Plate Tectonics in Puget Sound

Along the western coast of the continental United States sits the North American Plate, the Pacific Plate, and the Juan de Fuca Plate: all are shaping the landforms of the Pacific Northwest region. The North American Plate has a relative motion towards the south – southeast, while the relative motion of the Pacific Plate is to the north – northeast. The Juan de Fuca Plate, in the Pacific Northwest, is a remnant piece of the Farallon Plate. The majority of the Farallon Plate has been subducted under the North American Plate, helping to form much of the Basin and Range topography and the Sierra and Rocky Mountains in the western United States (see Figure 1).

The Pacific Plate has a spreading center, similar to the mid-oceanic ridge in the Atlantic only spreading at a much slower rate, called the East Pacific Rise. Portions of the East Pacific Rise have been subducted under the North American Plate, giving rise to the San Andreas Fault system. The Juan de Fuca Plate is currently being subducted under the North American Plate, mostly driven down by its own weight.

Figure 2. Plate interactions along the continental western United States. [Image courtesy of U.S. Geological Survey]

Another type of plate interaction in Puget Sound is the trench-ridge interaction. A trench-ridge interaction is when an ocean ridge collides with a subduction zone. The subduction zone marks the beginning of the trench, with which intense earthquake activity is associated. As one plate dives beneath the other along the subduction zone, magma works its way to the surface, creating arcs. When the ridge is subducted, the arc moves in an uphill fashion and becomes emergent as the ridge crest approaches the subduction zone. The Juan de Fuca Ridge is currently undergoing such an interaction. The ridge is a broad submarine mountain chain about 500 kilometers long (300 miles). Along the crest of the Juan de Fuca Ridge there is a broad valley less than 8 kilometers wide (5 miles) where young volcanoes, lava flows, and hot springs occur regularly, causing new ocean crust to develop.

Reference

Condie, Kent C. 1997. Plate Tectonics and Crustal Evolution. Butterworth-Heinemann. Oxford. 4th edition.

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