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Subduction Zones and Processes

Pamela Lassiter, Mary Ellen Ellis, Christianlly Cena
  • Author
    Pamela Lassiter

    Pamela Lassiter has taught middle school science for over 28 years. With an Ed.D. in instructional technology and a M.S. degree in science education from Nova Southeastern University, she has developed science curriculums, STEM projects and PBLs for many years and is certified in the State of Georgia.

  • Instructor
    Mary Ellen Ellis

    Mary Ellen is a science and education writer with a background in chemistry. She holds an M.S. in analytical chemistry and has worked as a high school science teacher.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master's degree in Physics and is currently pursuing his doctorate degree.

Where does subduction occur? In this lesson, learn what a subduction zone is, learn how the subduction process works, and understand why plates subduct. Updated: 09/23/2021

What is Subduction?

The word subduction might not be familiar to most people, but it is an essential process in geology. The definition of subduction is the process that occurs when two tectonic plates meet at convergent boundaries, and one of the plates moves under the other one due to gravity and differences in density. The boundary area where this occurs is the subduction zone. In the image below, an oceanic plate is moving forward and is sliding under the continental plate off Washington's west coast as an example of convergent subduction.

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.

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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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Convergent subduction involving an oceanic and continental plate

Subduction occurring at an oceanic and continental plate convergence

Where Does Subduction Occur?

Subduction occurs at specific convergent boundaries. To better understand convergent boundaries, know that the surface of Earth is covered with broken sections called plates. Each plate consists of a layer of crust and the top layer of the mantle combined to form the lithosphere, which moves very slowly and forms boundaries where one plate edge is in contact with another plate edge. The three types of boundaries and the plate movement occurring at each boundary are:

  • Divergent boundaries - plates move apart
  • Transform boundaries - plates slide past each other
  • Convergent boundaries - plates move together

The map below illustrates the locations of the three types of boundaries formed between the major tectonic plates. The green lines in the map indicate convergent boundaries. Not all convergent boundaries create subduction; the subduction process depends upon the type of plates involved. Subduction occurs when two oceanic plates or a continental plate and an oceanic plate converge.

Plate tectonics map showing the three boundary types and locations

Color-coded plate tectonics world map showing the three types of boundaries and location

The world map below highlights convergent boundaries where the subduction process is actively occurring on Earth today. Many subduction zones are located along the edge of the Pacific Ocean, such as the west coast of South America, the west coast of the United States, the east coast of Asia, and the islands of Indonesia. Using both maps, notice that this area of plate activity forms an irregular circle, called the Ring of Fire, as it is known for the significant number of Earth's volcanoes (75%) found in this region. The majority of Earth's earthquakes are located in the Ring of Fire as well.

Locations of subduction zones including depth of subduction

World map showing subduction zones

Subduction Process

Convection currents occur in the asthenosphere, which is the plastic-like layer below the lithosphere. Magma heats up, rises, cools, and sinks. This convection process slowly drives or moves the tectonic plates. As tectonic plates move toward each other at convergent boundaries, the force of the movement creates subduction zones at the plate boundaries due to density differences of the plates and the force of gravity. The denser and thinner plate called the subducting plate, will slide under the less dense, thicker plate, and continue to move into the mantle and melt over time.

If subduction is happening at one end of a tectonic plate, what is happening at the opposite end? At the plate boundary where convergence occurs, the opposite end of the plate will be diverging or moving away from another plate. At the boundary where two plates are diverging, magma is slowly released through volcanic activity. In a nutshell, divergent boundaries construct plate material through cooled magma, or new oceanic crust, while convergent boundaries destroy plate edges through subduction.

The most significant divergent boundary on Earth is the Atlantic Mid-Ocean Ridge. It is an underwater volcanic mountain range where the plates move apart at the rift valley between the diverging plates. This process is called seafloor spreading. The image below illustrates the processes of seafloor spreading at divergent boundaries and subduction occurring at convergent boundaries. As the seafloor moves towards the edges of the continents over millions of years, the rock is eventually destroyed at subduction zones as the subducting plate melts in the mantle. The rocks found at the mid-ocean ridge are as young as they are newly created; rocks near the continents are much older. The movement of tectonic plates recycles rock material and drives the rock cycle.

The process of seafloor spreading and subduction recycles rock

Diagram showing seafloor spreading and subduction

Oceanic-Continental Crust Subduction

In understanding subduction dynamics, it is essential to know the compositional features of the plates involved in the process. Oceanic plates are composed of heavy rock such as basalt. Due to the mass, the lithosphere below oceanic plates compacts, making the oceanic plate much thinner than a continental plate. Continental plates are composed of lighter rock, so the lithosphere below it is less compact. A candy analogy would be that oceanic plates are like Hershey bars while continental plates are like Three Musketeer bars.

In oceanic-continental crust subduction, the subducting plate is oceanic due to its greater density. As the oceanic plate subducts or moves under the continental plate, the descending portion of the oceanic plate melts due to the high temperatures in the mantle. The melted magma, being less dense and higher pressure than the surrounding mantle, rises to the surface, forming a volcanic arc or long series of inland volcanic mountains. The Cascade Mountains, Rocky Mountains, and Andes Mountains are topographical features formed by this subduction type. The angle of subduction is one factor that determines the distance between the subduction zone and mountain formation, as each of these mountain ranges is at varying distances from the subduction zones that formed them.

Oceanic-Oceanic Crust Subduction

To better understand oceanic-oceanic crust subduction, know that oceanic plates become more compact and thinner as they cool over time. Not all oceanic plates are the same age and have varying densities, although they are much denser and thinner than a continental plate. When two oceanic plates converge, the older oceanic plate subducts under a younger oceanic plate. The resulting magma from the melted plate rises to the surface, which creates a line of volcanoes called an island arc. The Aleutian Islands, the Philippines, Japan, and the Krakatoa Islands of Indonesia are island arcs formed by oceanic-oceanic subduction.

Effects of Subduction Zones

Subduction is a powerful process that changes the shape of continents, recycles plate material to create new rock, builds volcanoes and ocean trenches, causes earthquakes, and generates tsunamis. Convergent boundaries create subduction zones, but some of these boundaries do not result in convergent subduction. When two continental plates push towards each other, the result is that neither plate subducts due to the thickness of the plates, or one plate will only slightly subduct. As the force of the two plates increases, the rock material is forced upward, creating fold mountains such as the Himalayas, which were formed when the Indian-Australian Plate moved into the Eurasian Plate millions of years ago.

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

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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Video Transcript

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.
subduction

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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  • Activities
  • FAQs

Subduction Word Scramble Activity

For this activity, study the scrambled letters and try to unscramble or rearrange the letters to form a word or phrase that fits the given clues. To do this, you must right-click and print this page. With a pencil and an eraser, neatly write your answers in the blank space provided.

Scrambled Words

_________________________1. NCVNEREOTG NIDBRUSOAE

_________________________2. MELNAT

_________________________3. ADLINS RCA

_________________________4. CINOACE EPLTA

_________________________5. MAALHSIAY

_________________________6. SOTNDBIUCU

_________________________7. ECITTCON ELTAP

_________________________8. TICOCENONV

_________________________9. LEHOTPESIHR

_________________________10. CLIVONCA CAR

Clues

  1. These are areas of compressive stress and, depending on the nature of the plate interactions, the recycling or destruction of the lithosphere.
  2. It is known as the Earth's thickest layer where rocks flow without breaking.
  3. A long chain of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries.
  4. This plate sinks when an oceanic lithosphere meets a continental lithosphere in a subduction zone.
  5. The continental-continental collision of India with Asia produced, and is still producing, this mountain range.
  6. Pertains to the process in which one plate bends and descends beneath the other.
  7. It is made of both crust, or the outer layer of the earth, and a thin upper layer of the mantle.
  8. The transmission of heat in the mantle by the circulation of currents.
  9. The rigid, mechanically strong, outer layer of the Earth, including the entire crust and the uppermost part of the mantle.
  10. Refers to a chain of volcanoes formed from continent-oceanic interactions.


Answers

  1. CONVERGENT BOUNDARIES
  2. MANTLE
  3. ISLAND ARC
  4. OCEANIC PLATE
  5. HIMALAYAS
  6. SUBDUCTION
  7. TECTONIC PLATE
  8. CONVECTION
  9. LITHOSPHERE
  10. VOLCANIC ARC

Why do plates get subducted?

Oceanic plates subduct under continental or another oceanic plate due to density and gravity. Oceanic plates are composed of heavier material than continental plates. Older oceanic plates are cooler and therefore more compact and thinner. As the plates converge, the plate with the greater density will slide under the lighter plate.

What is an example of a subduction zone?

The coast of Peru is a subduction zone. The oceanic Nazca Plate is subducting below the South American plate creating an ocean trench off the coast of Peru as well as the Andes Mountains.

What is subduction and why is it important?

Subduction is part of the recycling process in plate tectonics. Plates melt to form volcanoes, trenches, and new rock at subduction zones, while new oceanic plates are formed at divergent boundaries such as the mid-ocean ridge.

What do subduction zones create?

Subduction zones can create earthquakes, island arcs, volcanic mountain ranges, and deep ocean trenches. Island arcs are found at oceanic-oceanic convergent boundaries. Volcanic mountain ranges (or volcanic arcs) are found at oceanic-continental convergent boundaries. Tsunamis may also form from earthquakes created by plate movement at subduction zones.

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