Subduction: Definition & Process

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  • 0:00 What Is Subduction?
  • 0:39 The Process Of Subduction
  • 2:18 Subduction And Volcanoes
  • 3:15 Examples Of Subduction Zones
  • 4:01 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
Subduction is a geological term for one of Earth's tectonic plates sinking under another. When this happens, we can get earthquakes, volcanoes, and a recycling of Earth's rocks. Let's explore this more in this lesson.

What Is Subduction?

Subduction is a kind of geological recycling. It occurs at convergent tectonic plate boundaries or where two tectonic plates come crashing together, in slow motion of course. At a convergent boundary, two plates can come together and rise up into mountains. This is how the impressive Himalayan Mountain range formed when India crashed into the rest of Asia. Another possibility for a convergent boundary is subduction. Instead of both plates crumpling upwards to form mountains, one sinks under the other and is recycled back into the mantle.

Subduction occurs when one plate sinks under another and is recycled into a deep layer of the mantle, called the asthenosphere.

The Process of Subduction

As you know, the earth's crust is not continuous but divided up into pieces, like a puzzle. These pieces, the tectonic plates, move around relative to each other, powered by circular convection cycles in the fluid mantle, a layer beneath the crust. Some plates move sideways past each other, some pull apart from each other, and others come together. The latter are convergent plates.

In a region in which one of two convergent plates sinks under the other, we call it a subduction zone. A tectonic plate is made of both crust, or the outer layer of the earth, and a thin upper layer of the mantle. Together, these two layers are called lithosphere. The two tectonic plates and the lithosphere involved in a subduction zone may both be oceanic, or one may be oceanic and the other continental.

When an oceanic lithosphere meets a continental lithosphere in a subduction zone, the oceanic plate always goes under the continental plate. This is the rule because the rock making up an oceanic lithosphere is denser than in a continental lithosphere. When two oceanic plates come together, one may sink under the other.

The mantle underneath the lithosphere is hot, fluid rock. When one plate sinks into it during subduction, it melts into the mantle. Essentially, the rock making up that plate is getting recycled. New plates form at tectonic boundaries that are diverging. At these boundaries, usually under the ocean, two plates pull apart and magma wells up and hardens, forming new rock and crust.

Subduction and Volcanoes

The movement of one plate under another and the recycling of rock into magma can lead to some exciting geologic activity. Deep earthquakes can occur as the two plates rub against each other and release energy, but they are not as common or as devastating as the earthquakes that occur at other types of plate boundaries. More interesting is the formation of volcanoes in subduction zones.

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