What Is Subduction?

An illustration of subduction zones
An illustration outlining the many different mechanisms of a subduction zone. Wikimedia Commons user MagentaGreen/ licensed under CC BY-SA 3.0

Subduction, Latin for "carried under," is a term used for a specific type of plate interaction. It happens when one lithospheric plate meets another—that is, in convergent zones—and the denser plate sinks down into the mantle.

How Subduction Happens

Continents are made up of rocks that are too buoyant to be carried much farther than about 100 kilometers deep. So when a continent meets a continent, no subduction occurs (instead, the plates collide and thicken).

True subduction happens only to oceanic lithosphere.

When oceanic lithosphere meets continental lithosphere, the continent always stays on top while the oceanic plate subducts. When two oceanic plates meet, the older plate subducts. 

Oceanic lithosphere is formed hot and thin at mid-ocean ridges and grows thick as more rock hardens underneath it. As it moves away from the ridge, it cools. Rocks shrink as they cool, so the plate becomes more dense and sits lower than younger, hotter plates. Therefore, when two plates meet, the younger, higher plate has an edge and does not sink.

Oceanic plates do not float on the asthenosphere 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." Where very old seafloor is being subducted, the slab falls almost straight down, and where younger plates are being subducted, the slab descends at a shallow angle.

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.

It's a mistake to picture subduction as a sumo match, a battle of plates in which the top plate forces the lower one down. In many cases it's more like jiu-jitsu: the lower plate is actively sinking as the bend along its front edge works backward (slab rollback), so that the upper plate is actually sucked over the lower plate. This explains why there are often zones of stretching, or crustal extension, in the upper plate at subduction zones.

Ocean Trenches and Accretionary Wedges

Where the subducting slab bends downward, a deep-sea trench forms. The deepest of these is the Mariana Trench, at over 36,000 feet below sea level. 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 of material, known as an accretionary wedge or prism, like snow in front of a plow. Slowly, the trench is pushed offshore as the upper plate grows. 

Volcanoes, Earthquakes and the Pacific Ring of Fire

Once subduction begins, the materials on top of the slab—sediments, water and delicate minerals—are carried down with it. The water, thick with dissolved minerals, rises into the upper plate.

There, this chemically active fluid enters an energetic cycle of volcanism and tectonic activity. This process forms arc volcanism and is sometimes known as the subduction factory. The rest of the slab keeps descending and leaves the realm of plate tectonics. 

Subduction also forms some of Earth's most powerful earthquakes. Slabs normally subduct at a rate of a few centimeters per year, but sometimes the crust may stick and cause strain. This stores potential energy, which releases itself as an earthquake whenever the weakest point along the fault ruptures.

Subduction earthquakes can be very powerful, as the faults they occur along have a very large surface area to accumulate strain. The Cascadia Subduction Zone off the coast of northwest North America, for example, is over 600 miles long. A magnitude ~9 earthquake occurred along this zone in 1700 AD, and seismologists think the area may see another one soon.

 

Subduction-caused volcanism and earthquake activity occur frequently along the outer edges of the Pacific Ocean in an area known as the Pacific Ring of Fire. In fact, this area has seen the eight most powerful earthquakes ever recorded and is home to over 75 percent of the world's active and dormant volcanoes. 

Edited by Brooks Mitchell