Major PNW Geologic Structures

The defining geologic structures I was listing in the previous section all exist in Washington, and show abundant evidence of continued activity.

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Firstly, a Volcanic Arc is a belt of volcanoes formed above a subducting oceanic plate, situated right above the area of the subduction zone where the oceanic plate begins to melt due to intense heat. Magma rises to the surface of the Earth in this area due to a plethora of complicated geochemical and geophysical factors that will be discussed at greater length in the homepage of the “Cascade Volcanoes” section of the website. The Volcanic Arc that exists in Washington and the greater PNW is the Cascade Range, with marquee peaks such as Mt. Rainier, Mt. St. Helens, Mt. Baker, and Mt. Hood all being active volcanoes that could erupt at any time and will erupt in the future.

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Accretionary Wedges, also referred to as Accretionary Prisms, are areas of subduction zones where masses of sedimentary material from the subducting plate has been scraped off and piled up at the edge of the non-subducting plate. As such, these accretionary wedges or prisms are often built into mountain ranges. In the PNW/Washington, the accretionary wedges/prisms that we see are the Olympic Mountains and the Coast Range, as masses of sediment and rock from the subducting Juan De Fuca Plate is accreted onto the North American Plate.

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Fore-Arc Basins, also known as Fore-Arcs, are basins between volcanic arcs and accretionary wedges/prisms where sediment eroded from volcanic arcs and accretionary wedges accumulate. Seattle, Tacoma, Portland, and the vast majority of cities built along the I-5 corridor in the PNW are built in fore-arc basins. The Puget Sound and Willamette Valley are textbook examples of these basins.

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Lastly, an Offshore Trench, also known as an Oceanic Trench or simply a Trench, are large depressions formed at the boundary of the two plates in a subduction zone, due to the weight of the down-going plate literally dragging the surface of the earth down with it. Trenches are the deepest parts of the ocean. Unlike most subduction zones, the Cascadia Subduction Zone doesn’t have a very deep trench, but it does have a trench off the shores of the PNW. It is not very deep because the angle at which the Juan De Fuca Plate is subducting underneath the North American Plate is not very high. Higher angle of subduction usually indicates deeper trenches.