Rootless Cones - volcanoes without plumbing
Rootless cones, or pseudocraters, are volcanic cones that form from phreatomagmatic volcanic eruptions. This type of eruption results from an interaction between magma/lava and water. Rootless cones differ from true volcanoes by their absence of a magmatic feeder system, or ‘root’, which transports magma from a reservoir to the surface. Instead, these eruptions are driven by steam created by superheating and subsequent vaporization of groundwater by an advancing lava flow. As the lava flows over a water-logged area the surface of the lava flow cools to form a hardened crust while hot, molten lava continues to flow beneath. At the same time, the water underneath the lava flow is being rapidly heated and converted into steam. When the pressure of this steam has built enough to exceed the strength of the overlying lava flow it forces its way through and explosive venting occurs, creating a tephra cone.