A lake is derived from latin word Lacus which means a body of water which is not a part of the ocean and is larger and deeper than a pond. The Laurentian great lakes of United States are located in eastern North America, on the Canada- United states border and are world’s largest system of world’s fresh water lakes (Columbian encyclopedia, 2007). Consisting of 5 major lakes Superior, Huran, Erie, Ontario and Michigan or St.
Clair, the lake superior alone is the world’s largest freshwater lake with an area of 31,820sq mi (82,414 sq Km) (Columbian encyclopedia, 2007) and together contain one fifth of the world’s surface fresh water providing water to at least 42 million Americans and Canadians (savage 2007). They are sometimes referred to as island seas or Canada and the United States third coast and also as the Sweetwater seas (DEQ 2008).
Known for their beauty and the wealth of resources they possess, they are large enough to influence the regional climatic changes, differentiating temperatures in summers and winters, and increasing amount of snow and rain in the region. The great lakes contain not only the five main lakes themselves, but also numerous minor lakes and rivers, and a series of small islands (savage 2007). Rivers are sources of connection between two great lakes such as the St Marys River connect lake Superior to lake Huron and, the St Clair River connects the Lake Huron to Lake St Clair.
Also, there are the straits of Mackinac that connect Lake Michigan to Lake Huron, which are considered a single lake hydrologically (savage 2007). The Saint Lawrence Seaway and the Great lakes waterway open the great lakes to the ocean for bigger vessels. The lakes are bounded by several states of the US such as Minnesota, Wisconsin, Michigan, Illinois, Indiana, Pennsylvania and New York, whereas, they touch the boundary of Ontario in Canadian province. Formation of the rock basin The answer to the formation of the great lakes lies in the Ice age.
Prior to the ice age, a fracture in the earth running from now what is known as lake Superior generated volcanic activity that almost split North America in two different parts (Columbian encyclopedia, 2007). The process started in Precambrian era occupying about fifth-sixth of all geological time and was a time of high volcanic activities and great stress. These volcanic eruptions later, created a basin for the great lakes. Over a period of 20 million years, lava intermittently flowed from the fracture.
Early sedimentary and volcanic rocks were formed into complex structures by repeated hot temperatures and folding due to it (UEPP 2008). The gemographic age then created mountain region now known as the Northern Wisconsin and the Laurentian mountain in the east Canada. Over the time, occasional volcanic activity continued while the rocks eroded. Molten magma below the highlands of what is now known as Lake superior spewed out of its sides, causing the high lands to sink and form a mammoth rock basin (DEQ 2008). Eventually the fracture stabilized and over time the rock tilted down from north to south.
Granitic rocks of the shield extended southwards beneath the Paleozoic, sedimentary rocks where they combined to form a basement structure of the southern and eastern portions of the basin (DEQ 2008). The intermediate era between the Precambrian era of volcanic activities and the Pleistocene era of glaciers lies the Paleozoic era. In this era the marine seas flooded most of the North America repeatedly, and deposited lime silts, clays, sand and salts, which eventually consolidated into limestone, shales, sandstone, halite and gypsum.
Marine seas were also inhabited by a multitude of life forms, including corals, crinoids, brachiopods and mollusks that in later years formed the fauna of the great lakes. Formation of the lakes The region went from fire to Ice with the arrival of glaciers and the lakes were formed at the end of last Ice age known as the Pleistocene Epoch, which occurred 1. 6 billion and 10,000 years ago (Worster 2005). The Pleistocene is known as the glacial period of earth’s history and is the 6th epoch of the Cenozoic ere of the geologic time.
Its ice sheets at one time covered all the large parts of Europe, North America and Antarctica. The Pleistocene glaciers up to 2000 meters high made important alterations in the topography of the glaciated regions, leveling hilly sections to convert them into low lying rolling plains caused by erosion and by the deposition of the drift, and emptying the hollows through erosion that later on became the lakes (DEQ 2008). The mountains steadily eroded, depositing layers of sediments which compacted and became became limestone,dolostone and taconite.
Also it forced rivers to cut new channels by filling the beds formerly created by the molten magma (Columbian encyclopedia, 2007). Among the characteristic surface features formed in the Pleistocene are the drumlin, karne, esker and the Moraine. The glaciations of the Pleistocene were not continuous but consisted of several glacial advances interrupted by interglacial stages, during which the ice retreated and a comparatively mild climate prevailed and during which vegetation and wildlife returned. In all probability there were actually only four glacial stages.
The interglacial stages were marked by the weathering of the till of the drift to form a sticky, heavy soil called the gumbotil and by the deposition of peat and loess (DEQ 2008). During the period of glaciations, giant sheets of ice flowed across the land, leveling mountains and carving out the large massive valleys that is a process known as glacial erosion. Where they encountered more resistant bedrock in the north which they could not level or erode, then only the overlying layers of the rocks were removed (UEPP 2008).
At least four times during the Pleistocene epoch, large masses of ice or glaciers advanced and retreated over the surface of what is known as the North America (Wilson et al. 2000). The last major glacier, called the Laurentide began to form around 10,000 years. To the south, the softer sandstones and Shale’s were more affected. As the glaciers melted and began receding, their leading edges left behind high ridges, some of which can even be seen today.
Huge lakes were formed between these ridges from the retreating ice fronts, and continually changed over time as the ice sheets moved (Wilson et al. 2000). Gradually with the changing temperature of the earth, as it entered a more warm climatic regime, the glaciers began to melt. The whole cycle was repeated several times. It left behind a large amount of melt water along with ice. The land was greatly depressed after recurring cycles, from the weight of the glaciers and hence, lakes much larger than the present day lakes were created (UEPP 2005).
Their legacy can still be seen in the form of beach ridges, eroded bluffs and flat plains located hundreds of meters above present lake levels and, are visible even today around Saginaw Bay and west and north of Lake Erie (UEPP 2008). Again the glaciers receded and the land began to rise. The uplift, which was at times very rapid, caused remarkable changes in the depth, size and drainage patterns of the glacial lakes (Worster 2008). Thus, the drainage system was formed through various river ways of Illinois River valley, Hudson River valley and the Ottawa River valley before entering their present outlet through the St Lawrence River valley.
Various studies indicate, that although the uplift has minimized considerably it is still occurring in the northern portion of the basin (Govt. of Ontario 2008). Also, because of the uneven nature of glacial erosion some higher hills like Niagara Escarpment became the great lake islands (UEPP 2008). Thus, the great lakes were created and the process took a thousand of years to complete. The long-term climatic changes along with the uplift, suggests that the lakes are not static and will continue to evolve (Govt. of Ontario 2008).
Department of environmental quality; accessed online on 19 Apr 2008 www. michigan. gov US environmental protection policy; accessed online on 20 Apr 2008 www. epa. gov Government of Ontario; accessed online on 20 Apr 2008; www. gov. on. ca Melissa savage 2007; saving the great lakes; magazine article; state legislature; Vol 33; May 2007 R. C Wilson et al 2000; Great ice age, climate change and life; 2000 Donald Worster 2005; a long cold view of history; magazine article; American scholar; 2005
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