Article Outline
Ice Ages, periods in Earth’s history when sea ice or glaciers have covered a significant portion of the planet’s surface and significant cooling of the atmosphere has occurred. Earth has existed for about 4.5 billion years. During that time it has experienced several ice ages, each lasting tens of millions of years. The total of these episodes may account for as much as 15 to 20 percent of the planet’s history. The icy cover has ranged from about 10 percent to about 30 percent of the entire surface of the planet.
The most recent ice age, the Pleistocene Epoch, lasted from about 1.6 million years to 10,000 years before present. During that time at least 20 glaciations, or periods when the ice cover increased, occurred. Each of these periods was followed by an interglaciation, or a period when the ice cover shrank. The most recent glaciation in North America, called the Wisconsin glaciation, lasted from about 115,000 years ago to 10,000 years ago. The climate during that time was much different from what it is today, with temperatures on the continents as much as 15° C (27° F) colder. In areas that are currently occupied by subtropical deserts, cooler and wetter climates caused large lakes to form from increased rainfall and glacial runoff. The past 10,000 years have been part of a relatively warm interglacial period. However, the presence of massive continental ice sheets on Greenland and Antarctica, along with numerous smaller glaciers in mountainous regions throughout the world, indicates that Earth is still in the grip of an ice age.
Glacial geologists can determine where ancient glaciers were located by studying the land. They examine the processes of glacial action to learn more about the impact glaciers have had, and continue to have, on Earth. Geologic features, such as glacial lakes, form when glaciers expand. Expanding glaciers also cause sea levels to decrease. Glacial erosion (wearing away) and glacial deposition (release of sediments) cause many geologic changes. Scientists study these processes, as well as ice cores from glaciers and sediment cores from lakes and oceans, to learn about ice ages that predate the Pleistocene Epoch. Scientists use these findings to determine what factors may influence the occurrence of future ice ages. See also Geology; Ice.
During an ice age several geologic changes occur. These alterations range from changes in the shape of the land to a decrease in sea level. Water freezes and settles within the growing glaciers. This process causes worldwide sea level to drop by as much as 150 m (500 ft) below the current sea level. When this process occurs, shallow ocean waters that cover the continental shelves, or the edges of the continents, recede and uncover the submerged land. Advancing ice sheets can block water drainage pathways and create glacial lakes. Elsewhere, rivers are diverted from their original pathways to courses along the ice margin. The added weight of glacial ice sheets causes Earth's crust to lower by as much as several hundred meters. The ground in some areas becomes frozen throughout the year and forms permafrost, or permanently frozen ground. When glaciers recede, the combined effects of rebound from crustal depression and the shifting of ice masses cause the redistribution of rivers and lakes.
Geologists study glacial erosion (see Erosion: Glacial Erosion) to determine the structure of former glaciers and to examine current glaciers. The freezing and thawing processes of glacial erosion loosen bedrock underneath and next to glaciers. The loose rock pieces may be carried away within the flowing ice or may be dragged along the bottom of the glacier and scratch the surface of the land. In mountainous regions, glaciers transform V-shaped river valleys into deeper and broader U-shaped valleys by erosion. Glacial erosion at the highest altitudes creates bowl-shaped hollows (cirques) at the heads of valleys. A series of peaks (horns) and narrow ridges (arêtes) are all that remain of once broad uplands.
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