Snow Accumulation and Ablation
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. We shall examine the climate at Mount Rainier and then, how climatic factors affect a glacier's lifespan.
Mount Rainier towers over the surrounding Cascade peaks and acts as an impediment to eastbound clouds blowing from the Pacific Ocean. It causes air to rise and cool, and to condense its load of moisture as rain or snow. The glaciers on the mountain's slopes are as dependent on the abundant precipitation as are the forests of fir and cedar on the slopes below. Some of the largest snowpacks recorded in the world occur high on the mountain. For example, at Paradise a record 1,122 inches of snow fell between July, 1971 and July, 1972 (written commun., U.S.Weather Service).
At altitudes above Paradise, scientists find it difficult to measure accumulation accurately. The wind makes mockery of their recording gauges by depositing snow unpredictably. Snow deposits may accumulate by direct precipitation, by avalanching, or by wind. For instance, during winter it is not uncommon to find barren wind-blown ridges on the slopes above valleys with deep snow deposits. In addition, wind, relative humidity and precipitation are variable around the mountain. By these capricious agents, snow layers are accumulated.
ABLATION is the loss of snow or ice by evaporation and melting. The rate
at which ablation occurs depends on the atmospheric conditions present, such as
air moisture content, solar radiation, temperature, and the reflectivity
(ALBEDO) of the snow or ice surface. Fresh snow has a high albedo (0.7 to 0.9),
indicating that 70 to 90 percent of the radiation received is reflected; glacier
ice has a lower albedo of 0.2 to 0.4 (Patterson, 1981).
Therefore, more radiation may be absorbed by glacier ice than by snow. Glaciers
around the mountain receive different amounts of sunlight, so each glacier has
its own characteristic ablation pattern.
Scientists and skiers alike can note that within a few days of falling, snowflakes have noticeably begun to change. If we could sit comfortably inside a crevasse for an entire year with a magnifying glass and examine a layer of newly fallen snow, we might see some of these processes taking place. We might see the points break off as they settle to the surface or as they are blown by the wind. 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.
Glaciologists measure snowpack DENSITY frequently so that they may
anticipate future water supplies, and to assess avalanche hazards. The density
of a fresh snowpack is about 0.1; firn has a density of about 0.55 and glacier
ice, of about 0.89 (Sharp, 1960). Each annual snow layer has a characteristic
grain size and density -- a record of the season's storm conditions and of
atmospheric contaminants. Buried pollen, dust, and volcanic ash give us clues
to understanding the age of each layer and the environmental conditions present
when it was formed.
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.
Using photographs taken in autumn of the year, you may examine the location of the equilibrium line during different years. A lowered equilibrium line indicates a larger accumulation area and more positive mass balance; a raised equilibrium line indicates a smaller accumulation area and a more negative mass balance.
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.
Because each glacier has a unique valley geometry and setting on the mountain,
each responds slightly differently to the same climatic event. Only after a lag
period unique to each glacier will a higher-than-normal snow accumulation cause
an advance at the glacier terminus. The general response of Mount Rainier's
larger glaciers to climate appears to be somewhat synchronous when viewed over a
period of years, though the extent of advance or reteat varies dramatically
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.
We have been successful in observing glacial movement, but only partially successful in understanding the mechanisms that control it. Some glaciologists say that ice is a PERFECTLY PLASTIC substance. (That is, brittle and capable of cracking like a solid, yet deformable and capable of flowing at other stresses.) (Patterson, 1981). Glaciologists have defined two distinct types of glacial movement -- deformation of the ice, and sliding of the glacier upon its rock bed. You can see where deformation has taken place by observing the wavelike flow patterns within the ice. Near the equilibrium line on Nisqually Glacier perhaps 5 to 20 percent of the glacier's movement is caused by ice deformation; 80 to 95 percent of its movement is caused by sliding of the entire glacier upon its bed (Hodge, 1974). Note the effects of glacial sliding where the ice has towed rocks that scratched the bedrock.
How fast a glacier moves is mostly dependent on the thickness of the ice, and on the angle of its surface slope (Patterson, 1981). 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 (Patterson, 1981). We can measure those KINEMATIC WAVES using instruments to survey the glacier surface. These waves leave a legacy of severely cracked ice and often advance the glacier terminus. Kinematic waves may occur on all large glaciers. At Mount Rainier, their behavior has been studied on Nisqually Glacier. One of the most complete records of kinematic wave movement is shown in figure 8 (not online). When measured during May 1969, at its equilibrium line, Nisqually Glacier moved about 18 inches per day; during the preceding November it moved only 8 inches per day (Hodge, 1974). Glaciologists disagree about exactly how water flows inside and beneath glaciers. However, they agree that these changes in speed are likely indicators of maximum and minimum water storage in cavities within the ice.
Look for crevasses -- cracks in the glacier surface that should not be
confused with crevices that formed in rocks.
Crevasses form where the speed of the ice is variable, such as in icefalls
and at valley bends. The surface may appear blistered with crevasses where the
ice flows over bedrock knobs and ridges.Crevasses are especially visible in
figures 5, 19, 16, 18, 19, and 34 (not online).
It has been said that good scenery and interesting rocks usually go together, and Mount Rainier is no exception. Here, a complex plot ahs been written where glaciers are reducing the mountain upon which they are sustained. Although volcanism and landslides are responsible for the overall shape of the mountain, glaciers are responsible for its fine sculpturing. The Emmons, Winthrop, and Fryingpan Glaciers exist in scars remaining from the landslide that produced the Osceola mudflow. Other glaciers, such as the Puyallup and Tahoma Glaciers, exist in the scars of smaller slides. Even the summit crater, formed by volcanism, is filled with snow and firn. All the glaciers on Mount Rainier are products of volcanism in that it has built a high altitude environment favorable to glacier formation.
But the glaciers themselves are slowly reducing the size of the mountain they depend on. Glacier ice alone is too soft to be a powerful rock-cutting agent. Many glaciers are armed with rock fragments embedded within the ice that are effective cutting tools. The rock-choked ice grazes over the glacier bed, removing rock obstacles and leaving the bedrock rounded and smoothed. In some places find-grained debris polishes the bedrock to a lustrous surface finish called GLACIAL POLISH. Coarser rocks may gouge scratches called STRIATIONS.
Bedrock knobs are commonly polished on their upper side and are quarried and broken on the lower. These rounded knobs are formed in all sizes. Observers in the 1700's thought they resembled fashionable wavy wigs of their day and named them ROUCHES MOUTONNEES(figure 3(not online))(Embleton and King, 1969). Look for them in the vicinity of Paradise Glacier. Look for glacial polish in regions of exposed bedrock, such as in front of Paradise Glacier, on valley walls above the trail to Carbon Glacier, at Box Canyon, and elsewhere in valleys around the park.
Frost action and running water are responsible for additional erosion. Broken rock and ice continually avalanche from walls surrounding glaciers, sandwiching rocks between fresh layers of snow on their journey toward the terminus.
Look for evidence of glacial erosion in the streams and rivers around the park. First, observe the streamwater originating in snowfields. Because no major erosion occurs here, the water appears clear. Those rivers originating beneath glaciers are choked with GLACIAL FLOUR, the silty fine-grained sediment produced by the abrasion of rocks at the glacier bed.
Not all glacier beds are alike. Some glaciers rest upon sturdy bedrock foundations, while others engulf so much rock that there is no distinct boundary between the glacier and its bed. As glaciers melt, their remaining load or rocks is distributed in several ways. Rocks may be dropped in place by the melting ice; they may be rolled to the ice margins, or they may be deposited by meltwater streams. Collectively, these deposits are called GLACIAL DRIFT. TILL refers to the debris deposited directly by the glacier (Sugden and John, 1976). Rock debris rolls off the glacier edges and builds piles of loose unconsolidated rocks called GLACIER MORAINE. LATERAL MORAINES form along the side of a glacier and curl into a TERMINAL MORAINE at the glacier's downvalley end. Drift and moraines are valuable to geologists because they outline the boundaries of past glaciations. At Mount Rainier look for aging lateral moraines perched precariously above the present glaciers with their terminal moraines breached by glacial streams.
TRIMLINES are the sharp vegetative boundaries that delimit the upper margins of a former glaciation. The age difference of the ground surfaces is commonly visible because of the corresponding age differences of the vegetation. Look for trimlines in all glaciated valleys of the park.
Scientists studying past climatic trends have developed ingenious techniques for determining the ages of drift and moraines. For instance, they measure the amount of weathering on the exterior of cobbles (Porter, 1975); ascertain the presence of absence of dated volcanic ash (Mullineaux, 1974); determine how long a moraine has been ice-free by noting the age of the oldest trees on it or by measuring the largest lichens on its surface (Sigafoos and Hendricks, 1972); and they examine historical accounts (Russell, 1898).
Rockfalls from Sunset Amphitheatre, Willis Wall and Little Tahoma Peak are, in part, responsible for the debris on the surfaces of glaciers below. Indeed, there may be enough debris to control glacier behavior. On the glacier surface the darker rock absorbs heat, which causes the ice to melt faster than if the ice were exposed. For example, a 1/4-inch dusting of volcanic ash may increase surface melting by 90 percent. When the ash layer is one inch or more thick, it will insulate the snow or ice below (Driedger, 1981). Other rockfall deposits are thick enough to insulate the ice and inhibit melting substantially.
One reason why glacial erosion occurs so quickly on the mountain is the crumbly nature of its rock. The porous volcanic rock may be structurally weakened by HYDROTHERMAL ALTERATION, a slow geothermal cooking of the rocks in the presence of ground water. The weakened rock often collapses and exposes cliffs yellow or rust in color (see figure 18(not online)). This unstable rock is, in part, responsible for the numerous rockslides and mudflows on the mountain.
The presence of hydrothermally-altered rock near glacial meltwater can create a devastating combination. Mudflows and debris flows (called LAHARS when on the slopes of a volcano) have left numerous deposits on the flanks of Mount Rainier (Crandell, 1971).
One of the largest lahar events -- indeed one of the largest known on earth -- was the Osceola mudflow, which occurred 5,700 years ago. It is likely that rocks and mud moved in surges down the White River valley to the present location of Kent -- a distance of 70 miles for its source on the mountain (Crandell, 1971). The Electron mudflow, 600 years ago, easily eroded the rock near the mountain's northwestern summit to form Sunset Amphitheatre (Crandell, 1971). These lahars may have originated with volcanic heating of the mountain (Crandell, 1971).
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 JOKULHLAUPS (an Icelandic term pronounced Yo-kul-hloips). They have occurred at numerous glaciers on the mountain. Jokulhlaups often become lahars when they incorporate the rock debris that lies within their path. Some of these events are described later in the book (not online).
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