Volcanic plug

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A volcanic plug, also called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if rising volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. In a plinian eruption the plug is destroyed and ash is ejected. Glacial erosion can lead to exposure of the plug on one side, while a long slope of material remains on the opposite side. Such landforms are called crag and tail. If a volcanic plug is later covered by softer, looser layers of tephra, such as ash and hydrothermally altered lava, then agents of erosion may in time remove that softer surrounding rock while the plug's harder, less erodible congealed magma remains, producing a distinctive upstanding, necklike landform.

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