Introduction
Volcanic necks and plugs are the solidified remains of volcanoes’ conduit and plumbing systems that remain after the rest of a volcano has been eroded away. Solidified conduits are usually more resistant to erosion than the surrounding volcanic edifice which may have consisted of loose tephra or hydrothermally altered rock.
No real differences exist between volcanic necks and plugs. Both are roughly cylindrical in shape and stand out in relief about the surrounding countryside. They appear as vertical pipelike bodies of dense rock that can form rock pinnacles or spires that may stand up to hundreds of feet (meters) in height.
Sometimes systems of radial dikes that were also part of the volcano’s plumbing system are found in association with volcanic necks.
Technically, volcanic necks and plugs are intrusive because they consist of rock that solidified within the Earth. But they were once part of volcanoes and solidified at very shallow depths, potentially even ascending to the level of the former eruptive vent.
Source: U.S. Department of the Interior, National Park Service
