A continental glacier, also known as an ice sheet, represents the largest type of glacier on Earth. Unlike smaller counterparts confined to mountain valleys, this immense mass of ice covers vast areas of a landmass. These colossal ice bodies hold the majority of the planet’s freshwater, making them a significant component of the global climate and sea level system. Their sheer scale and slow, continuous movement reshape continents over geologic time.
Defining Characteristics and Scale
The defining characteristic of a continental glacier is its horizontal extent, which must be greater than 50,000 square kilometers to be classified as an “ice sheet.” Ice masses below this threshold are termed “ice caps,” but they share the same general morphology and flow characteristics of the larger ice sheets.
Continental glaciers form a massive, high-relief dome shape, with the thickest ice accumulating near the center. This central area, known as the ice dome, can reach thicknesses of 3,000 meters or more in present-day ice sheets. The immense weight of the ice creates an elevated plateau from which the ice gradually slopes and flows outward toward its margins.
Formation and Ice Sheet Dynamics
The formation of a continental glacier begins with persistent, heavy snowfall that does not melt completely during the summer. As layers of snow accumulate, the weight of the overlying snow compresses the lower layers, expelling air and increasing density. This process transforms the fluffy snow into a denser, granular substance called firn.
With further burial and compression, the firn recrystallizes into solid, interlocking glacial ice. Once the ice mass is thick enough, the force of gravity causes it to spread out radially from the central dome. This outward flow is maintained by two primary mechanisms: internal deformation (creep) and basal sliding.
Internal Deformation and Basal Sliding
Internal deformation occurs when the pressure on the ice causes the crystals to change shape and slide past one another. Basal sliding happens when meltwater forms a thin lubricating layer between the bottom of the ice and the bedrock, allowing the glacier to slide more easily. The glacier maintains a balance between the zone of accumulation, where snow is added, and the zone of ablation, where ice is lost through melting and calving.
Distinguishing Them from Alpine Glaciers
Continental glaciers differ significantly from alpine glaciers, which are often called valley glaciers. Alpine glaciers are strictly confined to mountain valleys, which dictate their shape and linear flow path as they move downhill. The most notable distinction lies in the control exerted by the underlying topography.
In contrast, a continental glacier is so extensive that its own mass, not the underlying landscape, determines its movement and shape. This results in a radial flow pattern, where ice spreads outward in all directions from the central high point, regardless of hills or valleys beneath. Alpine glaciers are also vastly smaller, typically measuring a few kilometers in length, while continental glaciers cover areas the size of entire continents.
Present-Day Global Ice Sheets
Today, only two landmasses host continental glaciers, or ice sheets: the Antarctic and Greenland Ice Sheets. These two colossal bodies contain more than 99% of the planet’s glacial ice. The Antarctic Ice Sheet is the largest single mass of ice on Earth, covering nearly 14 million square kilometers.
The Antarctic Ice Sheet is commonly divided into the larger, more stable East Antarctic Ice Sheet (EAIS) and the smaller, more dynamic West Antarctic Ice Sheet (WAIS). The Greenland Ice Sheet, while substantial at approximately 1.7 million square kilometers, is dwarfed by its southern counterpart. Outside of these two polar regions, the largest persistent ice masses are classified as ice caps or ice fields, such as the Patagonian Ice Fields in South America.