The advance and retreat of ice sheets has been occurring for millennia. Ice sheets grow when snow accumulates faster than it melts. As the snowfall accumulates, its weight presses down on earlier snows and compacts this into glacial ice. An ice sheet is an accumulation of glacial ice on land covering more than 20,000 square kilometers or 50,000 square miles. Currently there are two ice sheets on Earth, one in Greenland and the other in Antarctica. In the past ice covered much of North America - this is called the Laurentide ice sheet; Europe's ice sheet is named the Weichselian; and in South America, the Patagonian ice sheet.
The motion in glacial ice is caused by stress from the weight of overlaying ice accumulations. The water molecules move from one ice crystal of greater stress to another of less stress. This motion is called glacial creep. In mountain glaciers, this creep tends to follow the force of gravity downhill and will follow valleys and land contours. In an ice sheet this creep is from the central regions where snow is accumulating to the edges where the stress is the least. The ice above this creeping or plastic ice, which is not under enough stress to creep, is fractured and carried along with accumulated dust and debris much like a raft on a stream. As the ice moves toward lower elevations or warmer areas, it melts, evaporates or caves into icebergs if over water. The faster moving extremities of ice sheets are called ice streams, glaciers, or when over water, ice shelves. When the melting at the edges matches the creep rates, the glacier is stationary. When the creep rate exceeds the melt rate, the glacier is said to be advancing. Retreating glaciers occur when the melt rate exceeds the creep rate.
The periods of extensive ice creation and growing ice sheets are called ice ages or glaciations. The periods in between are called interglacial. The creation or demise and motion of ice sheets follow general climate conditions. Colder temperatures at lower latitudes allow less snowfall to melt; warmer temperatures cause increased melting. Colder temperatures alone are not enough to cause an ice sheet to grow, another factor involved is the amount of snowfall. The Earth is at the end of a glaciation or ice age that peaked about 10,000 years ago and still has two remaining ice sheets. Even though the average global temperature has increased, the Greenland ice sheet is still growing due to increased snowfall during the winter and local conditions not fully explained that have kept Greenland cooler than it was only a thousand years ago when the Vikings settled it. At this time, it is worthy to note that the summer of 2012 saw the greatest surface melt in Greenland that has ever been observed.