Bodies of ice, ranging from vast ice sheets to valley glaciers move downhill because of gravity. In Antarctica the ice sheet moves by only a metre (39in) or so a year, while most valley glaciers move the same distance in a day.
Special circumstances can speed up the movement of the ice. In 1936-37, Alaska’s Black Rapids Glacier moved downhill at up to 60m (197ft) a day! This amazing rate was probably caused by an earthquake which shook huge amounts of snow on to the glacier’s source. The snow rapidly compacted into ice, and its sheer weight propelled the glacier downwards at speed.