GREAT LAKES GEOLOGY

A diagram of the formation of the Great Lakes. It has been estimated that the foundational geology which created the conditions shaping the present day upper Great Lakes was laid from 1.1 to 1.2 billion years ago, when two previously fused tectonic plates split apart and created the Midcontinent Rift. A valley was formed providing a basin that eventually became modern day Lake Superior. When a second fault line, the Saint Lawrence rift, formed approximately 570 million years ago, the basis for Lakes Ontario and Erie were created, along with what would become the St. Lawrence River.

The Great Lakes are estimated to have been formed at the end of the last ice age (i.e. about 10,000 years ago), when the Laurentide ice sheet receded. The retreat of the ice sheet left behind a large amount of meltwater (see Lake Agassiz) which filled up the basins that the glaciers had carved, thus creating the Great Lakes as we know them today. Because of the uneven nature of glacier erosion, some higher hills became Great Lakes islands. The Niagara Escarpment follows the contour of the Great Lakes between New York and Wisconsin. Land below the glaciers "rebounded" as it was uncovered. Because the glaciers covered some areas longer than others, this glacial rebound occurred at different rates. Some researchers believe that differential has contributed to fluctuating water levels throughout the Great Lakes basin.

Climate - Lake Effect The effect of Great Lakes on weather in the region is called the lake effect. In winter, the moisture picked up by the prevailing winds from the west can produce very heavy snowfall, especially along lake shores to the east such as Michigan, Ohio, Pennsylvania, Ontario, and New York. The lakes also moderate seasonal temperatures somewhat, by absorbing heat and cooling the air in summer, then slowly radiating that heat in autumn. This temperature buffering produces areas known as "fruit belts", where fruit typically grown farther south can be produced. Western Michigan has apple and cherry orchards, and vineyards adjacent to the lake shore as far north as the Grand Traverse Bay. The eastern shore of Lake Michigan and the southern shore of Lake Erie have many wineries as a result of this, as does the Niagara Peninsula between Lake Erie and Lake Ontario. A similar phenomenon occurs in the Finger Lakes region of New York as well as Prince Edward County, Ontario on Lake Ontario's northeast shore. Related to lake effect, is the occurrence of fog over medium-sized areas, particularly along the shorelines of the lakes. This is most noticeable along Lake Superior's shores, due to its maritime climate.

The Great Lakes have been observed to help strengthen storms, such as Hurricane Hazel in 1954, and a frontal system in 2007 that spawned a few tornadoes in Michigan and Ontario, picking up warmth from the lakes to fuel them. Also observed in 1996, was a rare subtropical cyclone forming in Lake Huron, dubbed the 1996 Lake Huron cyclone.