What is subduction? Subduction, from Latin for “carried underneath,” is a term used for a specific type of plate interaction. It occurs when one lithospheric plate meets another, that is, in convergent zones, and the denser plate sinks into the mantle.
How subduction happens
The continents are made up of rocks that are too floating to be transported more than 100 kilometers deep. So when a continent meets a continent, no subduction occurs (instead, plates collide and thicken). True subduction occurs only in the oceanic lithosphere.
When the oceanic lithosphere meets the continental lithosphere, the continent always remains above while the oceanic plate subducts. When two oceanic plates meet, the oldest plate is subducted.
The oceanic lithosphere forms warm and thin on the ridges of the middle ocean and becomes thick as more rocks harden beneath it. As it moves away from the ridge, it cools. The rocks shrink as they cool, so the dish becomes denser and sits lower than younger, hotter dishes. Therefore, when two plates meet, the younger and taller plate has an edge and does not sink.
Oceanic plates do not float in the asthenosphere like ice above water; they are more like sheets of paper on water, ready to sink as soon as an 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 the very old seabed is being subdued, the slab falls almost downwards, and where younger plates are being subdued, the slab descends at a shallow angle. Subduction, in the form of gravitational “slab drag,” is thought to be the greatest tectonic force of the drive plate.
At a certain depth, high pressure turns the basalt on the slab into a denser rock, eclogite (i.e. a mixture of feldspar – pyroxene becomes garnet – pyroxene). This makes the slab even more eager to descend.
It is a mistake to imagine subduction as a combination of sumo, a plate battle in which the upper plate forces the lower one down. In many cases, it’s more like jiu-jitsu: the bottom plate actively sinks as the curve along its front edge runs backwards (slab recoil), so that the top plate is actually sucked over the bottom plate. This explains why there are often stretching zones, or extension of the cortex, on the upper plate in the subduction zones.
Ocean trenches and accretional wedges: What is subduction?
Where the subducting slab bends downward, a trench is formed in deep water. The deepest of these is the Mariana Trench, more than 36,000 feet below sea level. The trenches capture a large amount of sediment from nearby land masses, much of which is transported along with the slab.
In about half of the world’s trenches, some of that sediment is scraped off. It remains at the top as a wedge of material, known as a wedge or cumulative prism, like snow in front of a plow. Slowly, the trench is pushed into the sea 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 transported with it. The water, thick with dissolved minerals, rises to the top plate. There, this chemically active fluid enters an energetic cycle of volcanism and tectonic activity. This process forms arc volcanism and is sometimes referred to as the subduction factory. The rest of the slab continues to descend and leaves the realm of plate tectonics.
Subduction also forms some of the most powerful earthquakes on Earth. The slabs normally subduct at a rate of a few centimeters per year, but sometimes the bark can adhere and cause tension. This stores potential energy, which is released like an earthquake whenever the weakest point along the fault is broken.
Subduction earthquakes can be very powerful, as the faults along which they occur have a very large surface area to build up stress. The Cascadia subduction zone off the coast of northwestern North America, for example, is more than 600 miles long. A magnitude~9 earthquake occurred along this area in 1700 AD, and seismologists think the area may see another soon.
Volcanism caused by subduction and seismic activity frequently occur 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 more than 75 percent of the world’s active and dormant volcanoes.