburst floods, also called by the Icelandic term "jökulhlaup" (pronounced "yo-kul-h-loip"). Outburst floods at Mount Rainier form from sudden release of water stored at the base of glaciers or within the glacier ice. Outburst floods have been recorded from four glaciers on Mount Rainier: the Nisqually, Kautz, South Tahoma, and Winthrop glaciers. ...

Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, often by triggering landslides that mix with the flood waters. The transformation from water flood to debris flow occurs in areas where streams have eroded glacially derived sediments and sediment-rich, stagnant glacier ice that was stranded in valleys as glaciers thinned and retreated earlier in this century. As the stagnant ice melts over the next several decades, it will release its charge of sediment into the stream valleys. That sediment will potentially be incorporated into more debris flows if it is mobilized by outburst floods. ...

Because outburst floods are unpredictable, you should be alert when visiting valleys with glacier-fed streams, particularly on unusually hot or rainy days. If you are near a stream and hear a roaring sound coming from upvalley, or note a rapid rise in water level, move quickly up the stream embankment, away from the stream channel and to higher ground. Do not try to escape by moving downstream; debris flows move faster than you can run. Observe Park Service regulations, especially those provided for your safety in areas prone to debris flows. Here, as in most areas in other national parks, natural processes such as floods and debris flows are allowed to occur without human intervention.

Mount Rainier Glaciers and Glaciations Menu

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