At times in the geologic past, almost a third of the surface of the earth was covered by thick glacial ice sheets. Glacier's movement across continents changed the landscape profoundly through extensive erosion, deposition of sediments and rock, and transportation. The deposition of glacial sediments formed extensive and gently rolling landscapes that we see today.
Let’s dig in and find out more.
By the end of this topic, you are expected to;
A glacier is a large, permanent mass of ice formed on land and moves under the gravity force. Glaciers grow when the accumulation is greater than the losses during the summer melting. They are also influenced by the steepness and elevation of the topography. For example, a steep mountain, even if above the snow line will not have a glacier since the snow can't stick and accumulate. Similarly, mountains at low elevations will not have glaciers.
Glaciers can be found in both polar and more temperate climates. They are the most abundant in the polar regions, where it remains so cold that only a minor amount of water is lost through melting or evaporation. They can also be found in the highest mountains in temperate or even tropical latitudes where temperatures remain cold throughout the year, such as in the Pacific Northwest of the United States and Canada, Alaska, and South America. More snow and ice accumulate during the winter months in these mountain ranges than is lost as meltwater in the summer.
About one‐tenth of the land surface on Earth is covered by glaciers today. Over 75 percent of this amount is on Antarctica, and 10 percent is on Greenland. The remainder occurs in mountain regions across the world.
Glaciers are categorized by their morphology, thermal characteristics, and behavior.
Alpine glaciers form on the crests and slopes of mountains. A glacier that fills a valley is called a valley glacier, or alternatively an alpine glacier or mountain glacier. A large body of glacial ice astride a mountain, mountain range, or a volcano is termed an ice cap or ice field. Ice caps have an area less than 50,000 km2 .
Piedmont glaciers are the forward-most extension of valley glaciers and form where the ice emerges at the front of the mountain range. The ice spreads out on the flat terrain to form a wide sheet at the mouth of the valley.
Glacial bodies larger than 50,000km2 are called ice sheets or continental glaciers. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70m (230ft). Portions of an ice sheet or cap that extend into water are called ice shelves; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called ice streams. In Antarctica, many ice streams drain into large ice shelves.
Tidewater glaciers are glaciers that terminate in the sea. As the ice reaches the sea, pieces break off, or calve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than those of other glaciers.
Glaciers are also classified by their thermal state.
Similarly, the thermal regime of a glacier is often described by its basal temperature.
A typical valley glacier adds snow at its head and loses to melt at its foot. Snow line refers to the line below which the annual snow cover is lost in summer. The region that is above the snow line is called the zone of accumulation; the region below is called the zone of wastage. In case it gains more than it loses, its terminus advances. In case it loses more than it gains, it retreats.
When a glacier experiences an increase in the slope of its bed, crevasses form where the surface is in tension and close where it is in compression. When a glacier encounters a steep slope in its bed flow may become chaotic as in an icefall. Seracs is the name given to the irregular ice blocks. They may be extremely unstable. Ice cannot hold a vertical wall more than a height of 40m (130ft). At the bottom of an icefall, the surface can be in strong compression and periodic waves known as ogives can form on the surface. The crevasse at the head of a glacier separating the moving ice from the stationary ice is known as bergschrund.
Glacial valleys normally have a characteristic U-shape with very little alluvial fill. They may have hanging tributaries. The steep-walled, semi-circular valley at the head of a glacier is called a cirque. Where two cirques intersect the narrow ridge is referred to as an arete. Aretes can intersect in a horn.
There is no general consensus on the causes of glaciation. Below are some of the leading hypotheses:
