Subduction

Subduction is a fascinating geological process where the oceanic lithosphere and some continental lithosphere get pulled back into the Earth's mantle at places called convergent boundaries between tectonic plates. When two of these giant plates move toward each other, the heavier one slides beneath the lighter one and slowly sinks down into the mantle. These areas are known as subduction zones, and they are very important because subduction has helped create most of the Earth's landmasses we see today. The process usually happens very slowly, at a rate of just a few centimeters each year.

Subduction works because the cold, stiff oceanic lithosphere is a little heavier than the hot, soft layer of rock underneath called the asthenosphere in the upper mantle. Once subduction starts, it keeps going mainly because the heavy sinking plate is being pulled down by its own weight. This moving slab of rock can trigger many interesting events on the Earth's surface.

One of the most noticeable effects of subduction zones is the frequent occurrence of earthquakes. Also, when the sinking plate releases fluids, it can cause volcanism or volcanic activity in the plate above it. Depending on how steeply the plate sinks, it can either create chains of mountains and deformed land or form special areas called back-arc basins. Subduction shapes our planet in many powerful and surprising 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. So far, 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. These volcanoes can erupt dangerously because of the water carried down with the sinking plate.

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 and sink all the way to the edge of the core.

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, while 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 create special kinds of rocks due to the high pressure and low temperature conditions that the sinking plate experiences. As the plate moves deeper, water is released from minerals, which can lower the melting point of rocks in the mantle and start melting processes. These changes help create new volcanic activity and shape the continents.

Subduction zones have unique rock types that form under these conditions. The process involves different stages, each with its own minerals, showing the changes happening as the plate sinks. These stages include zeolite, prehnite-pumpellyite, blueschist, and eclogite conditions, each marked by specific minerals.

Arc magmatism

Main article: Volcanic arc

Subduction zones create volcanic arcs, which are chains of volcanoes that form above where one tectonic plate dives beneath another. There are two main types: island arcs, which form over the ocean, and continental arcs, which form near continents. Famous examples include the Mariana Islands and the Cascade Volcanic Arc in North America.

Volcanoes in these arcs, like Mount St. Helens and Mount Fuji, occur about 100 kilometers from the trench where the plates meet. The process begins when the sinking plate releases water, causing the mantle rock to melt and rise, forming magma chambers. This magma can reach the surface as volcanoes or cool below to form other types of rocks.

Earthquakes

Main article: Megathrust earthquake

Subduction zones are also where some of the biggest earthquakes happen. These include deep earthquakes within the sinking plate, huge megathrust earthquakes near the trench, and outer rise earthquakes on the plate bending into the zone. Some of the largest earthquakes ever recorded, like the 1960 Great Chilean earthquake and the 2004 Indian Ocean earthquake, occurred in subduction zones. These quakes can cause tsunamis, large waves that travel across the ocean.

Orogeny

Main article: Orogeny

Subduction can also build mountains. When plates converge, oceanic material can be scraped off and added to the continent, forming new land. This process, called accretion, creates mountain ranges. In some cases, the angle at which the plate subducts can cause mountain building even without adding new material, leading to the formation of 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 lead to the continent itself being pulled down into the subduction zone. Studies show that entire sections of continents have been subducted, such as parts of Australia and India.

Intra-oceanic: ocean/ocean plate subduction

Ocean-ocean subduction zones make up about 40% of all subduction zones. These zones can involve the subduction of one oceanic plate beneath another, and scientists have proposed three main models to explain how this process starts and continues: retreating subduction, stable subduction, and advancing subduction. Each model describes different ways the plates interact and move.

Beginnings of subduction on Earth

See also: Archean subduction

Modern-style subduction is marked by low heat levels and the creation of special rocks like eclogite and blueschist. Evidence from eclogite pieces found in the North China Craton shows that this type of subduction happened at least 1.8 billion years ago during the Paleoproterozoic Era.

The presence of blueschist in modern subduction areas is common today, but older examples are rare. This absence suggests that Earth's ancient ocean floor had more magnesium, which changed into different rocks under heat and pressure. These findings show that Earth’s mantle was once hotter, but the subduction process itself wasn’t necessarily hotter. This evidence changes earlier 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 and 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.

Later, in 1964, George Plafker looked at 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, but this is not allowed by international rules due to the risks from big earthquakes.