Glacial Morphology: Formation and Features of Glaciers
Glacial Morphology
Glaciers
Glaciers can be defined as large natural accumulations of ice located on land that are animated by a translational motion.
Valley Glaciers and Alpine Glaciers
At high altitudes, glaciers can form because the temperature is low and the mountains receive a large amount of rainfall. Glaciers formed in high mountains are characteristically long and narrow because they occupy valleys formed earlier. They carry plastic ice formed at higher altitudes to lower levels where the temperature is higher and the ice melts (ablation).
Icecap
An icecap is a limited accumulation of ice towering over mountains and plateaus.
During glacial periods, ice caps advanced over adjacent low-lying areas, encompassing all forms of relief in their path. Consequently, large areas were buried under these giant masses of ice. Such an expansive mass of ice is called a Continental Glacier, Greenland Ice Sheet, or Inlandeis.
Glacier Flow
Snow accumulates at the top of a glacier in a cylindrical depression called a Cirque. This is the accumulation zone. The snow cover undergoes compaction and recrystallization to form névé. The flow of glacier ice carries excess ice out of the cirque and spreads it down the valley. Any major bedrock prominence creates a threshold that increases the speed of the ice and causes deep Seracs (tension cracks). The bottom of the glacier is the ablation zone. The glacier’s front or snout contains much debris. The most superficial layer of the glacier is brittle and fractures to become crevasses, while the ice below behaves like a plastic substance that moves slowly. The movement is faster in the center than at the edges. Different types of cracks and crevasses of varying sizes abound on the surface of glaciers. The intersection of crevasses can create blocks of ice called seracs. The cirque is a basin or depression with rocky and steep walls. This depression may contain a small lake or remnants of glacial action. A cirque originates from a pre-glacial, shallow, concave hollow located at a significant altitude and affected by erosion. Initially occupied by a small patch of ice, it gradually widens and deepens due to gelifraction and the abrasive action of the debris it carries.
Glacier Erosion
Most glacial ice is filled with rock fragments. These materials come from the bedrock on which the glacier moves or, in the case of alpine glaciers, from debris falling from the valley walls. Glaciers are capable of performing significant erosion:
- Nivation erosion: The process by which the glacier plucks and carries rocks crumbled by the recrystallization of ice within fractures.
- Abrasion erosion: Erosion caused by pieces of rock carried by the glacier rubbing against the bedrock.
Debris caused by erosion is located on the banks or in front of the glacier and deposited when the ice disappears.
Morphology due to Alpine Glaciers
Before glaciation, a mountainous region is carved entirely by weathering and river flows. The mountains are gently rounded without sharp dividing lines, although this is not always typical of mountains in wet climates. As glaciation progresses, snow accumulates in the headwaters of the higher valleys. In the first phase of glaciation, snow accumulates in depressions excavated by ice movement and the fragmentation of materials by the ice near the masses of compacted snow. The glaciers fill the valleys, and some tributaries merge with others. The rapid freezing and thawing of meltwater in the crevices above the cirque walls breaks the bare rock. Angular fragments fall or glide onto the snow and glacier, becoming incorporated into it. The cirque walls become increasingly steep and rough, replacing the smooth, gentle slopes of the original mountain mass. The intersection of opposite cirque walls is called an Arête. If three or more cirques develop together, a sharp peak arises at the intersection of their edges. In the Swiss Alps, such peaks are known as Horns. If the intersection of cirques forms over a pass, it is called a Col.
Moraines
Moraines are accumulations of rock fragments (blocks, pebbles, and gravel) and clay transported and deposited by a glacier.
- Debris carried by an alpine glacier within the ice and swept from the valley walls forms lateral moraines.
- When two lateral moraines of glacier tongues make contact, they give rise to a medial moraine, which lies on the ice in the center of the valley.
- At the end of the glacial tongue, the transported debris accumulates in a terminal moraine. These deposits have a concave, curved shape that points up the valley, and their ends are joined by the lateral moraines of each wall.
- If there are irregularities in the rate of retreat of the glacier front, causing temporary halts, recessional moraines are formed.
- The accumulation of rocky material under a glacier, carried by the glacier’s base, is called ground moraine.
The valley spurs that originally extended into the main channel before the ice age have been beveled by grinding ice and are called truncated spurs.
The motion of the glacier constantly digs and expands its channel. When the ice disappears, it leaves behind a deep glacial valley with a characteristic U-shaped cross-section. After the ice age, rivers flowing through these valleys erode them further, eventually transforming them into V-shaped valleys.
Tributary glaciers also carve U-shaped valleys, but their cross-sections are smaller, and their floors are at a higher altitude than the main valley floor. These are called hanging valleys.
Cirques and upper parts of valleys are often occupied by numerous small lakes called tarns. Larger valleys usually contain valley lakes.
Glacial Valleys and Fjords
When the base of a valley glacier opens to the sea below sea level, the sea penetrates the valley and pushes back the ice front, creating a narrow estuary known as a fjord. These can be formed by subsidence of the coast or by a rise in sea level after the glacial period.
Development of a Valley Glacier
- During the peak of glaciation, the U-shaped valley is filled with ice up to the level of small tributary valleys.
- After glaciation, the valley floor may be occupied by a river or lakes.
- If the river carries a heavy load of sediment, it can fill the valley floor with alluvium.
- If the glacial valley has been deepened below sea level, it is occupied by a stretch of water, resulting in a fjord.