Volcanic Landscape, Navajo Reservation, Arizona, 2004

(Click image to enlarge)

The rocky prominance in the foreground is a volcanic dyke. Dykes are a sheet of rock that is formed in a fracture of a pre-existing rock body that was infiltrated by molten lava. Volcanic plugs, also called volcanic necks or lava necks, are much more rare. These are created when magma hardens within a vent on an active volcano. There is an an enormous one on the horizon near the right side of the image that rises over 1500 feet (457 meters) above the surrounding terrain. Immediately to its left is another volcanic dyke. If you look carefully at the larger image you will notice several volcanic features throughout the scene. In all cases these formed underground and subsequently the exterior rock eroded away leaving the much harder lava exposed.

The large volcanic plug and two dykes I enumerated above can be seen from a different viewpoint in different conditions along with more information about their history here.