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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 pressure if rising volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. 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 plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive upstanding landform.
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An example of two volcanic plugs can be found at the Pitons, in Saint Lucia, as they rise abruptly out of the eastern Caribbean Sea.
In Africa
Near the village of Rhumsiki in the Far North Province of Cameroon, Kapsiki Peak is an example of a volcanic plug and is one of the most photographed parts of the Mandara Mountains. Spectacular volcanic plugs are present in the center of La Gomera island in the Canary Islands archipelago, within the Garajonay National Park.
In Australasia
There are several volcanic plugs in New Zealand, including the Pinnacles in the Coromandel Peninsula, Bream Head, Paritutu and the adjacent Sugar Loaf Islands, St. Paul's Rock at Whangaroa Harbour, and Piha's Lion Rock, which hosted a fortified Maori pa.
In New Zealand's South Island, Ōnawe Peninsula on Banks Peninsula is a prominent volcanic plug, and erosion of Otago's Saddle Hill has also revealed a plug.
In Australia, the The Nut in Tasmania is a further example, along with Mount Warning.
In Europe
Borgarvirki is a volcanic plug located in north Iceland. A volcanic plug is situated in the town of Motta Sant'Anastasia in Italy.
Saint Michel d'Aiguilhe chapel, whose construction started in 969, near Le Puy-en-Velay in France. The volcanic plug rises about 85 metres (279 ft) above the surroundings. Another building on a volcanic plug is the 14th century Trosky Castle in the Czech Republic.
Rockall, a small, uninhabited, remote islet in the North Atlantic Ocean, is also a volcanic plug.
In the United Kingdom, two examples of a building on a volcanic plug are the Castle Rock in Edinburgh, Scotland, and Deganwy Castle, Wales. The Law, Dundee, Ailsa Craig, Bass Rock, North Berwick Law and Dumgoyne hill are other examples of volcanic plugs located in Scotland. There are over 30 volcanic plugs in Northern Ireland, including Slemish in Ballymena, Tievebulliagh, Scawt Hill, Carrickarede, Scrabo and Slieve Gallion.
In the Americas
Examples of volcanic plugs in the United States include Morro Rock, California 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.
The origin of Devils Tower (featured in the 1977 movie Close Encounters of the Third Kind) is not totally clear. Although it is generally admitted that it was formed by igneous intrusion, it is not necessarily a volcanic plug, formed within the vent of an active volcano.
In New Mexico, Shiprock is a further example.
The Pitons of Saint Lucia, in the eastern Caribbean Sea are two volcanic plugs that rise abruptly out of the sea and are part of a UNESCO World Heritage site. The highest mountain plug rises to a height of nearly 760 metres (2,490 ft).
In Canada, the Castle Rock, located in British Columbia, is a volcanic plug that had volatile-charged magma still beneath it, leading to a Volcanic Explosivity Index 2 eruption.