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Quaternary Glaciation

           QUATERNARY GLACIATION


Quaternary glaciation, also known as the Pleistocene glaciation, the current ice age or simply the ice age, refers to the period of the last few million years (2.58 Ma to present) in which permanent ice sheets were established in Antarctica and perhaps Greenland, and fluctuating ice sheets have occurred elsewhere (for example, the Laurentide ice sheet).  The major effects of the ice age were 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.  The ice sheets themselves, by raising the albedo, effected a major feedback on climate cooling.

 

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, as evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period.  There have been approximately 80 glacial cycles over this time.  All of this time 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 present during the previous and earlier interglacials.  During the colder episodes (referred to as glacial periods) large ice sheets also existed in Europe, North America, and Siberia.  The shorter and warmer intervals between glacials are referred to as interglacials. 

 

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

 

  • Astronomical cycles
  • Atmospheric composition
  • Plate tectonics
  • Ocean currents

 

Effects

 

Lakes:  The Quaternary glaciation created more lakes than all other geologic processes combined.  The reason is that a continental glacier completely disrupts the preglacial drainage system.  The surface over which the glacier moved was scoured and eroded by the ice, leaving myriad closed, undrained depressions in the bedrock.  These depressions filled with water and became lakes. Very large lakes were created along the glacial margins.  The ice on both North America and Europe was about 3,000m (9,843 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins.  Crustal subsidence was greatest beneath the thickest accumulation of ice.  As the ice melted, rebound of the crust lagged behind, producing a regional slope toward the ice.  This slope formed basins that have lasted for thousands of years.  These basins became lakes or were invaded by the ocean.  The Great Lakes in Canada and the Baltic Sea of northern Europe were formed primarily in this way.

 

Pluvial lakes: The climatic conditions that cause glaciation had an indirect effect on arid and semiarid regions far removed from the large ice sheets.  The increased precipitation that fed the glaciers also increased the runoff of major rivers and intermittent streams, resulting in the growth and development of large pluvial lakes.  Most pluvial lakes developed in relatively arid regions where there typically was insufficient rain to establish a drainage system to the sea.  Instead, stream runoff in those areas flowed into closed basins and formed playa lakes.  With increased rainfall, the playa lakes enlarged and overflowed.  Pluvial lakes were most extensive during glacial periods.  During interglacial stages, when less precipitation, fell the pluvial lakes shrank to form small salts flats. 

 

 

Isostatic adjustment

 

Major isostatic adjustments of the lithosphere during the Quaternary glaciation were caused by the weight of the ice, which depressed the continents.  In Canada, a large area around Hudson Bay was depressed below sea level, as was the area in Europe around the Baltic Sea.  The land has been rebounding from these depressions since the ice melted.  Some of these isostatic movements triggered large earthquakes in Scandinavia about 9,000 years ago.  These earthquakes are unique in that they are not associated with plate tectonics.

                                 

                                    


Studies have shown that the uplift has taken place in two distinct stages.  The initial uplift following deglaciation was rapid (called "elastic”), and took place as the ice was being unloaded.  After this "elastic” phase, uplift proceed by "slow viscous flow” so the rate decreased exponentially after that.  Today, typical uplift rates are of the order of 1 cm per year or less.  In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST (Baseline Inferences for Fennoscandian Rebound, Sea-level, and Tectonics)   GPS network.  Studies suggest that rebound will continue for about at least another 10,000 years.  The total uplift from the end of deglaciation depends on the local ice lead and could be several hundred meters near the center of rebound.       

 

 

Winds

 

The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation.  Winds near the glacial margins were strong and persistent because of the abundance of dense, cold air coming off the glacier fields.  These winds picked up and transported large quantities of loose, fine-grained sediment brought down by the glaciers.  This dust accumulated as loess (wind-blown silt), forming irregular blankets over much of the Missouri river valley, central Europe, and northern China.

 

Sand dunes were much more widespread and active in many areas during the early Quaternary period.  A good example is the Sand Hills region in Nebraska, USA, which covers an area of about 60,000 square kms (23,166 sq mi).  This region was a large, active dune field during the Pleistocene epoch, but today is largely stabilized by grass cover.

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