Volcanic plug

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Edinburgh Castle in Scotland is built upon an ancient Volcanic Plug
Edinburgh Castle in Scotland is built upon an ancient Volcanic Plug

A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when lava hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive landform.

Volcanic plug in the Ochoco National Forest of Oregon, USA
Volcanic plug in the Ochoco National Forest of Oregon, USA
Volcanic plug near Rhumsiki, Far North Province, Cameroon
Volcanic plug near Rhumsiki, Far North Province, Cameroon

Examples of landforms created in this way in the United States include Morro Rock, California; Lizard Head, Colorado; and Shiprock, New Mexico. Devils Tower in Wyoming, as well as Little Devils Postpile located in Yosemite National Park, are also thought to be a volcanic plug by many geologists. The only example of a volcanic plug in the eastern USA is the highly eroded Stark's Knob basaltic structure located along the Hudson River near Saratoga Springs, New York.[citation needed] However, some geologists believe Stark's Knob is not a plug at all, but merely an outcrop of an ancient submarine lava flow. The Ailsa Craig, Bass Rock and North Berwick Law are examples of volcanic plugs located in Scotland. Borgarvirki is a volcanic plug located in north Iceland.

Subsequent glacial erosion can lead to exposure of the plug on one side, while a long slope of material remains on the lee side. Such landforms are called crag and tail. Examples include the Castle Rock in Edinburgh, Scotland.