Alpine glaciers

Alpine glaciers are masses of ice and snow that move slowly down from the peaks to produce the spectacular landforms associated with high mountain scenery. Steep horn-shaped or pyramidal peaks, rushing meltwater rivers, cliff-sided valleys with waterfalls—one associates these features with alpine mountains that have been eroded by glaciers. Where such glaciers are still active, they may threaten lives and property through catastrophic forward surges and floodwaters, or they may be essential sources of meltwater in dry areas.

Zones of Accumulation and Ablation

Alpine, or valley, glaciers are long, narrow streams of ice that originate in the snowfields and cirque basins of high mountain ranges and flow down preexisting stream valleys. They range from a few hundred meters to more than 100 kilometers in length. In many ways, they resemble river systems. They receive an input of water in the form of snow in the high parts of the mountains. They have a system of tributaries leading to a main trunk system. The flow direction is controlled by the valley that the glacier occupies, and, as the ice moves, it erodes and modifies the landscape over which it flows.

The essential parts of the mass balance of the alpine glacier system are the zone of accumulation, where there is a net gain of ice, and the zone of ablation, where the ice leaves the system by melting and evaporating. In the zone of accumulation, snow is transformed into glacial ice through a process of metamorphism, or change of form. Freshly fallen snow consists of delicate hexagonal (six-sided) ice crystals, or needles, with as much as 90 percent of the total volume as empty air space. As snow accumulates, the ice at the points of the snowflakes melts from the pressure of the snow buildup and migrates toward the center of the flake, where it refreezes. Eventually, many small, elliptical grains about the size of BB shot (about 0.45 centimeter in diameter) are formed of recrystallized ice. The accumulation of masses of these ice pellets is called firn, or névé. With repeated deposits, each year the loosely packed firn granules are compressed by the weight of the overlying snow. Meltwater, which results from daily temperature fluctuations and the pressure exerted by the overlying snow, seeps through the open pore spaces between the granules; when it refreezes, it adds to the recrystallization process. Air in the pore spaces is forced out. When the ice reaches a thickness of about 30 to 40 meters, it can no longer support its own weight and yields to slow plastic flow. The upper part of a glacier is thus rigid and tends to fracture, but the ice beneath moves by plastic deformation and flow.

On the surface of the alpine glacier, the boundary between the zone of accumulation and the zone of ablation is approximated by the equilibrium line. Above this line, the glacier's surface tends to be smooth and white because more new snow accumulates than is lost by melting, and all the irregularities are soon covered and smoothed with snow. These areas are dangerous to mountain climbers, who can fall into buried fractures or crevasses there. Below the equilibrium line, melting and evaporation exceed snowfall. There, the surface of the ice is rough and pitted and is commonly broken by open crevasses or streaked with rock debris.

The glacier ice flows and slides from the positive accumulation area to the negative wastage zone; each area's gain or loss constitutes the mass balance. Should a glacier have an excess of one over the other for tens to hundreds of years, it will advance or retreat in consequence. As glacial ice flows over the land beneath, it erodes, transports, and deposits vast amounts of rock and soil material. Glaciers erode in two ways: by glacial plucking, in which meltwater beneath the ice freezes blocks onto the passing ice, and by abrasion of the substrate by the rock blocks held fast in the overlying ice. Eroded material can be carried throughout the glacier, especially at the bottom and the top, and is later mainly deposited as moraines composed of till near the glacier's margins, where melting dominates.

Landforms

Alpine glacial landforms of the ice itself include various features attributable to the characteristic fracture and flow of the movement. Fracturing, or breaking, of the brittle ice occurs close to the glacier's surface, where the ice is under lower pressure. Flow of the ice occurs through recrystallization of ice crystals at depth in the glacier, where the greater pressure causes slow change and consequent movement.

At the head of an alpine glacier, where the ice pulls away from the wall of the mountain, a bergschrund (from the German word for “mountain crack”) crevasse develops in summer; during winter, it is filled with avalanche snow. Wherever a glacier moves over an irregular rock bed below, the ice on the surface fractures into various other crevasses. Above the equilibrium line, these features are usually covered with snow and thus are dangerous to traverse, but below, they are uncovered and pose far less hazard to mountain travelers. When crevasses, such as those described, create tumbled cliffs and icefalls, causing flow over a submerged cliff, the (originally French) term serac is used to describe them. Below such icefalls, where compressive flow occurs as the ice piles up, the glacier surface is commonly formed into a series of semicircular waves, or bands, called ogives. Ogives form when the broken crevasses collect dust and dirt during the summer melt season. In winter, only snow accumulates in the crevasses. Thus, when the dirty seracs close up and move away from the icefall, they form a dark band, whereas the snow-filled crevasses form a light band. Monitoring the formation and movement of these light and dark objects assists in investigating ice velocity.

At the front of an alpine glacier, some ice usually stagnates, and others may be forced to override it. The result commonly is sheafs of overthrust ice slabs that carry the debris forward into the terminal moraines. Ice that melts away in these regions produces considerable meltwater capable of eroding much sediment from the moraines. Because water is an efficient fluid, it sorts the sediment into different sizes: gravel, sand, silt, and clay. Redeposition of these sediments into layered deposits provides valuable sand and gravel supplies for construction in mountain areas.

Various landforms are produced by glacier meltwater processes. Kame terraces (mounds of sediment dropped from the meltwater) are formed between the glacier ice and the valley wall by the streams and lakes impounded there. Similarly, out in front of the ice, the valley train, or outwash, sediments are spread out into plains of sorted and stratified sediment. Commonly, blocks of ice are stranded in these sediments to melt away later and leave kettle holes and kettle lakes to pockmark the plain. Beneath the flowing ice of the alpine glacier, the bedrock will be abraded by the sediment carried along in the ice. This “glacial rasp” will groove, striate, and finally polish the bedrock over which it glides. Where blocks of bedrock are frozen onto the base of the ice and plucked out, a roche moutonnée (from the French for sheepskin-wig-shaped, or “curly,” rock) is formed, smoothed by abrasion on the up-ice side and rough and broken on the down-ice portion.

After retreat of an alpine glacier upvalley for a long time and wasting away of the ice, the eroded bedrock surface of roche moutonnée and polished and striated bedrock will finally be exposed. In the case of the surface beneath a former icefall, for example, a smoothed “cyclopean,” or glacial, stairway will emerge. The many intervening troughs eroded into the bedrock of the alpine valleys commonly become filled with small lakes that are linked by small overflowing streams. The resulting “paternoster” lakes are so named for their resemblance to beads on a chain. The valley walls themselves, having been undercut and eroded deeply by the ice, become exposed to show the characteristic U-shaped cross-sectional profile of glaciated valleys.

As alpine glaciers melt away entirely, finally the upper cirque basins high on the mountainsides will be exposed. These cirques commonly have steep headwalls, where the old bergschrunds were originally, and an overdeepened floor, where the flowing ice scooped out a basin. Many overdeepened cirque floors fill with water to form a small, steep-sided lake referred to as a tarn. The steep mountain peaks above the cirques are also steepened and undercut by the ice around them and form sharp glacial horns, the characteristic pyramidal peaks of alpine glacial regions. The famous Matterhorn of Switzerland is an example of this landform.

Where cirques have formed on opposite sides of mountain ridges, their headwalls may have merged through back-to-back or headward erosion of the glaciers on opposite sides. If not far advanced when the glaciers melt away, a knife-edged ridge may be the only result. In contrast, if the glaciation has continued for a long time, the cirques may merge and remove much of the intervening rock mass. After the ice has melted away, a low point, or col, will result. Many of the world's most famous mountain passes are cols formed in this fashion.

Principal Terms

ablation: the result of processes, mainly melting (evaporation is also involved), that remove ice and snow from a glacier

cirque: a steep-sided, gentle-floored, semicircular hollow produced by glacial erosion of bedrock high on mountain peaks

equilibrium line: the boundary between areas of mass balance gain and loss on a glacier's surface for any one year

Little Ice Age: a short-term cooling trend that lasted from about 1450 to 1850, during which mountain glaciers all over the world advanced considerably beyond their present limits

mass balance: the summation of the net gain and loss of ice and snow mass on a glacier in a year

moraine: a ridge of glacial-ice-deposited till

till: an ill-sorted mixture of fine and coarse rock debris deposited directly by glacial ice

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