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The three maars near Daun - Link to Wikimapia: http://wikimapia.org/#y=50175057&x=6844783&z=15&l=0&m=h&v=2

A maar is a broad, low relief crater that is caused by a phreatic eruption or explosion caused by groundwater contact with hot lava or magma. The maar typically fills with water to form a relatively shallow crater lake. The name comes from a local German dialect of Daun, which is in turn derived from Latin mare (sea). Maars are shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 60 meters (200ft) to 2000m (6,500ft) across and from 10 meters (30ft) to 200 meters (650ft) deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme.

Hole-in-the-Ground, in the US State of Oregon, a maar crater

Maars occur in the western USA, in the Eifel region of Germany, where they were originally described, and in other geologically young volcanic regions of the world. Kilbourne Hole and Hunt's Hole, near El Paso, Texas, are maars. The notorious, carbon dioxide saturated, Lake Nyos in Cameroun, Africa is another example. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet (1,980 m) across and 400 feet (120 m) deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks (sandstone, shale, limestone) of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths. The only maar-like volcano known in Canada is found in the Wells Gray-Clearwater Volcanic Field in east-central British Columbia.^[1]

• USGS Photo glossary of volcano terms: Maar
• USGS: Maars and Tuff Cones

• List of Maar Volcanoes
• Morale Claim Maar, Arizona

Ukinrek Maars, Alaska; the result of a 10-day eruption in 1977. Maar is about 300 metres (980 ft) in diameter.

Weinfelder Maar, one of the three maars originally described