An ice cap is a glacier that covers less than 19,000 square miles. These miniature ice sheets form in polar and subpolar regions that are high in elevation and possess a relatively flat surface. Glacial ice that covers an area greater than 19,000 miles is known as an ice sheet.
The glaciers that make up ice caps, ice fields and ice sheets are formed slowly over time. Annual snowfalls that accumulate and then begin to melt result in a hardening and compression of the lowest levels of snow. The hard underlayers of snow, known as "firn," continue to build up until the glacier grows thick enough to fuse them into a solid mass of ice.