How Are Volcanic Arcs Formed?

Volcanic arcs are formed when an oceanic plate is forced under a continental plate and the water-soaked crust melts as it's forced below the ocean floor, creating magma that feeds volcanoes on the continental side of the subduction zone. Stratovolcanoes are created in an arc around the subduction zone. The geologic phenomenon occurs all around the world but is most commonly seen around the Pacific Ocean tectonic plate in a formation known as the ring of fire.

In North America, the volcanoes of the Cascade range, including Mount Shasta, Mount Ranier, Mount Hood and Mount Saint Helens, form a volcanic arc where the Jaun de Fuca Plate subducts under the North American Plate. The arc is nearly 700 miles long and extends from the Mendicino Triangle in Northern California into British Columbia.

Farther north is the Aleutian Arc, formed by the subduction zone in the Aleutian Trench. The volcanic arc extends from Southeast Alaska 1,500 miles along the Aleutian Islands chain across the northern Pacific Ocean to the Kamchatka Peninsula of Russia. Over 100 volcanoes line the arc.

Volcanic arcs form a distinct pattern and lie directly over subduction zones. They should not be confused with hot spots, such as the Hawaiian Islands in the middle of the Pacific Ocean.