mud-volcano

from The Century Dictionary.

• noun A conical hill or miniature volcano surrounding an orifice or crater, and the result of the pressure and escape from below of steam or gases, given out either continuously or at intervals. Such accumulations of mud are not uncommon in regions of dyingout volcanism, the material being the result of the softening and decomposition of the lava or ashes by solfataric agencies. Somewhat similar mud-cones or mud-volcanoes sometimes occur in regions not volcanic, where they appear to be caused by the combustion of sulphur or of coal.