An ice cap is a mass of glacier ice covering a relatively extensive area of land and spreading outward in several directions, but it is smaller than an ice sheet under the conventional glaciological definition. The commonly used threshold distinguishes ice caps from ice sheets at approximately 50,000 square kilometres, although terminology can vary in particular geographical contexts. An ice cap can cover highlands, mountains and plateaus, with the ice generally flowing outward from a central or elevated accumulation region. Unlike a valley glacier, an ice cap is not confined primarily to a single valley. It may feed numerous outlet glaciers that descend from its margins into surrounding valleys or lowlands.
Ice caps occur in polar and subpolar environments where snow accumulation persists over long periods and annual losses do not completely remove accumulated ice. Examples include the Vatnajokull ice cap in Iceland, the Austfonna ice cap on Nordaustlandet in Svalbard and numerous smaller ice caps in Arctic Canada, Norway and other high latitude regions. Vatnajokull is particularly extensive and contains several active volcanic systems beneath its ice. An ice cap can therefore coexist with volcanic, mountainous and coastal environments. Its ice may be several hundred metres or more thick, although thickness varies greatly depending on climate, bedrock and ice dynamics.
Ice caps form through the accumulation and long term preservation of snow. Snow surviving successive summers becomes progressively compacted into firn and then glacial ice. Once sufficient thickness develops, gravity causes the ice to deform and flow outward. The flow pattern is broadly radial or multidirectional compared with the strongly channelised flow of a valley glacier. However, underlying topography can redirect the ice into preferred outlet routes. The ice cap therefore has its own broad flow system while simultaneously supplying smaller glaciers around its margins. This makes ice caps important sources of outlet glaciers and sediment transport.
The distinction between an ice cap and an ice sheet is primarily one of scale and geographical extent rather than a completely different physical process. Both are large masses of grounded glacial ice that flow under gravity and can develop outlet glaciers. An ice sheet exceeds the conventional 50,000 square kilometre threshold and has continental scale coverage, whereas an ice cap is smaller. The term icefield is related but different. An icefield generally covers extensive highland terrain while allowing underlying mountain peaks and ridges to protrude through the ice, whereas an ice cap may more completely bury the underlying topography. These definitions can overlap in informal usage.
Ice caps modify landscapes through both erosion and deposition. Their moving ice can abrade bedrock, quarry rock fragments, transport sediment and reshape valleys. At their margins, meltwater streams carry sediment downstream and can construct outwash plains, sandur systems, gravel deposits and braided channels. Retreating ice caps can leave moraines that mark former margins. Where outlet glaciers extend into valleys, they can produce classic glacial landforms such as U shaped valleys, cirques, hanging valleys and lateral and medial moraines. The resulting landscape records both the former extent and flow directions of the ice.
Volcanic interactions can be especially important beneath certain ice caps. Iceland's Vatnajokull covers several active volcanic systems, and eruptions beneath the ice can generate large quantities of meltwater through rapid heating and melting. Such sudden floods are called jokulhlaups and can transport enormous amounts of water and sediment. Subglacial volcanic activity can also create distinctive landforms and influence ice structure. This relationship between ice and volcanism demonstrates that an ice cap is not merely a climatic feature but can interact strongly with geological processes beneath and around it.
Ice caps contribute to regional hydrology because their seasonal melting supplies streams, rivers and lakes. In some regions, meltwater is important for ecosystems, agriculture and human water supplies. However, warming can alter this contribution. Initially, increased melting can raise seasonal runoff, but sustained ice loss eventually reduces the amount of stored ice available to generate meltwater. Ice caps can therefore act as natural reservoirs whose behaviour depends on accumulation, temperature, precipitation, altitude and atmospheric circulation. Their mass balance is monitored using field measurements, satellite observations, digital elevation models and repeated surveys of ice velocity and thickness.
Ice caps are also important indicators of climate change because relatively small ice bodies can respond noticeably to changes in temperature and precipitation. Their smaller size means that climatic signals can sometimes produce substantial proportional changes in area and volume. Retreat of an ice cap may expose previously buried terrain and alter local ecosystems, rivers and sediment systems. Nevertheless, individual ice caps do not all respond identically, because elevation, snowfall, aspect, volcanic activity, wind redistribution of snow and local oceanic or atmospheric conditions can alter their mass balance. An accurate interpretation therefore requires regional measurements rather than assuming that every ice cap changes at the same rate.
