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Volcanic Plug 

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Ailsa Crag Scotland Volcanic Plug Paul HartA 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.