A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar, or in an impact crater caused by a meteorite. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.
Crater lakes form as incoming precipitation fills the depression. The lake deepens until an equilibrium is reached between the rate of water coming in and the rate of water loss due to evaporation, subsurface drainage, and possibly also surface outflow if the lake fills the crater up to the lowest point on its rim. Surface outflow can erode the deposits damming the lake, lowering its level. If the dam erodes rapidly, this can produce a breakout flood.
A well-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon, USA. It is located in the caldera of Mount Mazama, hence the name "Crater Lake" is somewhat of a misnomer. It is the deepest lake in the United States with a depth of 594 m (1,949 ft). Crater Lake is fed solely by falling rain and snow, with no inflow or outflow at the surface, and hence is one of the clearest lakes in the world.
The highest volcano in the world, 6,893 metres (22,615 ft) Ojos del Salado, has a permanent crater lake about 100 metres (300 ft) in diameter at an elevation of 6,390 m (20,960 ft) on its eastern side. This is most likely the highest lake of any kind in the world.
Due to their unstable environment, some crater lakes exist only intermittently. Caldera lakes in contrast can be quite large and long-lasting; for instance, Lake Toba formed after its eruption around 70,000 years ago and has an area of over 1,000 square kilometres.
While many crater lakes are picturesque, they can also be deadly. Gas discharges from Lake Nyos suffocated 1,800 people in 1986, and crater lakes such as Mount Ruapehu's often contribute to destructive lahars.
Lakes can also fill impact craters, but these are not usually referred to as crater lakes except in a few isolated cases. Example of such impact crater lakes include Manicouagan in Canada, Lake Bosumtwi in Ghana and Siljan in Sweden.
