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The continents record Planet Earth’s history because they are old and contain innumerable accreted parts of old oceans. Continents are not subducted back into the mantle because they are made up of light, buoyant rock. The oldest parts of the continents are the cratons, usually located at the centers of continents. The continents grow by accreting terrains at their edges. For example, California lies at the western edge of the North American plate and at the eastern edge of the Pacific plate. The boundary is the San Andreas Fault. To the north are the Cascade Mountains, a volcanic arc resulting from subduction of the Pacific seafloor under Washington, Oregon, and Northern California. To the south lies the Imperial Valley, the northern edge of the Gulf of California, a seafloor spreading center. Over the past 40 million years, the North American plate has overrun the Farallon plate, subducting nearly all of it except remnants that were accreted on the North American plate to form the Coast Range of Western California. The past motion of the three plates - the Pacific, Farallon, and North American - can be traced using magnetic anomalies, hotspot trails, and fracture zone trends.
Plate boundary interactions on continents generally involve a much broader area with more diffuse results than oceanic plate interactions. Continents may form by collision, accretion, and/or volcanism, each of which emplace light rock on a plate. Deformation of a continent involves rifting, collision, or loading. Tilting and horizontal compensation to loading are termed epeirogeny, while vertical adjustments are called orogeny.
Each continent has one to several core cratons that are remnants of the differentiation event which formed the Earth's core. Surrounding the cratons are successive histories of collision, rifting, volcanism, and/or flexure. North America was once attached to Europe and Africa prior to the formation of the Atlantic Ocean. Africa collided with North America during the assembly of Gondwanaland to close the proto-Atlantic Ocean and form the Appalachian Mountains. Preceding that event, an ocean called Tethys, closed to form the Washita, Arbuckle, and Ouachita mountains of Oklahoma and Arkansas. This closure resulted in the deposition of salt and organic matter in the Gulf of Mexico, and now that organic debris exists as hydrocarbons – oil and gas.