In simple terms, ice caps form where annual snow accumulation exceeds annual losses by processes such as melting or ICEBERG calving (pieces of ice breaking off from a glacier).
Ice CapAn ice cap is a large mass of ICE that originates on land by compaction and recrystallization of SNOW. By definition, ice caps submerge most or all features of the underlying landscape. If a large mass of ice does not cover the surrounding land it is constrained by bedrock outcrops called NUNATAKS, and is referred to as an icefield. Ice caps are typically thickest in their central regions (up to approximately 1000 metres thick), and flow outwards from a central dome in a radial pattern. GLACIERS draining ice caps and icefields are referred to as outlet glaciers, and are typically constrained by bedrock to flow in one main direction. A single ice cap is usually drained by a series of outlet glaciers, which typically move at a higher rate than the ice cap interior.
How Ice Caps Form
In simple terms, ice caps form where annual snow accumulation exceeds annual losses by processes such as melting or ICEBERG calving (pieces of ice breaking off from a glacier). As a layer of snow is buried by subsequent snowfalls, it increases in density and is called firn once it is more than a year old. This firn is then transformed to ice as it gets further buried in the accumulation zone of the glacier, and the whole mass moves away from the ice cap centre under the effect of gravity. At lower elevations, in the ablation zone, all of the previous winter's snow and some of the ice are removed by melting and runoff, and in some cases by calving of icebergs into an ocean or lake.
On an ice cap in a stable climate, the surface profile does not change much from year to year because the ice that flows out of the accumulation area approximately balances that which is lost from the ablation area. However, if snowfall increases or summers become colder, so that melting is reduced, the ice starts to thicken and, after some years, the ice front will advance. Conversely, a reduction in snowfall or an increase in summer warmth produces a surface thinning and eventually a retreat at the ice cap margins. Interpretation of the record of advances and retreats is complicated because each ice mass has its own characteristic response time that depends on its size and speed. In recent years, measurements of changes in the surface height, mass and terminus position of ice caps and icefields indicate that they are undergoing widespread thinning and retreat in response to the warming of Earth's climate. For example, ice masses in the Canadian ARCTIC ARCHIPELAGO lost mass at a rate that tripled between 2004-2006 and 2007-2009 in direct response to warmer summer temperatures.
Canadian Ice Caps and Icefields
The largest ice caps and icefields in Canada are found in the Arctic Archipelago. For example, ELLESMERE ISLAND contains the Agassiz Ice Cap, Prince of Wales Icefield and Northern Ellesmere Icefield, all of which exceed 20 000 km2 in area. Other large ice caps and icefields are found on AXEL HEIBERG, DEVON and BAFFIN islands. The ice in some of these caps and icefields is up to one kilometre thick, and their physical characteristics often differ greatly across them. On their ocean margins, snowfall amounts are relatively high, which results in fast flowing outlet glaciers (up to one kilometre per year) terminating in the ocean, and which are incised deeply into the surrounding bedrock. In contrast, snowfall is less on their landward margins resulting in more slowly moving outlet glaciers (tens of metres per year), which are poorly constrained by topography and often terminate in broad lobes on land.
Mountain ranges in western Canada contain many icefields. Perhaps the best known is the COLUMBIA ICEFIELD, a major tourist attraction adjacent to the Banff-Jasper highway. It is the largest ice mass in the Rocky Mountains with an area of 230 km2 and is drained by several valley glaciers, including the Athabasca and Saskatchewan glaciers. Its surface elevation varies from about 2600 to 3500 metres, the large range being a reflection of the peaks and valleys in the bedrock underneath. The icefield's average thickness is probably no more than 100-150 metres, although the thickest ice is 365 metres thick. Velocities range from a few metres per year in the centre to about 100 metres per year on Athabasca Glacier. About four metres of ice is melted from the surface of the lower part of the Athabasca Glacier each summer, which is more than the amount being replaced by ice flow from above. This has resulted in long-term retreat of the glacier; it has lost approximately half of its volume and retreated more than 1.5 kilometres in length over the past 125 years.
The world's largest ice masses are called ice sheets if they are greater than 50 000 km2 in size, and today there are two of them: the Greenland and Antarctic ice sheets. During the last major GLACIATION, which peaked approximately 20 000 years ago, large ice sheets covered much of eastern Canada (Laurentide Ice Sheet), western Canada (Cordilleran Ice Sheet) and northern Europe (Eurasian Ice Sheet). Today the Antarctic Ice Sheet has an area of almost 14 million km2 and a maximum thickness of about 4.8 kilometres. If it were to melt, world sea level would rise by about 60 metres. The Greenland Ice Sheet has an area of 1.7 million km2 and a maximum thickness of about 3.3 kilometres, and would cause global sea level to rise by about 6 metres if it all melted. Glaciers flowing from the Greenland Ice Sheet move at speeds of up to ten kilometres per year or more, and most have been rapidly retreating since the 1990s. The glaciers draining from western Greenland produce the main source of icebergs seen in Canadian waters, with the ice caps and icefields of Devon and Ellesmere Islands also providing important sources. These icebergs can be hazardous to shipping and offshore oil drilling locations such as the Grand Banks off Newfoundland.
Record of the Past
Taking samples from the central part of ice caps and ice sheets by drilling through them provides a continuous record of past snowfalls and climate. Annual layers can sometimes be distinguished and counted; if not, the ice can be dated by other means such as the identification of distinctive horizons caused by volcanic eruptions. Chemical analyses of core samples from Greenland, Antarctica and arctic Canada have given detailed records of past climates, particularly temperatures, extending in some cases over more than 100 000 years in Greenland and 800 000 years in Antarctica. Air bubbles in the ice contain samples of the atmosphere at the time the ice formed from snow, and so past concentrations of gases, such as carbon dioxide and methane, which contribute to CLIMATE CHANGE, can be measured. The concentration of carbon dioxide before the industrial era was only 280 parts per million (ppm) compared with the present value of nearly 400 ppm, while the concentration of methane has more than doubled in the past 150 years. Concentrations of both gases were much lower during the last ICE AGE than they have been since.
Major volcanic eruptions are recorded by layers of increased acidity. The acid is mainly sulphuric acid produced as an aerosol from sulphur dioxide emitted in the eruption. A well-dated record of northern-hemisphere eruptions over the last 10 000 years has been obtained from Greenland cores. Ice cores are also valuable for pollution studies because the pre-industrial (circa 1750) background levels of trace elements such as lead and mercury can be measured. Nuclear bomb tests in the 1960s, as well as the 1986 Chernobyl and 2011 Fukushima Daiichi nuclear accidents, produce radioactive layers detectable in snowfall and ice cores.