Volcano Learning zone > Volcano Glossary
A volcanic plug is a hardened magma which blocks the main vent of a volcano. The plug can cause pressure to build up within the volcano as new magma rises. Eventually a violent volcanic eruption can occur as the plug is blasted out of the volcano. Erosion can remove the softer rocks leaving the plug as a prominant pinnacle of solid rock. Castle rock in Edinburgh is a volcanic plug as is Ailsa Crag island also in Scotland.