DIATREME

A diatreme is a volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes... The term 'diatreme' has been applied more generally to any concave body of broken rock or tuff-breccia, generally formed by explosive or hydrostatic forces, whether or not it is related to volcanism. Some diatreme explosions may result from the interaction of hot magma and relatively shallow groundwater.