How Volcanoes Work

 Image Described in Caption
 Dike from Shiprock, New Mexico -- Shiprock (background) is a large volcanic neck, a monolith composed of resistant volcanic rock that crystallized in the upper level of a volcano's central vent system. The volcano became extinct about 27 million years ago and has eroded away, leaving the volcanic neck exposed at the surface. Several dikes radiate away from the volcanic neck. These were radial fissures from which magma rose upward from the underling magma chamber. Having cooled and crystallized in the fissure, the resulting rock structure forms a dike. These hard rocks are also relatively resistant to erosion, and thus stand above the horizon as massive walls of lava. Photo by Vic Camp.