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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.
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 is 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. Also, the little devils postpile located in Yosemite National Park. However, some geologists believe Stark's Knob is not a plug at all, but merely an outcrop of an ancient submarine lava flow.
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.