Ocean floor is dense so it slides under the edge of an adjacent continent. This process is called subduction. Subduction forms a broad fault zone with many sheared fragments of ocean floor rock forming a mixture we call melange. The faults are like a cuisinart, cutting up and mixing different vegetables. Cool, descending crust acts like a heat sink, so conditions of high pressure but fairly low temperature prevail, forming odd metamorphic minerals.
Subduction pushes rock deeply into the hot regions of the earth which melts to form magma.
This results in an arc-like chain of volcanos on the surface. This magma is rich in silica and forms sticky magma with explosive volcanos such as Mount Saint Helens and the ancient volcanos that exploded to form the Petrified Forest. Many older parts of the California Coast Ranges are melange, and include large areas of unusual rocks that may have travelled long distances on the ocean floor. The ancient magnetism of a limestone in Laytonville tells us that it formed at 17 degrees south of the equator and traveled north on the moving ocean floor to the coast of California. We think that these are orphan fragments of foriegn continents, ocean floor, volcanic arcs and other rocks which travel on the ocean floor and eventually become accreted or stuck in the subduction zone.