- This page describes the geological feature. For the lake of the same name, see Crater Lake.
A crater lake is a lake that forms in a volcanic crater or caldera after the volcano has been inactive for some time. Incoming precipitation fills the depression to form a deepening lake, 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. Crater lakes covering active (fumarolic) volcanic vents are often known as volcanic lakes, and the water within them is typically 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 is often exceptional due to the lack of inflowing streams and sediment.
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 . Crater Lake is fed solely by falling rain and snow, with no inflow or outflow at the surface, and hence has the clearest water of any lake in the world.
The highest volcano in the world, Ojos del Salado, has a permanent crater lake about in diameter at an elevation of on its eastern side.<ref>http://www.andes.org.uk/peak-info-6000/ojos-info.htm</ref> 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 1000 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 Lake Bosumtwi in Ghana and Siljan in Sweden.
Notable crater lakes
- USGS Hawaiian Volcano Observatory: Water on volcanoes: heavy rain and crater lakes
- USGS Cascades Volcano Observatory: Volcanic Lakes