Subduction Zone

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Subduction Zone Meaning

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Let’s briefly understand what a subduction zone is. A subduction zone is perhaps the greatest crash scene on Earth. These boundaries mark the strike between two of the planet's tectonic plates. The plates are pieces of crust which gradually move across the planet's surface over millions of years. Where two tectonic plates collide at a subduction zone, one bends and slides below the other, curving down into the mantle (the hotter layer beneath the crust). Subduction zones are massive boundaries, thus they produce very large and impactful earthquakes.

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Amazing Events in Subduction Zones

The Earth’s couple of tectonic plates can be millions of miles across and underlie both oceans and continents. These plates slam into, slide past, and move apart from each other. Where they slam and one plate is thrust underneath another (a subduction zone), the most impactful earthquakes, landslides, tsunamis, and volcanic eruptions arise.

Events in Subduction Zones

Following are the Events That Take Place in Subduction Zones -

  1. Earthquakes

Cramming two gigantic slices of Earth's crust together is like stroking two pieces of sandpaper against each other. The gigantic scale of subduction zones implies they can induce cosmic earthquakes. The biggest earthquakes ever documented were on subduction zones, recording a magnitude of 9.5 in Chile (1960) and a magnitude of 9.2 in Alaska (1964).

  1. Tsunamis

Subduction zones are generally formed along coastlines, so tsunamis tend to always produce close to where people live. "There's a silver lining there.”If these earthquakes occur beneath a city, the city would just have no chance. But unfortunately, sometimes a tsunami is produced."

When a subduction zone earthquake strikes, Earth's crust bends and breaks like a freed spring. For earthquakes bigger than a magnitude 7.5, this can bring about a tsunami, a colossal sea wave, by abruptly moving the seafloor. However, not all subduction zone earthquakes will give rise to tsunamis. Also, some earthquakes activate tsunamis by sparking underwater landslides.

Whatever their cause, the tsunami from subduction zones may strike in minutes for coastal areas near an earthquake, or hours later, after the waves travel along the sea.

  1. Volcanoes

As plate tectonics subduction zones slide into the mantle, the hotter layer underneath Earth's crust, the heating disperse fluids trapped in the plate. These fluids, such as carbon dioxide and sea water, can partially melt the overlying crust, producing magma. And magma (molten rock) often indicates volcanoes.

The Toba volcanic explosion in Indonesia, the biggest volcanic eruption in the past 25 million years, had been from a subduction zone volcano.

  1. Abundant Landslides

Landslides are specifically profuse in subduction zones, where geologic mechanisms form steep rapidly evolv­ing topography. Onshore, high rates of rainfall on the seaward side of the mountain chains formed by the compressing of the plates make landslides more likely. Offshore, thick sediments amass, constructing steep unstable slopes.

  1. Land-Level Changes Over Massive Areas

In the hundreds of years between megathrust earthquakes, the compressing motions cause the upper plate to upsurge and uplift just above the locked region, over thousands of square kilometers. Almost spontaneously after a megathrust earthquake, the uplifted area drops by as much as a few meters (1 meter is about 3 feet), inducing sea level to rise by amounts that would take hundreds of years if because of climatic change.

  1. Magma Generation in Subduction Zones

When the (literally very old) oceanic crust sinks back into the mantle in a subduction zone, it comes continuously under huge pressure and temperature. The rocks contain considerable amounts of carbon dioxide, water and other fluids which are released into the blanketed mantle wedge.

  1. Magmas Change Composition

The magmas in subduction zone volcanoes are commonly explosive since they appear at the surface as very viscous (sticky) and gas-rich. Why?

On their way up to the surface, these magmas can (and essentially do) experience a variety of processes, such as cooling and crystallizing when they pool and cool in magma chambers at varying depths.

Fun Facts

  • Plate tectonics subduction zones can carry along both oceanic crust and continental crust, or they may be made of only either kind of crust.

  • Continental crust is less dense and condensed than the Oceanic crust.

  • At a subduction zone, the oceanic crust generally descends into the mantle underneath a lighter continental crust.

  • Scientists believe sometimes, the oceanic crust may grow so old and dense that it collapses and in a spur of a moment develops a subduction zone

  • With a similar type of crust collision, such as continent-continent, the plates may crash together without subducting and scrumple together like crashing cars.

  • The gigantic Himalaya mountain chain was created this way when India rammed into Asia.

FAQ (Frequently Asked Questions)

Q1. Why are Subduction Zone Earthquakes the Largest in the World?

Answer: The main reason for subduction zone earthquakes being largest is the size. The size of an earthquake is associated with the size of the fault that induces it, and subduction zone faults are believed to be the longest and widest in the world. The Cascadia subduction zone offshore of Washington is approximately 620 miles (1,000 kilometers) long and around 62 miles (100 km) wide.

Smaller earthquakes also strike all across the descending plate, also referred to as a slab. Seismic waves from these tremors and temblors enable scientists to "see" inside the Earth, same as a clinical CT scan. The quakes expose that the sinking slab is inclined to bend at an angle between 25 to 45 degrees from the Earth's surface, though some are steeper or flatter than this.

Sometimes, the slabs may rip off, like a tear in a wrinkled paper. Pieces of the sinking plate can also fragment and fall into the mantle, or get stuck.

Q2. What are the Types of Subduction Zones?

Answer: There are typically 2 types of subduction zones i.e.

  1. Oceanic-Oceanic Plate Boundaries: If the subducting plate subducts under an adjacent oceanic plate, an island arc is developed. Examples include the Kuriles, Aleutians, Japan, and the Philippines, all situated at the western and northern borders of the Pacific plate.

  1. Oceanic-Continental Plate Boundaries: if the subducting plate subducts underneath the continental lithosphere, then a similar belt of volcanoes will be formed on the continent. These are known as volcanic arcs. Examples include the Andes volcanic arc of South America, the Cascade volcanic arc of the United States, the Pacific Northwest, and the Hellenic arc in the southern Aegean.