A volcanic plug is a landform created when volcanic magma hardens inside a vent on an active volcano. They are sometimes called volcanic necks or puys. The plug is revealed when surrounding land is eroded.
As it forms, a plug may cause an extreme build-up of pressure if gas-charged magma is trapped beneath it. This sometimes leads to an explosive eruption. But if the explosion does not occur, then the volcano becomes a solid block made of hard material.
The plug is usually harder than the surrounding rock. 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. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains. This is what produces the distinctive upstanding landform.