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Why will a rock sink if we put it in sand? This is because the Earth’s gravity pulls everything down as far as it will go, and the rock is denser than the sand. Gravitational sinking also happens with bigger objects, like tectonic plates under our oceans! This happens in a process called subduction.

An image of a puppy sinking into sand on a beach.

If you ever walk in the mud and your feet sink, you can blame gravity!

The Whats and Where of Subduction

This process takes place at convergent plate boundaries. This is when two tectonic plates meet and push against each other. On land, where the lithosphere doesn’t give away easily, convergent plates push each other upwards and sideways. This creates mountain ranges, like the mighty Himalayas! Underwater, however, convergent plate boundaries do something different.  

An illustration of 3 types of tectonic plate movement.

The different kinds of plate boundaries.

This type of plate movement happens between two oceanic plates, or an oceanic and a continental plate. When these two plates collide head-on, they don’t push each other and rise. Instead, the one that is denser sinks below the other one! The sinking plate slides down into the asthenosphere, and gets recycled back into the Earth’s mantle. Wherever this sinking process occurs is a subduction zone. 

Diagram of subduction.

If one tectonic plate is denser, it will sink.

Subduction under the ocean isn’t easy to judge with the naked eye. You can only predict which one will sink if you can measure its density. When an oceanic plate meets a continental plate, however, the oceanic one always sinks. That’s because the oceanic plate is always denser!
This might seem a little counterintuitive: after all, doesn’t rock sink in water? But think carefully! We have to remember that a tectonic plate doesn’t just include the surface, but encompasses parts that are below the Earth’s crust! 

Subduction, Beware!

Subduction zones create a variety of different landforms. Oceanic trenches, for instance, form in the boundary between a sinking plate and the plate above it. But subduction zones can be very dangerous as well! Arcs of composite volcanoes can form on the plate that lifts up. While these don’t tend to be as active as shield volcanoes, they can still erupt and cause a lot of damage!  

An image of a subduction zone .

Subduction gives rise to both oceanic trenches and volcanoes!

Subduction zones are also known for very bad earthquakes. The largest earthquakes tend to happen in between two convergent plates. If earthquakes happen underwater, they can cause tsunamis – massive waves of water that can completely cover shoreline towns!

A subduction zone notorious for both earthquakes and volcanic activity is the Ring of Fire in the Pacific Ocean. That’s a very fitting name, given the horrible disasters that take place there. 

An illustration of icons representing many types of natural disasters.

Several natural disasters take place at subduction zones.