Subduction facts for kids

Diagram of the geological process of subduction

Subduction is a super important process on Earth where one of the planet's huge tectonic plates slides *under* another plate and sinks into the Earth's hot, gooey middle layer called the mantle. Think of it like a giant conveyor belt that recycles old parts of the Earth's outer shell.

This amazing process happens at places called subduction zones. When plates crash into each other, the heavier plate (usually the one under the ocean) dives down. This sinking movement is mostly because the cold, stiff oceanic plate is a bit heavier than the warmer, softer rock below it.

Subduction zones are busy places! They often have lots of Earthquakes, and they're where most of the Earth's volcanoes are found. The water trapped in the sinking plate helps create the magma that feeds these volcanoes. Over millions of years, subduction has helped build most of the land we live on, called the continental crust. Plates usually move at speeds of a few centimeters per year, which is about as fast as your fingernails grow!

How Subduction Works
Parts of a Subduction Zone
Effects of Subduction
Why Subduction Matters
Images for kids
See also

How Subduction Works

Earth's Moving Plates

The Earth's outer shell, called the lithosphere, isn't one solid piece. It's broken into about sixteen large pieces, plus many smaller ones, like a giant jigsaw puzzle. These pieces are called tectonic plates, and they are always moving very, very slowly.

This movement is mostly caused by the heavy, sinking parts of plates at subduction zones. As a plate dives down, it pulls the rest of the plate along with it. This is part of a bigger process called mantle convection, which is like a slow-motion boiling of the Earth's mantle. This convection helps heat escape from deep inside our planet.

Oceanic vs. Continental Plates

The lithosphere includes the Earth's outer crust and the very top part of the mantle.

Oceanic lithosphere is thinner, usually about 100 kilometers (60 miles) thick. It's also generally heavier.
Continental lithosphere is much thicker, up to 200 kilometers (120 miles) thick, and lighter.

Subduction zones are where the cold, heavy oceanic lithosphere sinks back into the mantle to be recycled. This happens when an oceanic plate crashes into another plate that is lighter (either another oceanic plate or a continental plate). The sinking plate, often called a "slab," usually dives at an angle between 25 and 75 degrees. As it sinks, it carries down sediments and trapped water.

Earth is the only planet we know of where subduction happens. It's super important because it's the main engine that drives plate tectonics. Without subduction, the Earth's surface wouldn't be nearly as active or dynamic as it is!

Water's Role in Subduction

Ocean water seeps into cracks in the oceanic plate. It reacts with minerals to form "hydrous minerals" (minerals that store water in their structure). As the plate sinks deeper and gets hotter, these hydrous minerals break down and release their water. This water, now a super-hot fluid, rises into the mantle above the sinking plate. This fluid acts like a "flux," lowering the melting temperature of the mantle rock, which then creates magma. This magma is what feeds volcanoes!

Parts of a Subduction Zone

Arc-Trench Complex

The surface features of a subduction zone are called an "arc-trench complex."

Outer Trench High: Before the plate dives, it often bulges up slightly, forming a gentle rise on the ocean floor.
Oceanic Trench: This is where the plate starts to plunge downwards. Trenches are the deepest parts of the ocean, like the famous Mariana Trench.
Forearc: This is the area on the overriding plate, between the trench and the volcanoes. Sometimes, sediments scraped off the sinking plate pile up here, forming an "accretionary wedge."
Volcanic Arc: Beyond the forearc, you'll find long chains of volcanoes. These are the "arc volcanoes," like Mount Vesuvius or Krakatoa. They are often very explosive because their magma is rich in water from the sinking plate.
Back-Arc Region: This area is behind the volcanic arc. Depending on how steeply the plate sinks, this region can either be squeezed, forming mountains, or stretched, creating a "back-arc basin" (a new small ocean basin).

Deep Structure of Subduction Zones

Even though we can't see deep inside the Earth, scientists use tools like geophysics to study the deep parts of subduction zones.

Wadati–Benioff Zone: This is a special zone of earthquakes that dips away from the trench and goes deep into the Earth, sometimes hundreds of kilometers down. These deep earthquakes are unique to subduction zones.
Slab Penetration: Using a technique called "seismic tomography" (like an X-ray of the Earth), scientists can see that some sinking plates can go all the way down to the boundary between the mantle and the Earth's core!

Subduction Angle: Flat or Steep?

The angle at which a plate sinks can vary a lot:

Flat-Slab Subduction: Sometimes, the plate sinks almost horizontally (less than 30 degrees). This can happen if the plate is extra buoyant (lighter) due to thick crust or if it's warmer. When a slab is flat, it can drag the upper plate with it, causing mountains to form far inland, away from the coast. The Rocky Mountains in the United States were formed this way long ago. Flat-slab subduction zones are also where the biggest earthquakes, called "megaquakes," tend to happen.
Steep-Angle Subduction: In other places, the plate sinks very steeply (more than 70 degrees). This usually happens when the oceanic plate is very old, cold, and heavy, like in the Mariana Trench. Steep subduction can cause the upper plate to stretch, creating new small seas behind the volcanic arc.

Effects of Subduction

Building Mountains (Orogeny)

Subduction is a major way mountains are built.

Accretion: As an oceanic plate sinks, it can carry along islands, underwater plateaus, or thick layers of sediment. Instead of sinking, this material often gets scraped off and added to the edge of the overriding continent. This piling up of material causes the crust to thicken and form mountains.
Flat-Slab Mountain Building: As mentioned before, if a plate sinks at a shallow angle, it can drag the bottom of the continental plate, causing it to fold, fault, and build mountains far from the trench.

Earthquakes and Tsunamis

Subduction zones are responsible for the most powerful earthquakes on Earth.

Deep Earthquakes: Unlike most earthquakes that happen close to the surface, subduction zones have quakes that can occur hundreds of kilometers deep.
Megathrust Earthquakes: These are the biggest earthquakes, like the 2004 Indian Ocean earthquake or the 2011 Tōhoku earthquake in Japan. They happen where the two plates are stuck together, building up huge amounts of stress. When they finally slip, it releases enormous energy.
Tsunamis: If a megathrust earthquake causes a sudden movement of the seafloor, it can create a massive wave called a tsunami. The 2004 Indian Ocean earthquake caused a devastating tsunami that affected many countries.

Scientists have found that the flatter the contact between the two plates, the more likely it is for mega-earthquakes to happen.

Volcanoes and Magma (Arc Magmatism)

Subduction zones create two main types of volcanic arcs:

Island Arcs: These form when one oceanic plate sinks beneath another oceanic plate, creating a chain of volcanic islands, like the Mariana Islands.
Continental Arcs: These form when an oceanic plate sinks beneath a continental plate, creating volcanoes along the coast of a continent, like the Cascade Volcanic Arc in North America.

Volcanoes like Mount St. Helens and Mount Fuji are examples of arc volcanoes. The magma that feeds these volcanoes forms when water released from the sinking plate causes the mantle rock above it to melt. This magma then rises to the surface, sometimes erupting violently. Arc volcanoes have played a huge role in building the Earth's continents over time.

Why Subduction Matters

Subduction zones are incredibly important for many reasons:

Plate Movement: The sinking of cold, heavy oceanic plates is the strongest force that drives the movement of all tectonic plates. It's the main way the Earth's mantle moves heat around.
Earth's Chemistry: As plates sink, they release water and other chemicals into the mantle. This process helps create new continental crust and moves elements between the Earth's surface and its deep interior.
Natural Hazards: Subduction zones are where the biggest earthquakes and most explosive volcanic eruptions happen. These events can cause huge damage to people, homes, and the environment.

Scientists have even thought about using subduction zones to get rid of nuclear waste by letting the Earth's natural processes carry it deep into the mantle. However, this idea is currently banned by international agreements because the huge earthquakes that happen in these zones make it too risky and unpredictable.

Images for kids

The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone
The simplified model of mantle convection: Oceanic plates are subducted creating oceanic trenches.
Global map of subduction zones, with subducted slabs contoured by depth