volcano: Occurrence

Volcanoes are found in association with midocean ridge systems (see seafloor spreading) and along convergent plate boundaries, such as around the Pacific Ocean's Ring of Fire (see plate tectonics), the ring of plate boundaries associated with volcanic island arcs and ocean trenches surrounding the Pacific Ocean. Continental volcanoes are also associated with converging plate boundaries, such as the volcanoes of the Cascade Range along the W coast of the United States. Isolated volcanoes also form in the midocean area of the Pacific apparently unrelated to crustal plate boundaries. These sea mounts and volcanic island chains, such as the Hawaiian chain, may form from magma plumes, called hot spots, that are believed to rise from the core-mantle boundary. An example of a continental hot spot is found at Yellowstone National Park, though the source of its magma may be from an ancient subducted tectonic plate.

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