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. If rising volatile-charged magma is trapped beneath a plug, the resulting extreme build-up of pressure can sometimes lead to an explosive eruption. Eons later, erosion can reveal the more resistant plug forming residual landforms. For example, Edinburgh Castle in Scotland is built upon an ancient volcanic plug.
Two Colorado examples are shown below.
Needle Rock Plug on the left and Tater Heap Laccolith on the right.