Glacier

What is a Glacier?

A glacier is a persistent body of dense ice that moves downhill under its own weight. Imagine it as a giant river of ice, slowly carving the landscape over centuries. It forms where snow accumulation exceeds ablation (melting and sublimation) over many years, often spanning decades or even centuries.

Glaciers: A Global Perspective

Glaciers cover about 10% of Earth’s land surface, with vast ice sheets in polar regions containing 99% of glacial ice. The Himalayas, Andes, and a few high mountains in East Africa are the only places between latitudes 35°N and 35°S where glaciers occur. Pakistan has more glacial ice than any other country outside the polar regions.

Glacial Ice: A Vital Freshwater Reservoir

Glacial ice is the largest reservoir of fresh water on Earth, holding about 69% of the world’s freshwater. Glaciers store water as ice during colder seasons and release it later as meltwater when warmer temperatures cause them to melt. However, in high-altitude environments, the temperature difference may not be sufficient to release meltwater.

Glacial Features and Behavior

A glacier appears blue due to the absorption of light by water molecules and the lack of air bubbles. The word ‘glacier’ comes from French and is derived from Latin. Glaciers are categorized by their size, shape, and behavior, with alpine glaciers forming on mountain crests and slopes.

Glacial Types

A glacier that fills a valley is called a valley glacier or alpine glacier. A large body of glacial ice astride a mountain is termed an ice cap or ice field. Ice caps have less than 50,000 km2 in area by definition. Glaciers larger than 50,000 km2 are called ice sheets or continental glaciers.

Glacier Movement and Erosion

The rate of internal flow can be modeled by the Glen-Nye flow law: Σ = kτ^n, where σ is shear strain, τ is stress, n is a constant between 2-4 (typically 3), and k is a temperature-dependent constant. Glaciers move downhill due to gravity and internal ice deformation.

Glacial Processes and Landforms

A glacier consists of three zones: the accumulation zone where new snow exceeds loss, the ablation zone where loss exceeds gain, and the equilibrium line separating them. The accumulation zone accounts for 60-70% of the glacier’s surface area.

Glacier Health Indicators

Glacier health is assessed by its mass balance or terminus behavior. Healthy glaciers have large accumulation zones, vigorous flow at the terminus, and significant snow cover. A glacier forms where accumulation exceeds ablation in a cirque landform.

Subglacial Processes

Subglacial processes include movement of glacial motion occurs in ice-bed contact due to bed temperature, roughness and softness defining basal shear stress. Soft beds allow for sediment sliding, while hard beds require basal sliding with meltwater forming between ice and bed.

Glacial Impact on Topography

Pre-glacial hollows will be deepened and topography will be amplified by glacial action; nunataks barely erode at all due to low erosion rates. Fjords can reach a kilometer in depth as ice is steered into them, making inland fjord extension sensitive to changes in climate and ocean.

Glacial Crevasses and Features

Crevasses can form transversely or longitudinally, depending on the direction of glacier movement; they can create isolated peaks in ice called seracs and form bergschrunds where moving ice separates from stagnant ice. Crevasses make travel over glaciers hazardous.

Glacial Meltwater and Sediment Transport

Glacier meltwater pools in stream channels below equilibrium line or descends into moulins. Streams flow in englacial or sub-glacial tunnels that sometimes reemerge at the surface. Subglacial processes include motion of glacial motion occurs in ice-bed contact due to bed temperature, roughness and softness defining basal shear stress.

Glacial Deposition

Glaciers erode terrain through two processes: plucking and abrasion. Plucking occurs when subglacial water freezes and expands, lifting rock blocks into the ice, while abrasion happens as the ice and debris slide over bedrock, smoothing it out.

Glacial Deposits

Glacial deposits are of two types: 1. Glacial till: mixture of undifferentiated material ranging from clay size to boulders; 2. Fluvial and outwash sediments: sediments deposited by water, stratified by size.

Glacial Moraines

Glacial moraines form where deposition of material from a glacier occurs. They appear as linear mounds of till. Types include terminal, lateral, medial, and ground moraines. Drumlins: asymmetrical, canoe-shaped hills made mainly of glacial sediments.

Glacial Valleys and Other Features

Glaciation widens, deepens, and smooths mountain valleys creating U-shaped glacial valleys or glacial troughs. Creates truncated spurs and paternoster lakes. Fjords form when a glacial valley runs into a large body of water. Therefore, when glaciers recede, the valleys of the tributary glaciers remain above the main glacier’s depression and are called hanging valleys.

Cirques and Arêtes

At the start of a classic valley glacier is a bowl-shaped cirque, which have escarped walls on three sides but is open on the side that descends into the valley called the ‘lip.’ Two glacial cirques may form back to back and erode their backwalls until only a narrow ridge, called an arête is left. This structure may result in a mountain pass.

Roches Moutonnées

Passage of glacial ice over an area of bedrock may cause the rock to be sculpted into a knoll called a roche moutonnée, or ‘sheepback’ rock. Roches moutonnées may be elongated, rounded and asymmetrical in shape.

Glacial Depositional Features

Passage of glacial ice over an area of bedrock may cause the rock to be sculpted into a knoll called a roche moutonnée. As the water that rises from the ablation zone moves away from the glacier, it carries fine eroded sediments with it. The water thus gradually deposits the sediment as it runs, creating an alluvial plain.

Glacial Deposits and Isostatic Rebound

When a glacier’s size shrinks below a critical point, its flow stops and it becomes stationary. Meanwhile, meltwater within and beneath the ice leaves stratified alluvial deposits in the forms of columns, terraces and clusters, remaining after the glacier melts.

Polar Ice Caps on Other Planets

The polar ice caps of Mars show geologic evidence of glacial deposits. Topographical features and computer models indicate more glaciers existed in the past. Martian glaciers are affected by the thin atmosphere and sublimation due to low atmospheric pressure.

Glaciers and Climate Change

Glaciers used to track climate change are melting due to global warming caused by human activities. The concentration of carbon dioxide and other greenhouse gases in the air has increased, causing current global warming. The flow velocity of glaciers accelerates and slows down due to climate change.

Impacts of Glacial Melting

Water runoff from melting glaciers causes global sea level rise. Impacts include encroachment on coastal settlements, existential threats to small islands, losses of coastal ecosystems, and compounding damage from tropical cyclones.

Isostatic Rebound

The polar ice caps of Mars show geologic evidence of glacial deposits, with topographical features and computer models indicating more glaciers existed in the past. Martian glaciers are affected by the thin atmosphere and sublimation due to low atmospheric pressure.

Glaciers are not just frozen rivers; they are dynamic forces that shape our landscapes and tell the story of Earth’s climate. As we face the challenges of global warming, understanding glaciers is crucial for predicting future changes and protecting our planet.