How can the ocean floor keep getting wider and wider? The answer is that the ocean floor generally does not just keep spreading. Instead, the ocean floor plunges into deep under-water canyons called deep-ocean trenches. At a deep-ocean trench, the oceanic crust bends downward. What occurs at trenches? In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.
The process by which ocean floor sinks beneath a deep-ocean trench and back into the mantle is called subduction (sub duk shun). As subduction occurs, crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean trench. Sea-floor spreading and subduction work together. They move the ocean floor as if it were on a giant conveyor belt.
New oceanic crust is hot. But as it moves away from the mid-ocean ridge, it cools and becomes more dense. Eventually, as shown in Figure 19, gravity pulls this older, denser oceanic crust down beneath the trench. The sinking crust is like the washcloth in the Discover activity at the beginning of this section. As the dry washcloth floating on the water gets wet, its density increases and it begins to sink.
The processes of subduction and sea-floor spreading can change the size and shape of the oceans. Because of these processes, the ocean floor is renewed about every 200 million years. That is the time it takes for new rock to form at the mid-ocean ridge, move across the ocean, and sink into a trench.
The vast Pacific Ocean covers almost one third of the planet. And yet it is shrinking. How can that be? Sometimes a deep ocean trench swallows more oceanic crust than a mid-ocean ridge can produce. Then, if the ridge does not add new crust fast enough, the width of the ocean will shrink. In the Pacific Ocean, subduction through the many trenches that ring the ocean is occurring faster than new crust can be added.
On the other hand, the Atlantic Ocean is expanding. Unlike the Pacific Ocean, the Atlantic Ocean has only a few short trenches. As a result, the spreading ocean floor has virtually nowhere to go. In most places, the oceanic crust of the Atlantic Ocean floor is attached to the continental crust of the continents around the ocean. So as the Atlantic’s ocean floor spreads, the continents along its edges also move. Over time, the whole ocean gets wider.