Subduction

Subduction is a geological process where parts of the Earth's surface get pulled back into the Earth's mantle. This happens at places called convergent boundaries between tectonic plates. When two plates move toward each other, the heavier one slides beneath the lighter one and sinks down slowly.

These areas are called subduction zones. They are important because subduction has helped create most of the Earth's land.

Subduction happens because the cold, stiff rock is heavier than the hot, soft rock underneath. Once it starts, the heavy plate keeps sinking down by its own weight. This moving rock can cause interesting events on the Earth's surface.

One of the most noticeable effects of subduction zones is earthquakes. The sinking plate can also cause volcanism or volcanic activity in the plate above it. Depending on how the plate sinks, it can create mountains or special areas called back-arc basins. Subduction shapes our planet in many powerful ways.

Subduction and plate tectonics

See also: Plate tectonics

According to the theory of plate tectonics, the Earth's lithosphere, its rigid outer shell, is broken into sixteen larger tectonic plates and several smaller plates. These plates move slowly because of the pull from subducting lithosphere. Subduction happens when the heavier oceanic lithosphere of one plate moves under another plate and sinks into the mantle.

Subduction zones are where cold oceanic lithosphere sinks back into the mantle. They are found where plates come together, and the sinking plate, called the slab, goes down at an angle. This process helps move the plates and recycle material into the deep Earth. Earth is the only planet where we know subduction occurs, and it is a key part of how plate tectonics works.

Structure of subduction zones

The surface features of subduction zones are called arc-trench complexes. On the side where the ocean meets the zone, there might be a small rise in the ocean floor before it drops sharply into a deep oceanic trench, the deepest part of the ocean.

Beyond the trench lies the overriding plate, which can have a pile of sediments scraped off the sinking plate. Further away, volcanoes often form in chains called volcanic arcs.

Deep below, subduction zones can be identified by inclined zones where earthquakes occur, extending far down into the Earth. Some parts of the subducting plates may even reach the lower mantle.

Life cycle of subduction zones

Subduction zones are places where one piece of Earth's outer layer moves under another. Scientists are still learning how these zones start. Sometimes, the heavier piece can sink on its own. Other times, the movement of nearby pieces can push it down. Over time, this process can keep going as the sinking piece changes and becomes even heavier.

Eventually, subduction can stop. When a big, less dense piece of land reaches a subduction zone, it can stop the process. This can lead to big changes in how the pieces of Earth's outer layer move. Some thick pieces of land or underwater land can also stop subduction, but smaller pieces might only cause small changes.

Characteristics and effects

Metamorphism

Subduction zones make special rocks because of the high pressure and low temperature. As the plate sinks deeper, water comes out of minerals. This water can help rocks melt deeper in the Earth. These changes create new volcanoes and shape the land.

Subduction zones have special kinds of rocks that form here. The process has different stages, each with its own minerals. These stages include zeolite, prehnite-pumpellyite, blueschist, and eclogite conditions.

Arc magmatism

Main article: Volcanic arc

Subduction zones create chains of volcanoes called volcanic arcs. These form above where one tectonic plate goes under another. There are two types: island arcs, which form in the ocean, and continental arcs, which form near land. Famous examples are the Mariana Islands and the Cascade Volcanic Arc in North America.

Volcanoes in these arcs, like Mount St. Helens and Mount Fuji, are about 100 kilometers from the trench where the plates meet. The sinking plate releases water, which makes the mantle melt. This melted rock rises and can form volcanoes or cool to make other rocks.

Earthquakes

Main article: Megathrust earthquake

Some of the biggest earthquakes happen in subduction zones. These include deep earthquakes in the sinking plate, huge earthquakes near the trench, and earthquakes on the plate bending into the zone. Big earthquakes like the 1960 Great Chilean earthquake and the 2004 Indian Ocean earthquake happened in subduction zones. These quakes can cause tsunamis, big waves that move across the ocean.

Orogeny

Main article: Orogeny

Subduction can build mountains. When plates come together, ocean material can be pushed onto the continent, making new land. This process, called accretion, creates mountain ranges. In some cases, the way the plate sinks can build mountains even without adding new material, making mountain chains like those in western North America.

Subduction of continental lithosphere

When continents are pulled into subduction zones by the sinking oceanic plates they are attached to, the thick layers of sediments and volcanic rocks are often scraped off and added to the edge of the continent. This process can sometimes pull the continent itself down into the subduction zone. Studies show that parts of continents like Australia and India have been subducted.

Intra-oceanic: ocean/ocean plate subduction

Ocean-ocean subduction zones make up about 40% of all subduction zones. These zones happen when one oceanic plate goes under another. Scientists have three main ideas about how this process starts and continues: retreating subduction, stable subduction, and advancing subduction. Each idea describes different ways the plates move and interact.

Beginnings of subduction on Earth

See also: Archean subduction

Modern-style subduction has low heat and makes special rocks like eclogite and blueschist. Pieces of eclogite found in the North China Craton show this type of subduction happened more than 1.8 billion years ago during the Paleoproterozoic Era.

Blueschist is common in subduction areas today, but older examples are rare. This suggests Earth’s ancient ocean floor had more magnesium, which turned into different rocks under heat and pressure. These findings show Earth’s mantle was once hotter, but the subduction process wasn’t always hotter. This evidence changes older ideas that modern subduction began only about 1 billion years ago in the Neoproterozoic Era.

History of investigation

Harry Hammond Hess studied the Mid-Atlantic Ridge during World War II. He proposed a theory called seafloor spreading. He suggested that as new seafloor forms at the ridge, older seafloor must be consumed somewhere else. He thought this happened at deep oceanic trenches, where the seafloor melts back into the Earth's mantle.

In 1964, George Plafker studied the Good Friday earthquake in Alaska. He found that the earthquake was caused by the Pacific oceanic crust being forced under the Alaskan continental crust. This process, called subduction, helped scientists develop the theory of plate tectonics.

Importance

Subduction zones are important because they help move Earth's tectonic plates. When an oceanic plate sinks into the mantle, it releases water that can create new volcanoes and help form the continents we live on.

These zones can also be dangerous because they are where huge earthquakes and powerful volcanic eruptions happen. Scientists have studied these areas for ways to safely store nuclear waste deep underground. However, this is not allowed by international rules because of the risks from big earthquakes.