A glacier is a persistent body of dense ice that forms on land and moves under its own weight. Glaciers develop where snow accumulation exceeds melt over many years, compacting and recrystallizing into glacial ice. They come in various forms and sizes, shaped by their environment and physical properties, leading to diverse classifications.
Glaciers Defined by Their Geographic Setting
Glaciers are broadly categorized by their geographic location and shape, primarily into continental and mountain glaciers. Continental glaciers are vast, unconfined ice masses covering extensive land areas, often obscuring underlying topography. They flow outwards from a central accumulation zone. Mountain glaciers, also known as alpine glaciers, originate in high mountainous regions. Unlike continental glaciers, they are confined by surrounding terrain, flowing down valleys and through mountain passes.
Major Forms of Continental Glaciers
Continental glaciers include immense ice bodies spreading across large landmasses. The most expansive are ice sheets, masses of glacial ice covering over 50,000 square kilometers. Earth has two primary ice sheets: the Antarctic and Greenland Ice Sheets, containing over 99 percent of the planet’s land ice and much of its freshwater. These ice sheets can be several kilometers thick, with the Antarctic Ice Sheet reaching nearly 4.9 kilometers at its thickest. They spread outwards from their thickest central regions.
Smaller than ice sheets are ice caps, dome-shaped glacier masses covering less than 50,000 square kilometers. Ice caps form in high-latitude polar and subpolar mountain regions, often covering plateaus or islands. They flow outwards in all directions, similar to ice sheets, but are less constrained by underlying topography than mountain glaciers. Examples include Iceland’s Vatnajökull and Norway’s Austfonna ice caps.
Major Forms of Mountain Glaciers
Mountain glaciers exhibit several distinct forms. Valley glaciers resemble frozen rivers, flowing down pre-existing mountain valleys. They are longer than wide and carve characteristic U-shaped valleys as they move, eroding the landscape through plucking and abrasion.
Cirque glaciers occupy bowl-shaped depressions called cirques, or corries, on mountain sides. These amphitheater-like basins, with steep walls, accumulate snow and ice, often augmented by avalanches. If a cirque glacier grows sufficiently, it can flow out and develop into a valley glacier.
Piedmont glaciers form when valley glaciers flow out of their confining mountain valleys onto broad, flat plains at the base of mountains. As the ice exits the constricted valley, it expands laterally, creating a distinctive lobe-like or fan-shaped formation. Alaska’s Malaspina Glacier is a well-known example.
Tidewater glaciers extend to the ocean or a large body of water. They lose mass primarily through calving, where large ice chunks break off their terminus and fall into the water, forming icebergs. Tidewater glaciers can be grounded, in contact with the seafloor, or floating, with their terminus resting on the water. Alaska’s Hubbard Glacier is an example with a calving face over ten kilometers long.
Glaciers Classified by Temperature
Beyond physical form and location, glaciers are also classified by internal temperature, which influences their behavior and movement. Temperate glaciers have ice at or near its melting point throughout their mass. Liquid water within the ice allows for faster movement through basal sliding. This type of glacier is common in regions with seasonal temperature fluctuations.
In contrast, polar glaciers, also known as cold glaciers, remain well below freezing from surface to base year-round. Without internal meltwater, their movement is primarily through internal deformation, a slower process. These glaciers are found in cold, high-latitude environments where temperatures rarely rise above freezing.
Subpolar glaciers represent an intermediate thermal classification, possessing characteristics of both temperate and polar types. They have a cold, frozen surface layer, but their deeper sections or base can be at the melting point. This allows for basal sliding, where a thin film of meltwater at the ice-ground interface can facilitate movement, even if upper layers remain frozen.