Lakes Essay

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Lakes are b odies of water surrounded by land. They cover about one percent of the surface of the earth and contain about 0.02 percent of the water of the earth. Lakes are relatively new from a geological point of view. They are very transitory because they are continually being formed or destroyed. Some are destroyed because they are dried up by declines in their water source(s), or by silting, or by earthquakes, or by other geologic actions. Humans have also played a major role in the destruction of lakes. Excessive drawing down of the water levels of lakes by humans can easily destroy their unique ecosystems. For example, Lake Huleh on the Jordan River above the Sea of Galilee (Lake Tiberius) has been drained nearly dry in the last 50 years to supply human consumption.

Lakes vary widely in their biological, chemical, and physical features. Each lake has its own origin, occurrence, size, shape, sedimentation attributes, circulation patterns, water chemistry, water depth, and life forms. The water chemistry of lakes varies widely. In Croatia, the Plitvice Lakes have unique limestone water chemistries. The dissolution of the limestone rock has created mildly acidic water. The lakes are in the karst region where there are few other surface sources of water. In contrast, many other lakes have high acid concentrations or in other cases have a basic pH number. Some lakes are shallow; others such as Lake Superior and Lake Baikal in Siberia are thousands of feet deep. Some cover only small areas and others are vast seas.

The sedimentation patterns (lacustrine deposits) in lakes are very important to geologists. Lacustrine deposits in lakes that have not been subjected to the actions of glaciers are stable because they remain at the bottom of the lake forming undisturbed layers of sediment. The sediment characteristics of modern lakes are used to interpret the sedimentary records of ancient lakes. From these sediments in ancient lakes, conclusions can be inferred about the earth’s climate and life forms in earlier geologic times.

Limnologists have classified lakes into 76 types. The major categories are tectonic, glacial, volcanic, dissolution, landslide-influenced, fluvial-influenced, wind-formed, coastal, and meteorite impacts.

Tectonic Lakes

Great movements of the earth’s crust have formed tectonic lakes. Lake Tanganyika, Lake Malawi, and Lake Victoria are African rift valley lakes. They are located in the Great Rift Valley, which also includes the Dead Sea and the Sea of Galilee (Lake Tiberius). Other tectonic lakes are Lake Baikal in Russia, which is in the Baikal Rift Zone of the Siberian Platform. In the United States, Lake Tahoe and Pyramid Lake in Nevada are in the Basin and Range Province of the west. There are other smaller lakes in the region that are also the result of great shifts in the earth’s crust including Lake Bonneville and the Great Salt Lake. Some tectonic lakes are shallow. Reelfoot Lake in northwestern Tennessee was one of several lakes created by the New Madrid Earthquake (1811-12). Centered near New Madrid, Missouri, the quake is believed to have been of a magnitude of 8.0 on the Richter scale.

Glacial Lakes

When the glaciers retreated after the last ice age, they created numerous periglacial lakes. These were lakes formed by the ice sheet or glacier, and the ice created obstructions that blocked the flow of water. Many lakes in Scandinavia are glacial. The Great Lakes-Lakes Ontario, Erie, Huron, Michigan, and Superior-are glacial lakes. In addition, many lakes in Canada, including the Great Slave Lake and the Great Bear Lake, are glacial lakes. In North America, the retreat of the glaciers left depressions in the ground that have since filled with water. These potholes or kettle lakes are very important ecologically for the breeding of ducks, geese, and other waterfowl. In Europe, glacial lakes formed by glacial sediment creating natural dams include Lakes Geneva, Lucerne, Zurich, Constance, and many others in Austria and Slovenia. In New York, the Finger Lakes were formed from glacial action. Subglacial lakes are in liquid form even though they are covered with ice. Lake Vostok in Antarctica is the world’s largest. The ice acts as a thermal insulator so that energy is retained in the water. In other subglacial lakes, pressure from the mass of the ice sheet keeps the water liquid. In some cases, geothermal heating causes the water under the ice to melt or to remain liquid.

Volcanic Lakes

Volcanic holes left by volcanic explosions or by collapse of a volcano provide collection points for volcanic lakes. Crater Lake in Crater Lake National Park, Oregon, was formed in a volcanic pipe or caldera created when the 12,000 ft. (3,660 m.) high Mount Mazama collapsed 7,700 years ago following a large eruption. Caldera or volcanic lakes are found around the world. Many volcanic lakes are very beautiful such as the Caldera de Taburiente on the island of La Palma in the Canary Islands.

Dissolution Lakes

The erosion of bedrock forms dissolution lakes. Usually the rock is limestone. As it dissolves a cave is formed, and if the cave’s roof eventually collapses, a sinkhole is created. Many lakes in Florida were formed as sinkholes. Silver Springs is one of the best known. Lake Skadar is a dissolution lake located at the southern end of the Dinaric Alps on the border of Yugoslavia and Albania.

Other Lakes

Eudorheic lakes are terminal or closed lakes with insignificant amounts of water flowing in or out. The primary method for water to leave is by evaporation. These lakes are usually located in desert areas. Lake Eyre in central Australia and the Aral Sea are examples. The Hueco Tanks in western Texas are natural rock basins containing trapped rainwater. The water leaves by evaporation or is removed by travelers. Meromictic lakes have a water chemistry that is unique. The various layers of water in the lake remain stationary and unmoved so that they do not mix. As a result, the bottom of the lake has not dissolved oxygen. It is often saline as well. These kinds of lakes will have bacteria that feed on sulfur compounds. Earthquakes can disturb the equilibrium in the layers of water and cause a limnic eruption. When this happens, the water in the lake overturns. There may be sudden triggering of carbon dioxide. In 1984, Lake Monoun in Cameroon overturned and caused the death of 37 people. In 1986, nearby Lake Nyos overturned killing 1,800 people.

Coastal lakes include those formed by water trapped behind sand dunes, such as the Okefenokee Swamp in Georgia. In northern Poland on the Baltic Coast, Lakes Lebsko, Gardno, Dolgie Wielkie, and Dolgie Male are coastal lakes formed by the isolation of part of a bay on the Baltic.

Meteorites, asteroids, and comets striking the earth are also a source of lake formation. Mistastin Lake in Newfoundland and Labrador, Canada, are thought to be impact crater lakes as is Lake Lappajarvi in central Finland.

Wind forms lakes, especially in hot or cold desert regions. Some lakes on the north slope of Alaska may be forming in this way. In hot desert areas playa lakes are standing bodies of water left after rare periods of rainfall. These lakes are soon dried by evaporation and leave precipitated salts on their shores to resemble a beach. The meandering of rivers can form lakes. Many ox bow-shaped lakes that are the remnants of the old riverbed parallel the lower Mississippi River.

Bibliography:

  1. D. Alt, Glacial Lake Missoula and Its Humongous Floods (Mountain Press Publishing Company, 2001);
  2. P.M. Bethke and L. Hay, Ancient Lake Creede: Its Volcano-Tectonic Setting, History of Sedimentation and Relation to Mineralization in the Creede Mining District (Geological Society of America, 2000);
  3. Jerry Dennis, The Living Great Lakes: Searching for the Heart of the Inland Sea (St. Martin’s Press, 2004);
  4. Richard Wold and W.J. Hinze, Geology and Tectonics of the Lake Superior Basin (Geological Society of America, 1988).

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