An ice cap is a large, permanent mass of glacial ice situated on land. It exists in regions where temperatures remain low enough for snow accumulation to consistently outweigh summer melting. The ice mass is characterized by a central dome from which ice flows outward in all directions due to the pressure of its own weight. These formations represent a substantial reservoir of freshwater that influences global systems.
Defining Ice Caps and Differentiating Features
An ice cap is formally defined by its size, covering a land area of less than 50,000 square kilometers. This definition distinguishes it from an ice sheet, which covers more than 50,000 square kilometers, such as the formations in Greenland and Antarctica. Ice caps generally possess a dome-like shape that largely obscures the underlying terrain, with the ice flowing radially from the highest central point, known as the ice divide.
The flow dynamics and size differentiate ice caps from other glacial features. A glacier is typically much smaller and is constrained by the topography of a valley, flowing only in one direction. Ice fields are similar in scale to ice caps but are distinct because their flow is strongly dictated by the underlying mountains, often resulting in rock peaks, called nunataks, protruding through the ice.
Geographic Distribution and Formation Processes
Ice caps are found in high-latitude polar and subpolar regions, and at high altitudes in mountain ranges. Key examples include the Vatnajökull ice cap in Iceland and the Austfonna ice cap in the Svalbard archipelago of Norway. The world’s largest ice cap is the Severny Island ice cap, located in the Russian Arctic.
The formation of an ice cap requires a long-term climate balance where annual snowfall consistently exceeds the amount of ice lost to melting and sublimation. The process begins with fresh snow accumulating. The weight of the overlying snow compacts the lower layers, expelling air and causing the snowflakes to recrystallize into a denser, granular material called firn.
This process, known as firnification, continues as the pressure increases with new snowfall. When the firn is buried to a depth of approximately 45 to 60 meters, the air pockets are sealed off, and the material transforms into dense, impermeable glacial ice. The continuous accumulation and compression create the mass that subsequently begins to flow outward, giving rise to the characteristic dome shape of the ice cap.
The Global Role of Ice Caps
Ice caps regulate global temperatures through the albedo effect. Their highly reflective white surface reflects a large portion of incoming solar radiation back into space, limiting the amount of heat the planet absorbs.
Ice caps are a significant component of the global water cycle, serving as vast reservoirs of frozen freshwater. While the ice sheets of Greenland and Antarctica hold the majority of the world’s ice, the combined mass of all ice caps and glaciers represents a substantial reserve that feeds downstream ecosystems and water supplies.
The melting of land-based ice caps is a primary driver of global sea level change, as the meltwater adds new volume to the oceans. Despite their smaller size compared to ice sheets, glaciers and ice caps contribute a significant percentage of sea level rise. Furthermore, the layered ice within these formations contains trapped air bubbles and dust, which provide scientists with historical records of past atmospheric composition and temperature fluctuations.
Monitoring Ice Cap Change
Scientists monitor ice caps to track changes in their mass balance, which is the net difference between snow accumulation and ice loss. One effective method is satellite gravimetry, using the Gravity Recovery and Climate Experiment (GRACE and GRACE-FO) missions. These satellites measure shifts in Earth’s gravitational field that are proportional to the mass of the ice beneath them, allowing scientists to track mass change in gigatons.
Another technique is satellite altimetry, which uses laser or radar to precisely measure the changing height of the ice cap’s surface. This provides volume change data, which complements the mass change data from gravimetry. High-resolution satellite imagery and remote sensing are also used to track the physical retreat of the ice cap’s margins and measure the flow speed of the ice.