A volcanic neck, also known as a volcanic plug, is a geological landform representing the solidified core of an ancient volcano’s main conduit. This feature is created when magma hardens within the central vent, or “throat,” of a volcano. The resulting structure is a dense, erosion-resistant mass of rock that stands alone long after the rest of the mountain has disappeared. This geological remnant provides a direct view into the inner workings of a long-extinct volcanic system.
Anatomy of a Volcanic Neck
The physical structure of a volcanic neck is essentially a vertical, pipe-like body of igneous rock that filled the central passageway of the former volcano. This rock formed when molten material cooled and crystallized slowly beneath the surface. The composition is typically fine-grained, dense rock such as basalt or phonolite, but can also be a mix of congealed magma and consolidated volcanic breccia. These hardened cores can range significantly in size, with some necks rising hundreds of feet in height and having a diameter of over a kilometer. In some cases, the volcanic neck is found in association with radial dikes, which are sheets of igneous rock that solidified in cracks extending outward from the central conduit.
How Differential Erosion Creates the Feature
The exposure of a volcanic neck begins after a volcano becomes extinct and is subjected to the forces of nature. An active volcano is primarily built from relatively soft materials, including layers of loose ash, cinders, and less consolidated lava flows. The central column, however, is filled with magma that cools slowly into a solid, crystalline mass of rock, making it significantly harder and more cohesive than the outer cone material. Over millions of years, the surrounding cone material is gradually worn away by wind, rain, and ice in a process called differential erosion. This mechanism preferentially removes the softer rock, allowing the dense, tough volcanic neck to remain standing as a prominent, often towering, pillar or spire.
Geological Importance and Famous Sites
Volcanic necks offer a unique cross-section view into the internal structure and plumbing of ancient volcanoes. The rock composition within the neck can reveal details about the magma source and the conditions of crystallization deep beneath the earth’s surface. In rare instances, the neck rock contains fragments of the Earth’s deep crust or upper mantle that were carried upward by the ascending magma, providing insights into the planet’s interior. Shiprock in New Mexico is a well-known example that rises about 550 meters (1,800 feet) above the desert floor and is accompanied by its prominent radial dikes. Devil’s Tower in Wyoming, which stands approximately 265 meters (867 feet) tall, is another famous site, distinguished by its spectacular vertical columns.
Famous Sites
The formation beneath Edinburgh Castle in Scotland is also an example of a volcanic neck upon which a historic structure was built.