PLIESTOCENE GLACIATIONS

 

 

Pleistocene glaciation, also known as the Quaternary glaciation, refers to a series of glacial events separated by interglacial events during the Pliestocene Epoch.  The Pleistocene Epoch is the geological period which lasted from about 2,588,000 to 11,700 years ago, spanning a time of 25,57,300 years, representing the world's most recent period of repeated glaciations. During this period, ice sheets expanded, notably from out of Antarctica and Greenland, and fluctuating ice sheets occurred elsewhere (for example, the Laurentide ice sheet). The major effects of the ice age are erosion and deposition of material over large parts of the continents, modification of river systems, creation of millions of lakes, changes in sea level, development of pluvial lakes far from the ice margins, isostatic adjustment of the crust, and abnormal winds. It affected oceans, caused large-scale flooding, and disrupted biological communities. The ice sheets themselves, by raising the albedo, effected a major feedback on climate cooling.

The Pleistocene geological record gives evidence of 20 cycles of advancing and retreating continental glaciers.  During most of the Pleistocene glaciers were far more extensive than they are today. Much of this glaciation occurred at high latitudes and high altitudes, especially in the Northern Hemisphere. Up to 30% of the Earth's surface was glaciated periodically during the Pleistocene. Large portions of Europe, North America (including Greenland), South America, all of Antarctica, and small sections of Asia were entirely covered by ice. In North America during the peak of the Wisconsinan glaciation, approximately 18,000 years ago, there were two massive yet independent ice sheets. Both the eastern Laurentide and the western Cordilleran ice sheets were over 3900 meters thick. In Europe, ice covered Scandinavia, extended south and east across Germany and western Russia, and southwest to the British Isles. Another ice sheet covered most of Siberia. In South America, Patagonia and the southern Andes mountains were beneath part of the Antarctic ice sheet. Because so much water was taken up as ice, global sea level dropped approximately 140 meters.

The causes of the Pleistocene cycle of glacial and interglacial episodes are still being debated. It appears that continental positions, oceanic circulation, solar-energy fluctuations, and Earth's orbital cycles combined to generate these glacial conditions, so perhaps it is inappropriate to pinpoint any single cause. Some scientists have calculated that changes in the concentration of greenhouse gases were a partial reason for large (5-7 C) global temperature swings between the ice ages and interglacial periods.

Two scientists greatly influenced how Pleistocene glaciations were interpreted. In the 1800s, geologists were studying widespread surface deposits called diluvium. This archaic term referred to deposits that could not be explained by the normal action of rivers and seas, but instead were believed to have been produced by extraordinary floods of vast extent. Louis Agassiz, a Swiss geologist who initially worked on fossil fish, demonstrated that diluvium was actually a ground moraine formed by continental glaciation. The other influential figure, the Yugoslav mathematician M. Milankovitch, showed that variation in Earth's orbital motions could explain periodic climate changes, including continental glaciation.

As evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period, during the Quaternary Period, the total volume of land ice, sea level, and global temperature has fluctuated initially on 41,000- and more recently on 100,000-year time scales.  Over the past 740,000 years there have been eight glacial cycles.  The entire Pliestocene and Holocene Period, starting 2.58 Ma, is referred to as an ice age because at least one permanent large ice sheet Antarctica has existed continuously. There is uncertainty over how much of Greenland was covered by ice during the previous and earlier interglacials. During the colder episodes referred to as glacial periods large ice sheets at least 120 m (394 ft) thick at their maximum also existed in Europe, North America, and Siberia. The shorter and warmer intervals between glacials are referred to as interglacials.

Currently, with the beginning of the Holocene, the earth is in an interglacial period. The current interglacial began between 10,000 and 15,000 years ago, which caused the ice sheets from the last glacial period to begin to disappear. Remnants of these last glaciers, now occupying about 10% of the world's land surface, still exist in Greenland, Antarctica and mountainous regions. The retreat of glaciers since 1850 is largely a consequence of anthropogenic warming of the climate system during the period.

During the glacial periods, the hydrologic system was completely interrupted throughout large areas of the world and was considerably modified in others. Due to the volume of ice on land, sea level was approximately 120 meters lower than present. The evidence of such glaciation in the recent past is robust. Over the last century, extensive field observations have provided evidence that continental glaciers covered large parts of Europe, North America, and Siberia. Maps of glacial features were compiled after many years of fieldwork by hundreds of geologists who mapped the location and orientation of drumlins, eskers, moraines, striations, and glacial stream channels. These maps revealed the extent of the ice sheets, the direction of flow, and the locations of systems of meltwater channels, and they allowed scientists to decipher a history of multiple advances and retreats of the ice. Even before the theory of worldwide glaciation was generally accepted, many observers recognized that more than a single advance and retreat of the ice had occurred. Extensive evidence now shows that a number of periods of growth and retreat of continental glaciers occurred during the ice age, called glacials and interglacials. The interglacial periods of warm climate are represented by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.

Causes

No completely satisfactory theory has been proposed to account for Earth's history of glaciation. The cause of glaciation may be related to several simultaneously occurring factors, such as astronomical cycles, atmospheric composition, plate tectonics, and ocean currents.

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