Two oceanic rises, present near the coast of the northwestern United States, are bordered on the east by the continental rise, on the west by abyssal hills and plains, and on the north and south by fracture zones. A third fracture zone separates and smaller zones transect the rises. Physiographic provinces on the rises include a crest province with a median valley, a flank province, and a transition province. The crest of Gorda Rise has greater relief and a better-developed median valley and associated positive magnetic anomaly than the Cobb Rise. Both may be Miocene or older. Their physiographic provinces, particularly those of Gorda Rise, are more similar to those of the Mid-Atlantic Ridge than to those of the East Pacific Rise.
In previous studies, Cobb and Gorda rises have been interpreted as northern extensions of the East Pacific Rise in the southeast Pacific. Alternative interpretations are that the northern rises may be a rejuvenated segment of an older arcuate rise off the west coast of the United States, or that they may form an auxiliary ridge associated with the Mendocino Fracture Zone and are possibly unrelated to the East Pacific Rise.