What is the Cascadia Subduction Zone? I know the San Andreas Fault, but not heard of the Cascadia Subduction Zone?
Hi TFM,
A subduction zone is a place on the ocean floor where one tectonic plate—a large piece of the earth’s crust—is being pushed under another tectonic plate.
New crust is being continually made by magma—molten rock—coming to the surface in cracks in the crust. The magma flows away on both sides of the crack and cools to form the basalt crust. This process takes place at various ocean ridges such as the Mid-Atlantic that runs south from Iceland and the Ninety East ridge that runs south of India (actually, Bangladesh) in the Indian Ocean.
As the new crust is pushed slowly away from the ridge, it pushes against a continental plate—a piece of lighter, granite, crust under a continent. In southern California, this forces a small plate that Los Angeles, San Diego, etc. sit upon to slide northward along the San Andreas Fault.
Off the coast of Oregon and Washington, however, a small piece of ocean crust—the Juan de Fuca plate—is pushed under the edge of the continental plate at a place about sixty miles offshore called the Cascadia Subduction Zone.
Over Geologic time this has built our Coast Range and the volcanic peaks of the Cascade Range. The Coast Range is partly a piece of crust that was tilted up by the Pacific plate sliding beneath it, and partly from sediment scraped off the top of that plate.
As the edge of the Pacific plate is forced downward, it eventually reaches a depth where the temperature is high enough to re-melt the basalt, forming pools of magma about sixty miles inland.
That magma finds a way to the surface to create the lava flows and the volcanoes of the Cascades.
Regards,
Marty
