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. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. A good example is Castle Rock (volcano), a volcanic plug that had volatile-charged magma still beneath it, leading to a Volcanic Explosivity Index 2 eruption. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive upstanding landform. Examples include Shiprock, New Mexico; Rockall, The Nut, Australia; the Pitons of Saint Lucia, and Deganwy Castle, Wales.
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. An example is the Castle Rock in Edinburgh, Scotland.
Further examples of volcanic plugs in the United States include Morro Rock, California; Lizard Head, Colorado; and Laurel Hill, New Jersey. Devils Tower in Wyoming, as well as Little Devils Postpile located in Yosemite National Park, are also thought to be a volcanic plug by many geologists. Another example of a volcanic plug in the eastern USA is the highly eroded Stark's Knob basaltic structure located along the Hudson River near Saratoga Springs, New York. However, some geologists believe Stark's Knob is not a plug at all, but merely an outcrop of an ancient submarine lava flow.
Ailsa Craig, Bass Rock, North Berwick Law and Dumgoyne hill are examples of volcanic plugs located in Scotland. There are over 30 volcanic plugs in Northern Ireland, including Slemish, Tievebulliagh, Scawt Hill, Carrickarede, Scrabo and Slieve Gallion.
There are several volcanic plugs in New Zealand, including the Pinnacles in the Coromandel Peninsula, Bream Head, Paritutu and adjacent Sugar Loaf Islands, and Piha's Lion Rock, which hosted a fortified Maori pa. Australia's Mount Warning is a volcanic plug.
Another example is Sigiriya (Lion's rock), the hardened magma plug from an extinct and long-eroded volcano. The rock rises 370 m (1,210 ft) and is sheer on all sides, in many places overhanging the base. It hosts an ancient rock fortress and ruins of a castle, one of the eight World Heritage Sites of Sri Lanka (Ref:202).
Another building on a volcanic plug is the 14th century Trosky Castle in the Czech Republic.
The origin of the famous Devils Tower (featured in the 1977 movie Close Encounters of the Third Kind) in Wyoming (USA) is not totally clear. Although it is generally admitted that it was formed by igneous intrusion, it is not necessarily a volcanic plug (stricto censu) which formed within the vent of an active volcano.
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