pseudocrater
https://en.wikipedia.org/wiki/Pseudocrater
rootless vent
A volcanic landform which resembles a true volcanic crater, but differs in that it is not an actual vent from which lava has erupted. A pseudocrater is characterised by the absence of any magma conduit which connects below the surface of the earth. Pseudocraters are formed by steam explosions as flowing hot lava crosses over a wet surface, such as a swamp, a lake, or a pond. The explosive gases break through the lava surface in a manner similar to a phreatic eruption, and the tephra builds up crater-like forms which can appear very similar to real volcanic craters.
crater