Ice sheet

In glaciology, an ice sheet (Danish: «Indlandsis», meaning «inland ice»), usually called continental glacier and sometimes ice shell or ice layer, is a large mass of glacier ice with an average thickness of over 2 km that covers a large continental area in the polar regions of the Earth. They're located at extreme latitudes with a conventional area of more than 50,000 square kilometres (19,000 sq mi).^[1][2] In other geologic time spans, there was a major number and they were covering an larger area, but currently they only cover Antarctica and Greenland. This concept might not be confused with the sea ice (the floating layer of ice of variant extension that forms in the polar seas), neither the ice shelf (barrier of ice of glacier origin that spans from the coast to the inner of the ocean) nor the polar cap.

Ice sheets are larger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km² are called an ice cap. An ice cap will usually feed a series of glaciers around its periphery.

Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat. In some places, melting occurs and the melt-water lubricates the ice sheet so that it flows it rapidly. This process produces fast flowing channels inside the ice sheet: these are ice streams.

The two current ice sheets are relatively young in geological terms. The Antarctic ice sheet first formed in the early Oligocene. It retreated and advanced many times until the Pliocene, when it came to occupy almost all of Antarctica. The Greenland ice sheet did not develop at all until the late Pliocene, but apparently developed very quickly. This had the unusual effect of allowing fossils of plants that once grew on present-day Greenland to be much better preserved than with the slowly forming Antarctic ice sheet.