Volcanic Necks and Plugs: The Hardened Plumbing Left When a Volcano Erodes Away

Volcanic necks and plugs are the solidified remains of a volcano's conduit, its plumbing, left behind after the rest of the volcano has eroded away. The magma that froze in the conduit is usually much tougher than the volcano around it, which may have been loose tephra or rock weakened by hot fluids.

There is no real difference between the two terms. Both are roughly cylindrical, vertical, pipe-like bodies of dense rock standing out above the countryside as pinnacles or spires, sometimes hundreds of feet high. Sometimes they are surrounded by radial dikes, sheets of rock that were also part of the plumbing.

Strictly, necks and plugs are intrusive rock, since they solidified underground; but they were part of a volcano and froze at very shallow depth, perhaps right up to the old vent.

Beacon Rock, an 848-ft volcanic neck in the Columbia River Gorge and a landmark for Lewis and Clark. National Park Service.

In the national parks

At least ten units of the National Park System contain volcanic necks or plugs.

• Columbia River Gorge: Beacon Rock, along the Ice Age Floods National Geologic Trail and the Lewis and Clark National Historic Trail, is a neck standing 848 feet, now a Washington state park.
• Big Bend (Texas): the Chisos Mountains were volcanically active in the Cenozoic, and several features, such as Elephant Tusk, are probably necks.
• Devils Tower (Wyoming): about 867 feet tall and about 40.5 million years old, and still not fully understood. It may be a shallow intrusion or a volcanic neck; one idea is that it is what remains of a maar-diatreme volcano.
• Grand Canyon (Arizona): eruptions in the western canyon happened between about 75,000 and 725,000 years ago. Vulcan's Anvil, a neck, rises from the Colorado River just above Lava Falls, one of the hardest rapids in the canyon.
• Grand Canyon-Parashant (Arizona): the oldest volcanism here was in the Grand Wash field, active about 4 to 6 million years ago.
• Katmai (Alaska): Devils Desk is the neck of a composite volcano active nearly 250,000 years ago; erosion has removed almost everything but the conduit and its radial dikes.

Devils Tower. National Park Service.

Vulcan’s Anvil in the Colorado River, upstream of Lava Falls. Public domain.

A volcanic neck in the Grand Wash field. National Park Service.

Sources

• National Park Service, "Volcanic Necks and Plugs"; rewritten in hubnx's own words. Photographs: National Park Service and public domain. An openly licensed photograph by a private photographer is not reproduced here.