A volcanic neck forms when lava inside a caldera cools after magma stops feeding the mountain, and then the outside of the extinct volcano erodes after millions of years. The cooled lava remains intact because it is harder than the surrounding rock that eroded. Perhaps the most famous volcanic neck in the United States is Devil's Tower in Wyoming.
Volcanic necks are also known as plugs because they seem to plug a spot in Earth's crust where magma was once active. Most volcanic plugs are cylindrical in shape. The rock is composed of the same material as volcanic ash, but volcanic necks also contain higher-than-normal amounts of magnesium and iron from deeper inside Earth's crust and possibly from the mantle.
Another famous volcanic plug formation in the United States is Ship Rock in New Mexico, which is 1,700 feet higher than the surrounding landscape. Other volcanic plugs are in the western United States, Germany, South Africa, Siberia and Tanzania.
Volcanic necks are rare because most volcanic plugs cause cataclysmic eruptions within mountains, such as the 1980 eruption of Mount St. Helens in the state of Washington and the explosive discharge of Krakatoa in the 1880s. Volcanic plug formations do not contain enough pressurized magma to blow the top off of a mountain. Instead, the interior of the volcano slowly cools over time.