A volcanic plug, also known as a volcanic neck, is a steep-sided landform that represents the hardened core of an ancient volcano. This geological feature is created when magma solidifies within the volcano’s central conduit, or vent, and is later exposed by the forces of erosion. The resulting structure is a tall, isolated rock column that stands prominently above the surrounding landscape.
How a Volcanic Plug Forms
The process of a volcanic plug forming begins deep beneath an active volcano as magma rises from a reservoir below through the main vertical channel. During an eruption, this magma travels upward through the conduit to the surface, but sometimes activity ceases before the entire magma column can be expelled. When the volcano becomes dormant, the molten rock remaining in the narrow vent begins to cool very slowly.
This slow cooling process allows the magma to crystallize into a mass of dense, intrusive igneous rock, which acts as a solid stopper in the volcano’s throat. The cooling material is much harder and more resistant to weathering than the materials that form the outer cone of the volcano. These outer materials, such as layers of ash, fragmented rock, and less dense lava flows, are relatively soft and easily broken down.
Over immense stretches of geologic time, often millions of years, the volcano’s conical exterior is relentlessly worn away by wind, water, and ice. This process is called differential erosion, where softer rock erodes faster than harder rock. The surrounding volcanic cone is stripped away, but the column of hard, solidified magma remains, standing tall and exhumed in bold relief.
The final, exposed volcanic plug is the remnant of the ancient volcano’s feeding system. This dense structure persists long after the rest of the mountain has vanished. The exhumation of the plug provides geologists with a glimpse into the deeper structure of a volcanic system.
Identifying Physical Characteristics
A volcanic plug’s most defining characteristic is its morphology, typically presenting as a tower-like, columnar, or cylindrical structure with steep, often near-vertical, sides. This shape directly reflects the diameter and vertical orientation of the original volcano’s vent, which can range from a few hundred meters to over a kilometer across. The height of the exposed plug can vary dramatically, from low hills to massive towers hundreds of feet high.
The rock composition of a plug is another distinguishing feature, consisting of dense igneous rock types such as basalt, phonolite, or trachyte. Since the magma cooled slowly beneath the surface, the resulting rock is much more compact and durable than the surrounding extrusive material that was eroded. In some plugs, the cooling process results in columnar jointing, where the rock contracts and fractures into distinct, polygonal columns, such as hexagons, running vertically down the sides.
These structures often taper downward, sometimes taking a funnel shape at depth, and may have dikes of the same hard rock radiating outward from the central mass. These radiating dikes represent magma that solidified in fractures leading away from the main vent, appearing today as linear walls of rock extending into the surrounding plain. The hardness of the rock allows the plug to resist erosion for so long.
Famous Volcanic Plugs Worldwide
Many famous landmarks around the world are the hard remnants of ancient volcanic plugs. Shiprock in New Mexico, USA, is one of the most spectacular examples, towering over 1,700 feet above the desert plain. This massive rock formation, which is sacred to the Navajo people, clearly shows the remnants of radiating rock walls, or dikes, that once extended from the main core.
Another globally recognized example is Devil’s Tower in Wyoming, which was the first United States National Monument. This immense, cylindrical plug is characterized by distinct, vertical columns caused by columnar jointing in the phonolite porphyry rock. In Scotland, Edinburgh Castle is perched atop Castle Rock, an ancient volcanic plug that provided a naturally defensible position for centuries.
The twin peaks of Gros Piton and Petit Piton in Saint Lucia are also prominent plugs that rise abruptly from the Caribbean Sea. These mountains soar over 2,400 feet and are recognized as a UNESCO World Heritage Site. These examples illustrate how the dense, solidified rock of a volcanic plug remains long after the volcano that created it has vanished.