A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma 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 upstanding landform. Examples include Shiprock, New Mexico; The Nut, Australia; and the Pitons of Saint Lucia.

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. An example is the Castle Rock in Edinburgh, Scotland.

Further examples of volcanic plugs in the United States include Morro Rock, California; Lizard Head, Colorado; and Laurel Hill, New Jersey. 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. Another 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.