A pseudocrater looks like a true volcanic crater, but is not. These distinctive landforms are created when flowing hot lava crosses over a wet surface, such as a swamp, a lake, or a pond causing an explosion of steam through the lava. The explosive gases break through the lava surface in a manner similar to a phreatic eruption, and flying debris builds up crater-like feature which can appear very similar to real volcanic craters. Pseudocraters are also known as rootless cones, since they are characterized by the absence of any magma conduit which connects below the surface of the earth.
A classic locality for pseudocraters is the Lake Myvatn area of northern Iceland that was formed 2,300 years ago by basaltic lava eruption. The lava flowed down the Laxárdalur Valley to the lowland plain of Aðaldalur where it entered the Arctic Ocean about 50 km away from Mývatn. There was a large lake in the area at the time, a precursor of the present-day Mývatn. When the glowing lava encountered the lake some of the water-logged lake sediment was trapped underneath it. The ensuing steam explosions tore the lava into small pieces which were thrown up into the air, together with some of the lake.
By repeated explosions in a number of locations, groups of craters built up and now dominate the landscape on the shore of Lake Mývatn and also form some of the islands in the lake. The Myvatn pseudocraters occur in several groups around the lake and as islands within the lake. A group of such craters at Skútustaðir on the south shore of the lake is protected as a natural monument and is frequented by tourists. Other pseudocrater groups in this lava field are in the Laxárdalur Valley and Alftaver district. Pseudocraters have also been discovered in the Athabasca Valles region of Mars, where lava flows superheated groundwater in the underlying rocks.
I've been to a few of these craters. Pretty cool.
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