Geomorphologically, a volcanic neck represents the skeletal remains of an extinct volcano. When a volcano stops erupting, the magma remaining in its conduit cools and hardens into dense igneous rock like basalt or rhyolite. Because this “neck” is much harder than the layers of ash and lava that form the rest of the mountain, it resists erosion far better. Eventually, the mountain’s exterior is stripped away, leaving the vertical pipe standing alone. Shiprock in New Mexico and the Devil’s Tower in Wyoming are world-famous geographical examples of these striking, isolated volcanic monoliths.