Subduction in a Nutshell

What happens when a lithospheric plate must go down

Subduction is a scientific Latin word meaning "carried under." Basically, it happens when one plate meets another—that is, in convergent zones—and one of them goes down into the mantle. (more on convergent zones)

Subduction happens only to oceanic lithosphere. Continents are too buoyant to be carried much farther than about 100 kilometers deep. So when a continent meets a continent, no subduction occurs. When oceanic lithosphere meets continental lithosphere, the continent always wins and the oceanic plate subducts. When two oceanic plates meet, the younger one wins. Here's why.

Oceanic lithosphere is formed hot and thin at the mid-ocean ridges, and as it moves away from the ridge it cools and grows thicker. It also sits lower, because like all rocks it shrinks as it cools. With age, the cold oceanic lithosphere becomes denser than the hot asthenosphere beneath. Therefore when two plates meet the younger, higher plate has an edge. It's a mistake to picture oceanic plates as floating like ice on water—they are more like sheets of paper on water, ready to sink as soon as one edge can start the process. They are gravitationally unstable.

Once a plate begins to subduct, gravity takes over. A descending plate is usually referred to as a "slab". In the western Pacific where very old seafloor is being subducted, the slab falls almost straight down. In the eastern Pacific where very young plates are being subducted, the slab descends at a shallow angle. (Another theory explains this through motions of the underlying mantle, not slab density.) Subduction, in the form of gravitational "slab pull," is thought to be the largest force driving plate tectonics.

At a certain depth the high pressure turns the basalt in the slab to a denser rock, eclogite (that is, a feldspar-pyroxene mixture becomes garnet-pyroxene). This makes the slab even more eager to descend.

_{U.S. Geological Survey image}

Where the subducting slab bends, a deep-sea trench forms. Trenches capture a lot of sediment from nearby land masses, much of which is carried down along with the slab. In about half the world's trenches, some of that sediment is instead scraped off. It remains on top as a wedge or prism of material, like snow in front of a plow, and slowly the trench is pushed offshore as the upper plate grows by accretion.

Once subduction begins, the materials on top of the slab—sediments, water, delicate minerals—are carried down with it, then rise into the upper plate. There they enter an energetic cycle of volcanism and tectonic activity, sometimes called the subduction factory. The rest of the slab keeps descending and leaves the realm of plate tectonics. (more on the death of plates)

Back to Plate Tectonics in a Nutshell

Subscribe to the Newsletter
Name	Email

Free Geology Newsletter!Sign Up