The remnant of an ancient
volcano's central magma pathway that has been exposed by
erosion.
The magma in a volcano's central plumbing system cools much more slowly than
extrusive igneous rocks because it is cooling beneath the surface. These slower-cooled rocks tend to be more resistent to erosion. Therefore, after a volcano has become extinct and is exposed to weathering and erosion, the flanks get eroded to a greater degree than the old central
shaft. This leaves the
neck standing exposed.
One example of a volcanic neck is
Devil's Tower in
Wyoming which is well known due to its appearance in
Steven Spielberg's
Close Encounters of the Third Kind. Another example is
Shiprock Peak in
New Mexico. Shiprock Peak is one of hundreds of volcanic necks that can be seen when driving along Route 64 near the
Four Corners.