Volcanic plug

A volcanic plug—also known as a volcanic neck or lava neck—forms when magma solidifies inside a volcano's vent. If this plug blocks the vent, rising magma charged with gas can become trapped beneath it, building up extreme pressure that may trigger an explosive eruption. In a Plinian eruption, the plug is destroyed and ash is blasted out.

Glacial erosion can expose a plug on one side while leaving a long slope of material on the opposite side, creating a landform known as crag and tail. When a plug is later buried by softer, looser layers of tephra—such as ash and hydrothermally altered lava—erosion can wear away that surrounding rock, leaving the harder, more resistant congealed magma standing as a distinctive, neck-like feature.

Examples of volcanic plugs are found worldwide. In Africa, Kapsiki Peak in Cameroon, plugs in La Gomera's Garajonay National Park, and Pico Cão Grande in São Tomé and Príncipe are notable. Asia's Sigiriya in Sri Lanka is a 180-meter-high rock fortress and UNESCO World Heritage Site. European examples include Borgarvirki in Iceland, a plug in Motta Sant'Anastasia, Italy, Saint Michel d'Aiguilhe chapel in France, Trosky Castle in the Czech Republic, Strombolicchio in the Aeolian Islands, and Rockall in the North Atlantic. In the United Kingdom, Castle Rock in Edinburgh, Deganwy Castle in Wales, and numerous Scottish plugs such as The Law, Ailsa Craig, and Bass Rock exist, along with over 30 in Northern Ireland, including Slemish and Tievebulliagh. North America and the Caribbean feature Morro Rock in California, Devils Elbow in Oregon, Thumb Butte in Arizona, Agathla Peak, Shiprock, and Cabezon Peak in the U.S. Southwest, Devils Tower in Wyoming, Little Devils Postpile in California, Castle Rock in British Columbia, and the Pitons in Saint Lucia. South America's Pinnacle Rock is in the Galápagos, Ecuador. Oceania has many in New Zealand's North Island—such as the Pinnacles, Bream Head, Paritutu, Lion Rock, and Mount Pohaturoa—and in the South Island, Onawe Peninsula, Saddle Hill, and Mount Cargill. Australian examples include The Nut in Tasmania, Mount Warning, peaks in the Warrumbungles, and the Glasshouse Mountains in Queensland.