Shiprock, in northwestern New Mexico, USA, is a volcanic plug protruding 480 meters out of the surrounding flat plain. Shiprock is a volcanic neck, the central basaltic cone that remained after the softer sediments surrounding the ancient volcano disappeared by erosion. Several dykes of the same basaltic rock, one of which is at left, fan out from the central cone into the surrounding plain. Monadnock is an overall name for this type of mountain.