Ice accumulates over hundreds of thousands of years near the poles and at high altitudes, where more snow falls in winter than melts in summer. As a result, snow accumulates over time, with layers added year after year. Summer melt removes some of the snowpack; what remains compresses into pebbly bits of ice called firn. As new layers of snow are added year after years, the weight of the added layers of firn press down on those below, causing them to compact further. The ice undergoes a metamorphosis, reforming into ice crystals of various sizes. Because of this process, the US Geological Survey (USGS) classifies glacial ice as a mono-mineralic metamorphic rock. Naturally formed ice, because it is solid, is considered a mineral.
Over time, as long as snowfall continues to outpace losses, ice accumulates until the glacier begins to flow downhill under the forces of gravity and its own weight. These rivers of ice flow downhill at a truly glacial pace. It can take hundreds of years for ice that formed at the top to flow the full length of the glacier; the rate of movement depends on the thickness of the ice and the steepness of the slope down which it travels. Depending on conditions, glaciers can last for hundreds of thousands of years. The oldest glacier, in Antarctica, is thought be be almost one million years old.
What Are Ice Sheets?
When glacial ice covers at least 50,000 square kilometers (20,000 square miles) of land, it is called an ice sheet. Earth is currently home to two ice sheets. One covers most of Greenland, the other most of Antarctica. Together they cover more than 10 million square kilometers of land and contain more than 99 percent of the freshwater ice on the planet. Continue reading from Woods Hole Oceanographic Institution
The world's glaciers have an estimated total area of about 700,000 square kilometes (270,00 miles). Most of the world's glaciers are in Antarctica. Glaciers are mainly found near the poles, but glaciers exist on all the world's continents, even Africa. Australia does not have any glaciers; however, it is considered part of Oceania, where glaciers can be found in New Zealand and the large islands of Indonesia and Papua New Guinea.
Most glaciers are found in regions of high snowfall and low temperatures in summer. These conditions ensure that the snow that accumulates in the winter is not lost during the summer. Examples of such regions are Alaska, Patagonia, and the Himalayas. The Arctic islands of Canada get much less snowfall, but it is cold enough through the year that glaciers can form. In short, glaciers form at high latitudes or high elevations. The closer to the equator, higher elevations are required for needed snowfall and lower temperatures. Continue reading from National Snow and Ice Data
What Is a Glacier (U.S. Geological Survey)
What Are Glaciers? (Discovering Antarctica)
Common Questions and Myths about Glaciers (National Park Service)
Climate Change Indicators: Glaciers (United States Environmental Protection Agency)
Glacial History of Connecticut (Yale-New Haven Teachers Institute)
