What Is Subduction?
In geological terms, subduction is the act of one tectonic plate moving under another tectonic plate at the point of their convergent boundary. As the subducting plate moves under its neighboring tectonic plate, gravity pushes it further down and into the mantle layer of the earth. The mantle layer is hotter than the crust, although it is generally found in a solid state, and allows the subducting plate to sink at angles of between 25 and 45 degrees. The exact angle of the subduction depends on the age of the subducting plate; older plates fall at a sharper angle. The hotter temperatures and increased pressure found at these depths cause the basalt rock of the sinking plate, also referred to as a slab, to morph into eclogite rock.
Subduction occurs very slowly. In fact, geologists have identified the average rate of convergence at between 2 and 8 centimeters per year. This speed is slow enough that subduction often goes unnoticed. Although tectonic plates may be oceanic or continental, the act of subduction (sliding under another plate) only happens to oceanic plates. When two continental plates collide, the result is an upward movement of rock and other material. Many mountain ranges around the world were created in this manner. The place where subduction has occurred, however, does not result in the formation of mountains. This place is known as a subduction zone. The subduction process is credited with being the single, largest contributor behind the theory of plate tectonics.
Why Does Subduction Happen?
As previously mentioned, an oceanic plate is the only tectonic plate that actually experiences subduction, or downward, movement. Why exactly does this geological process occur? Knowing what subduction is, does not explain why it happens. Geologists explain that subduction occurs to oceanic plates because they are denser and cooler than continental plates. If the subduction occurs between two oceanic plates, it is the older plate that will move underneath the younger tectonic plate. The reason for this is that oceanic plates, also known as the oceanic lithosphere, start out as thin and hot sections of earth. Over time, these plates gradually move away from the mid-ocean ridge where they were born. This movement causes the overall temperature of the oceanic lithosphere to drop, which serves to solidify material found on the underneath side of the plate. This solidification process causes the resulting solid rock to shrink (compared to its original size when liquid), leading to its increased density. This density causes the plate to sink under continental plates or younger, less dense, oceanic plates. Although this process is generally attributed to the movement of tectonic plates, some geologists have theorized that very old plates may sink quickly and without warning given their significantly denser state.
Effects of the Subduction Process
The movement of continental and oceanic plates does not go unnoticed on the surface of the earth. The explanation of subduction perhaps makes the process sound relatively smooth, with one plate slowly sinking and falling off into the mantle layer below. Most scientists, however, describe subduction as a rough, scraping activity that is characterized by high amounts of friction as the two plates rub past each other. As the subducting slab moves under the less dense plate, some of its pieces may become caught on the upper plate. This obstacle results in spent up energy that can only be released in one way: earthquakes.
Because subduction occurs along very long plate boundary lines, the potential for a very strong earthquake increases. In fact, the largest earthquakes ever recorded all occurred at subduction zones. Some examples of this include the Great Chilean Earthquake of 1960 (a magnitude of 9.5), the Indian Ocean Earthquake of 2004 (a magnitude of between 9.1 and 9.3), and the Japanese Tohoku Earthquake of 2011 (a magnitude of between 9 and 9.1).
The reason for these extreme earthquake magnitudes is attributed solely to the size of the fault line. Scientists have confirmed a positive correlation between the size of a fault line and the size of an earthquake. Some of the largest fault lines (by both width and length) in the world are located at subduction zones. Smaller plate boundaries typically generate smaller tremors.
Another side effect of the subduction process is the creation of volcanoes, as well as increased volcanic activity above subduction zones. These subduction zone-created volcanoes occur in one of two formations: island arc or continental arc. An island arc results when one oceanic plate moves under another oceanic plate. A continental arc results when an oceanic plate moves under a continental plate.
Volcanoes and volcanic activity often occur as a result of the subduction process because the subducting slab releases liquids as it moves into the extreme temperatures of the mantle layer. These extremely hot liquids, which primarily consist of carbon dioxide and ocean water, effectively melt the plate that has remained on top. This melted material is known as magma or lava.
Three-quarters of the volcanic activity on earth is confined to an area known as the Pacific Ring of Fire. This ring moves along the western coastlines of the Americas and along the eastern coastlines of Asia and the Pacific Islands, forming an upside down u-shape. Geologists rely on this zone to extract valuable information concerning the link between subduction zones, volcanoes, and earthquakes.
In addition to earthquakes and volcanic activity, the process of subduction is also credited with causing severe tsunamis around the world. Tsunamis, large and dangerous waves, are the result of earthquakes (and other geologic activity) on or near coastlines. Because subduction zones are typically located along coastlines, the resulting earthquakes caused by the tectonic plate movement often cause tsunami waves to ravage coastal environments and urban settlements. This wave activity occurs because earthquakes cause the crust of the earth to snap and rebound. This sudden movement on the ocean floor results in displaced water, which moves toward the shore in extremely high and long waves. Tsunamis may occur over a period of just a few minutes or even hours, as the displaced water rushes onto nearby land. This wave activity may occur for hours after a subduction zone earthquake has been registered.