Farallon plate
The Farallon Plate was an ancient, wholly oceanic tectonic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic period. Over time the central part of the Farallon Plate completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca Plate subducting under the northern part of the North American Plate, the Cocos Plate subducting under Central America and the Nazca Plate subducting under the South American Plate. A large fragment of the subducted plate is believed to presently be in the mantle under eastern North America.