volcanic plug

A volcanic plug is the stopper in the genie bottle of volcanic extrusion, left as a record of gravity reversed from below. A volcanic vent may spew ash and build a cinder cone, the vent pipe being filled last by a denser column of molten rock. If this hardens as a vertical core, the ash cone may be eroded in time to leave the core plug as a standing spire. Well-known examples are Smith Rock, Oregon; Ship Rock, New Mexico; and the Boar’s Tusk in Wyoming’s Jack Morrow Hills.

Kim Stafford