|A subduction zone is a boundary where two tectonic plates collide
and, because of differences in density, one dives beneath the other. This
occurs frequently where an oceanic plate meets a continental plate. The
denser and thicker oceanic plate is shoved underneath the less dense continental
plate. This is currently occurring off the Pacific Northwest coast of North
America from Cape Mendocino in California north to British Columbia. Below
is a USGS
diagram describing this type of boundary.
The subduction process is not a smooth one. It produces intense friction, raising coastal mountains and scraping off sediment and ocean crustal material. These scrapings, called a "melange" (from the French word for "mixture"), consist of millions of years of sediments that were collected at the continent's edge and sea-floor volcanic and sedimentary rocks. These are added to the leading edge of the continental plate and increase the continent's width. This spectacular image from Lamont-Doherty Earth Observatory shows the deformed seafloor sediments off of Oregon. The present, contrasting continental margin off Central California that is moving in the same direction as the adjacent ocean floor can be seen here.
At one time, or perhaps even a number of times, a subduction zone existed in the area where Ring Mountain is located. Although its precise location is not known the zone may have stretched for hundreds of miles along the length of the present-day California Coast Ranges.