Ice Ages.

Ice Ages. I INTRODUCTION 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. II THE STUDY OF ICE AGES 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. A Geologic Features 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. B Glacial Erosion 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. C Glacial Deposits Geologists also study features made from glacial deposits, called glacial till (see Deposit). Ridges made of glacial till, called moraines (from the French morena, or mound), offer the most important clues to the exact locations of former glaciers. Moraines are formed by the broken-down particles of rock that ice flows have carried to the glacier margin (terminus), where the material is released from the ice and forms deposits. When glacial ice melts, the ice flow deposits particles ranging in size from the smallest clay grains to boulders as large as 10 m (30 ft) across. The ice flows may deposit the glacial till in a thin, flat sheet beneath a glacier or into a series of elongated hills called drumlins molded beneath the flowing ice. Moraines may be more than 100 m (300 ft) high and 5 km (3 mi) broad. As a glacier shrinks because of climatic change, recessional moraines form around each successive position of the ice margin. Other deposits include glacial outwash, composed of sand and gravel deposited by glacial runoff streams. III SEDIMENT CORES Geologists often examine sediment cores to learn about glaciation and climate fluctuation. These scientists take core samples to study sediment layers that provide a continuous, undisturbed record of past ice ages. They acquire these samples from the ocean floor, from lakes, or as ice cores from glaciers (see Sedimentary Rock). A Pre-Pleistocene Ice Ages The evidence for the earliest ice age dates to about 2.3 billion years ago. The combined marine and land records indicate that several pre-Pleistocene ice ages occurred. Most features of earlier ice ages have been buried or eroded away over time by surface geologic processes. Most of the glacial landforms and deposits visible today are the results of the most recent (Wisconsin) glaciation. Some ancient glacial features partially survived erosion, and ancient glacial till deposits that also withstood erosion changed into rock. These till deposits, or tillites, provide the best evidence of the age and distribution of ancient ice ages. Tillites from Ontario and elsewhere in North America may have been formed from the same glaciation that deposited tillites in South Africa and possibly Australia. These tillites are evidence of one of the first ice ages, which occurred more than 950 million years ago. The next ice age occurred between 950 and 600 million years ago, and included at least three separate glacial-interglacial intervals. These continental glaciations were widespread and included western North America and portions of South America, Africa, and Australia. Another expansion of continental ice, between 450 and 400 million years ago, was centered in northern Africa and spread to adjacent portions of northern Europe and South America. Between 330 and 240 million years ago, an extensive glaciation affected Africa, Antarctica, Australia, Asia, and South America. Despite evidence for the widespread distribution of this particular glaciation, smaller, localized icecaps were probably more common than massive continental ice sheets. This is because during this time sea level fluctuated only slightly, rather than the 130 m (430 ft) or more of fluctuation that would be expected if great volumes of ice alternately formed and melted. B Future Ice Ages The record of previous glacial activity is the best indicator for future ice ages. Scientists examine the evidence for the numerous 100,000-year glacial-interglacial cycles within the present ice age to attempt a forecast of future ice ages. Since all previous ice ages lasted tens of millions of years, our present ice age will likely continue for a considerable amount of time. Each glaciation begins slowly and may take 80,000 years or more to reach its maximum extent. A rapid melting of these expanded glaciers within just a few thousand years follows. Then the next glaciation begins to build, only 10,000 to 20,000 years after the maximum of the previous glaciation occurred. Evidence from both land and sea environments indicates that, at least prior to the human-induced global warming of the last two centuries, the worldwide climate has been cooling naturally for several thousand years. Ten thousand years have already passed since the end of the last glaciation, and 18,000 years have passed since the last maximum. This may indicate that Earth has entered the beginning of the next worldwide glaciation. IV POSSIBLE CAUSES Several possible causes of ice ages exist. Scientists have proposed numerous theories to explain their occurrence. In the 1920s Yugoslav scientist Milutin Milankovitch proposed the Milankovitch Astronomical Theory, which states climatic fluctuations and the onset of glaciation can be caused by variations in Earth's position relative to the Sun. Milankovitch calculated that this deviation of Earth's orbit from its almost circular path occurs every 93,408 years. The movement of Earth's crustal plates, called plate tectonics, is also linked to the occurrence of ice ages. The positions of the plates in polar regions may contribute to ice ages. Changes in global sea level may affect the average temperature of the planet and lead to cooling that may cause ice ages. Other theories explaining the causes of ice ages--such as significant variations in the heat output of the Sun, the presence of an interplanetary dust cloud that occasionally blocks some of the Sun's heat from reaching Earth, and meteorite impacts--have not yet been supported by any solid evidence. A Milankovitch Astronomical Theory The Milankovitch Astronomical Theory best explains regular climatic fluctuations. The theory is based on three variations in the position of Earth relative to the Sun: the eccentricity (elongation or circularity of the shape) of Earth's orbit, the tilt of Earth's axis toward or away from the Sun, and the degree of wobble of Earth's axis of rotation. The total effect of these changes causes one region of Earth--latitude 60° to 70° north, near the Arctic Circle--to receive low amounts of summer radiation about once every 100,000 years. These cool summer periods last several hundred to several thousand years and thus provide sufficient time to allow snowfields to expand and merge into glaciers in this area, signaling the beginning of a glaciation. B Sea Level Changes When glaciers expand during an ice age, the sea level drops because the water that forms glaciers ultimately comes from the oceans (see Water Cycle). Global sea level affects the overall temperature of the planet because solar radiation, or heat, is better absorbed by water than by land. When sea levels are low, more land surface becomes exposed. Since the land is not able to absorb as much solar radiation as the water can, the overall average temperature of the planet decreases, or cools, and may contribute to the onset of an ice age. V PLEISTOCENE ICE AGE A map showing Earth during an ice age would look very different from a map of the modern world. During the Wisconsin glaciation of 115,000 to 10,000 years ago, two ice sheets, the Laurentide and the Cordilleran, covered the northern two-thirds of North America, including most of Canada, with ice. Other parts of the world, including Eurasia and parts of the North Atlantic Ocean, were also blanketed in sheets of ice. The Laurentide continental ice sheet extended from the eastern edge of the Rocky Mountains to Greenland. The separate Cordilleran Ice Sheet was composed of mountain ice caps and valley glaciers that flowed onto the surrounding lowlands in parts of northern Alaska, in parts of the Sierra Nevada, and in the Cascade Range and the Rocky Mountains as far south as New Mexico. Where the continental shelf between Alaska and Siberia was uncovered, the Bering land bridge formed. In northern Eurasia, continental ice extended from Great Britain eastward to Scandinavia and Siberia. Separate mountain glacial systems covered the Alps, the Himalayas, and the Andes. The extensive ice sheets on Antarctica and Greenland did not expand very much during each glaciation. Sea ice grew worldwide, particularly in the North Atlantic Ocean. Contributed By: Barry Goldstein Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.