When groundwater is heated below the surface to boiling by hot igneous rocks what forms?

Hot springs or geysers are created when groundwater gets heated by hot intrusive rocks and can ascend via fractures in the crust.

In the most extreme case so called phreatomagmatic eruptions take place. When magma ascends and reaches groundwater at shallow depths, the resulting steam expands dramatically and fractures the overlying rock thus producing a conical hole in the ground. These volcanic craters are referred to as maars.