Shiprock is composed of fractured volcanic breccia and black dikes of igneous rock called minette, or lamprophyre. It is the erosional remnant of the throat of a volcano, and the volcanic breccia formed in a diatreme. The rock probably was originally formed 2,500-3,000 feet (750-1,000 meters) below the Earth's surface, but it was exposed after millions of years of erosion. Wall-like sheets of minette, known as dikes, radiate away from the central formation. Radiometric age determinations of the minette establish that these volcanic rocks solidified about 27 million years ago. Shiprock is in the northeastern part of the Navajo Volcanic Field—a field that includes intrusions and flows of minette and other unusual igneous rocks that formed about 30 million years ago. Agathla (El Capitan) in Monument Valley is another prominent volcanic neck in this volcanic field.