The triple junction offshore the west coast is formed by which plates?

• A. Pacific plate, North American Plate, Gorda Plate
• B. Pacific plate, Juan de Fuca Plate, Cocos Plate
• C. Pacific plate, Australian Plate, North American Plate
• D. Pacific plate, North American Plate, Juan de Fuca Plate

Answer: (A)

A triple junction is the meeting point of three tectonic plates where their boundaries intersect. Off the west coast, the three plates that come together at this junction are the Pacific Plate on the west, the North American Plate to the east, and the Gorda Plate between them. The Gorda Plate lies between Pacific and North American crust, so their boundaries converge offshore northern California, forming the Mendocino triple junction.

The other plates listed would not define this offshore junction. The Juan de Fuca Plate sits between Pacific and North American farther north and forms its own junction with those two plates up near Cascadia, not at the Mendocino site. The Cocos and Australian plates are located in different regions and do not participate in this west-coast triple junction.