lake

The crater of a long-dead volcano in Iceland, which has filled with water to form a crater lake. Iceland's system of volcanoes, many of them active, has been created as a result of the movement in this area of two of the Earth's tectonic plates.

Sunset over Indian Lake, Wisconsin, USA. Approximately 39% of the total surface area of the state of Wisconsin is occupied by lake water. Its two best-known lakes are Lake Michigan and Lake Superior, but there are thousands of other smaller lakes formed by glaciers during the last Ice Age.

An aerial view of the lakes near Selfoss, a small town in southern Iceland. It is a centre for food processing and manufacture. Since the building of a bridge over the River Ölfusá, the town has evolved into the commercial centre of the region, providing a variety of financial, legal, and retail services.

Body of still water lying in depressed ground without direct communication with the sea. Lakes are common in formerly glaciated regions, along the courses of slow rivers, and in low land near the sea. The main classifications are by origin: glacial lakes, formed by glacial scouring; barrier lakes, formed by landslides and glacial moraines; crater lakes, found in volcanoes; and tectonic lakes, occurring in natural fissures.

Crater lakes form in the calderas of extinct volcanoes, for example Crater Lake, Oregon, USA. Subsidence of the roofs of limestone caves in karst landscape exposes the subterranean stream network and provides a cavity in which a lake can develop. Tectonic lakes form during tectonic movement, as when a rift valley is formed. Lake Tanganyika was created in conjunction with the East African Great Rift Valley. Glaciers produce several distinct types of lake, such as the lochs of Scotland and the Great Lakes of North America.

Lakes are mainly freshwater, but salt and bitter lakes are found in areas of low annual rainfall and little surface run-off, so that the rate of evaporation exceeds the rate of inflow, allowing mineral salts to accumulate. The Dead Sea has a salinity of about 250 parts per 1,000 and the Great Salt Lake, Utah, about 220 parts per 1,000. Salinity can also be caused by volcanic gases or fluids, for example Lake Natron, Tanzania.

In the 20th century large artificial lakes have been created in connection with hydroelectric and other works. Some lakes have become polluted as a result of human activity. Sometimes eutrophication (a state of overnourishment) occurs, when agricultural fertilizers leaching into lakes cause an explosion of aquatic life, which then depletes the lake's oxygen supply until it is no longer able to support life.