Introduction to Subduction Zones: Amazing Events in Subduction Zones

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The Earth’s many tectonic plates can be thousands of miles across and underlie both continents and oceans. These plates collide, slide past, and move apart from each other. Where they collide and one plate is thrust beneath another (a subduction zone), the most powerful earthquakes, tsunamis, volcanic eruptions, and landslides occur.

subduction zone graphic

Schematic Cross Section of a Typical Subduction Zone:  When tectonic plates converge (illustrated by the thick black arrows on either side of the image), one plate slides beneath the upper plate, or subducts, descending into the Earth’s mantle at rates of 2 to 8 centimeters (1–3 inches) per year (red-brown slab with skinny arrow shows direction of motion). (Public domain.)

Subduction

When tectonic plates converge, one plate slides beneath the other plate, or subducts, descending into the Earth’s mantle at rates of 2-8 centimeters (1–3 inches) per year.

Giant Earthquakes

At shallow depths, less than about 25 kilometers (16 miles), the interface between the plates may become stuck, or “locked,” and stresses build along these giant "megathrust" faults.  Eventually stresses exceed the fault’s strength and it breaks free, releasing the stored energy as seismic (shaking) waves in an earthquake. The massive size of these faults produce the largest earthquakes on Earth.

Other More Frequent Smaller Earthquakes

Earthquakes occur elsewhere in subduction zones, within the subducting plate (“intra-plate”) that often are deeper than about 30 kilometers (19 miles) below the surface, or at the “outer-rise” just a few kilometers below the surface where the plate begins its descent. They also occur within the crust of the upper plate, often just beneath our feet.

Inundating Tsunamis

When the surface of the seafloor moves vertically, a tsunami is born. This can happen either when earthquake faults move vertically just below the surface, or when submarine landslides transport large masses.

Land-level Changes over Vast Areas

In the hundreds of years between megathrust earthquakes, the squeezing motions cause the upper plate to bulge and uplift just above and inboard of the locked region, over thousands of square kilometers. Almost immediately following a megathrust earthquake, the uplifted region drops by as much as several meters (1 meter is about 3 feet), causing sea level to rise by amounts that would take hundreds of years if due to climate change.

Erupting Volcanoes

Simple illustration of the various parts of a subduction zone where one oceanic plate is thrust beneath another.

Schematic diagram of a subduction zone, showing the location of the outer rise and tensional stresses within the subducting plate.(Public domain.)

Trenches form where the subducting plate begins its descent and can be as much as 11 kilometers (7 miles) deep. Thick layers of sediment may accumulate in the trench, and these and the subducting plate rocks contain water that subduction transports to depth, which at higher temperatures and pressures enables melting to occur and 'magmas' to form. The hot buoyant magma rises up to the surface, forming chains of volcanoes.

Abundant Landslides

Landslides are particularly abundant in subduction zones, where geologic processes create steep rapidly evolv­ing topography. Onshore, high rates of rainfall on the seaward side of the mountain chains created by the squeezing of the plates makes landslides more probable. Offshore, thick sediments pile up, creating steep unstable slopes.

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