Sea floor spreading is the engine of Continental Drift, the heat-driven process that causes the geography of Earth's surface to undergo constant change. This spreading occurs along a network of mid-oceanic ridges that lace the globe; separating tectonic plates, they do not necessarily run down the middle of our geographic oceans. As the oceans open, the diverging plates cause other tectonic plates to collide, subduct or scrape along one another, lifting mountain ranges, igniting volcanoes and triggering earthquakes.
Spreading rates along these plate boundaries vary. Divergence along the Mid-Atlantic Ridge, which does follow the general midline of that ocean, is among the slowest on Earth, averaging 2-3 cm per year, while that along the East Pacific Rise, which cuts across the southeastern quadrant of the Pacific Ocean, is 5 times that rate; indeed, the divergence between the Pacific and Nazca Plates, off the west coast of South America, is currently the most rapid sea floor spreading on our planet, exceeding 17 cm per year. At the other extreme, there is no current spreading at some oceanic crust boundaries, such as that between the South American and Antarctic Plates. Continent rift zones, such as the Rio Grande Rift of the American Southwest and the East African Rift are young spreading zones that, if not aborted, will eventually become mid oceanic ridges.
Ocean crust forms at these diverging plate boundaries and is recycled into the mantle along subduction zones, where it dips below more buoyant continental crust or another segment of oceanic crust. Based on the rate of spreading and the maximum distance between a mid oceanic ridge and its corresponding subduction zones, oceanic crust has a current maximum age of 200 million years; in other words, the oldest oceanic crust on planet Earth, about to subduct in the northwest Pacific, formed near the end of the Triassic Period, when dinosaurs ruled the land. Continental crust, on the other hand, while constantly changing its shape due to the combined effects of faulting, rifting and accretion, is very old, having initially formed at least 4 billion years ago; of course, most surface rock formations, deposited and molded atop the ancient Precambrian basement, are much younger, dating from 600 million years ago to recent volcanic, stream and glacial deposits.
