Juan de Fuca Plate Map

The Juan de Fuca Plate is a small oceanic plate in the northeast Pacific, between the Pacific and North American Plates. It is much smaller than the other plates on this list. Its whole boundary with the North American Plate is convergent in PB2002.¹

Its boundary with the Pacific Plate is the Juan de Fuca Ridge, a spreading ridge off the coast of Oregon and Washington, offset by transform faults. USGS reports that in June 1993, seismic signals typical of submarine eruptions were detected along part of this ridge, and repeated surveys there have mapped fresh lava. To the south, the San Andreas fault connects this ridge system with the East Pacific Rise.

The plate is what is left of a much bigger one. USGS describes the Juan de Fuca Plate as a remnant of the formerly much larger oceanic Farallon Plate. It will someday be entirely consumed as it continues to sink beneath the North American Plate off the coast of Oregon and Washington (USGS). Plates made partly or entirely of oceanic lithosphere can sink under another plate in this way and disappear completely.

Its sinking edge feeds a line of volcanoes on land. USGS names the Cascade Range in the Pacific Northwest as an example of active volcanoes sustained by oceanic-continental convergence (USGS). Mount St. Helens in Washington State is one of them, a composite cone. Volcanoes of this kind typically erupt with explosive force, because their magma is too stiff to let volcanic gases escape easily (USGS).

The ridge on its western side is part of a much larger system. USGS notes that the global mid-ocean ridge system is about 50,000 km long, and that about three quarters of all lava erupted on Earth comes out unseen beneath the ocean, mostly along spreading centers like this one.