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. A plug can cause an extreme build-up of pressure if magma is trapped behind it, and this can sometimes lead to a devastating eruption.
Often, erosion leads to the disappearance of the surrounding rock, while the erosion-resistant plug remains. Examples of landforms created in this way in the United States include Morro Rock, California; Chimney Rock, Nebraska; and Shiprock, New Mexico. Devils Tower in Wyoming is also thought to be a volcanic plug by many but not all geologists.
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.