Subduction zone

Geography glossary

Also called: Subduction · Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

A subduction zone is the part of a convergent boundary (where two plates move towards each other) where the denser plate, usually oceanic, bends and sinks under the other plate into the mantle. It matters because this is where crust is destroyed. It also makes deep trenches, a belt of shallow to deep earthquakes, and a line of volcanoes along the trench, such as the Andes, Japan and India's Andaman-Nicobar arc.

Explanation

Why one plate sinks (the cause)

Not all crust is equally heavy.
Oceanic crust is made of basalt, so it is heavier (denser).
Continental crust is made of granite-type rock, so it is lighter.
When two plates meet, the heavier one goes down.
Ocean meets continent: the oceanic plate sinks.
Ocean meets ocean: the older, colder and denser plate sinks.
It is the opposite of a divergent boundary.
At a divergent boundary, new crust is made at mid-ocean ridges.
At a subduction zone, old crust is pushed back into the mantle and destroyed.

How subduction works (cause → process → result)

The plate bends and dives. The dense plate bends and slides down into the mantle at an angle.
A trench forms. Where the plate bends down, the sea floor sinks very deep. This makes a trench (a long, narrow and very deep valley in the ocean floor).
Earthquakes happen.
The sinking plate grinds against the plate above it.
This causes earthquakes at shallow, middle and deep levels.
Volcanoes form.
Deep down, heat and pressure partly melt the rock.
This molten rock (magma) rises and erupts at the surface.
Result: a trench at sea, a belt of earthquakes, and a line of volcanoes running parallel to the trench.

Types of subduction and what each one builds

Type	Which plate sinks	Main landforms	Examples
Ocean-continent	Oceanic plate	Trench offshore, plus volcanic fold mountains on land	Andes, Cascades
Ocean-ocean	The older, denser oceanic plate	Deep trench, plus an island arc	Japan, Aleutians, Marianas, Indonesia, Andaman-Nicobar arc

Ocean-continent: the edge of the continent is squeezed and lifted, and volcanoes grow on it. Together these make volcanic fold mountains.
Ocean-ocean:
Magma rises through the upper plate.
Lava and volcanic debris pile up on the ocean floor until a submarine (underwater) volcano rises above sea level [2].
This makes an island arc (a curved chain of volcanic islands). It runs parallel to the trench, on the side of the upper plate.
The chain is curved because the Earth's surface is curved, so a sinking plate bends along an arc.
Continent-continent: both plates are light, so neither sinks easily.
Only the ocean floor between them subducts first, and then the two continents crash.
The crust crumples and thickens into fold mountains instead (Himalayas, Alps).

The Wadati-Benioff zone: proof that the plate sinks

Wadati-Benioff zone: a sloping band of earthquakes that dips from the trench down into the mantle.
It is named after Kiyoo Wadati (Japan) and Hugo Benioff (USA).
Pattern:
Earthquakes near the trench are shallow.
Farther from the trench, under the land or the island arc, they get deeper and deeper.
On a cross-section, they form a slanting line. That line is the sinking plate (slab) itself.
Depth limit: about 700 km.
Below this, the slab is too hot and soft to break, so earthquakes stop.
Example: a 636 km deep earthquake in Bolivia in 1994 [2].
Why it matters: it explains deep-focus earthquakes (earthquakes that start deep inside the Earth) near trenches. It is direct proof that plates really do sink into the mantle.

Where it occurs

World

Andes, South America (ocean-continent):
The Nazca plate sinks under the South American plate and forms the Peru-Chile trench [2].
The South American edge is pushed up into the Andes. The region has strong, destructive earthquakes, and the mountains rise quickly [2].
Deep earthquake: on 9 June 1994, a magnitude 8.3 earthquake struck about 320 km north-east of La Paz, Bolivia, at a depth of 636 km [2]. It caused no major damage because it was so deep [2].
Cascade Range, Pacific north-west of the USA (ocean-continent): active volcanoes made the same way as the Andes [2].
Mariana Trench, western Pacific (ocean-ocean):
It runs alongside the Mariana Islands.
It is where the fast-moving Pacific plate meets the slower-moving Philippine plate [2].
Challenger Deep, at its southern end, is nearly 11,000 m deep. That is deeper than Mount Everest is high above sea level [2].
Other island arcs: Japan, the Aleutian Islands [2] and Indonesia.
Pacific "Ring of Fire": a belt of trenches, island arcs and volcanic mountains around the Pacific. It has most of the world's volcanoes and deep earthquakes.

India

Andaman-Nicobar arc (Bay of Bengal / Andaman Sea):
The Indian plate sinks eastward under the Burma plate along the Andaman Trench.
Barren Island lies on this arc. It is India's only active volcano.
The arc is part of the long Sunda arc chain that runs south to Sumatra and Java (Indonesia).
The 2004 Indian Ocean tsunami started on this subduction zone, off Sumatra.

Don't confuse with

Divergent boundary: plates move apart and new crust is made at mid-ocean ridges. At a subduction zone, crust is destroyed.
Continent-continent collision: neither plate sinks easily, so there is no true subduction and no major volcanoes. The crust thickens into fold mountains instead. For example, the Himalayas have no active volcanoes.
Island arc vs fold mountain belt: an island arc (Japan, Marianas, Andaman-Nicobar) comes from ocean-ocean subduction. The Himalayas and Alps come from continent-continent collision and are not island arcs.
Trench vs mid-ocean ridge: a trench is a deep valley where a plate sinks. A ridge is a raised line on the ocean floor where plates pull apart.

Prelims Hooks

Peru-Chile trench and Andes: the Nazca plate sinks under the South American plate. This is ocean-continent convergence [2].
Mariana Trench: the Pacific plate meets the Philippine plate. Challenger Deep is nearly 11,000 m deep, the deepest point on Earth. This is ocean-ocean convergence [2].
Island arc examples: Japan, Aleutians, Marianas, Indonesia, Andaman-Nicobar arc. Trap: the Himalayas and Alps are not island arcs.
Barren Island (Andaman Sea): India's only active volcano. It sits where the Indian plate sinks under the Burma plate.
Wadati-Benioff zone: a sloping band of earthquakes tracing the sinking slab, down to about 700 km. Earthquakes are shallow near the trench and deeper farther from it.
Which plate sinks: basalt (oceanic) crust sinks under granite-type (continental) crust. In ocean-ocean meetings, the older, colder, denser plate sinks.

Mains Points

Distribution pattern (GS-I):
Subduction zones explain the Pacific "Ring of Fire": the Andes, Cascades, Aleutians, Japan, Marianas and Indonesia.
The Andaman-Nicobar arc brings this belt into Indian territory.
Hazard link (GS-III, disasters):
Subduction zones produce the biggest earthquakes and tsunamis. The 2004 tsunami started on the Sumatra-Andaman zone.
India's response:
- Andaman and Nicobar is placed in the highest seismic zone.
- INCOIS in Hyderabad runs a tsunami early-warning centre.
- Building rules are designed to resist earthquakes.
Landform building (GS-I):
Subduction builds volcanic fold mountains on continents, such as the Andes, which rise quickly [2].
It also builds island arcs at sea.
Before India hit Asia, the floor of the ocean between them also subducted. The two continents then collided and the Himalayas rose.

Related concepts

Convergent boundaries: subduction, island arcs and fold mountains

Sources

1Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 4 "Distribution of Oceans and Continents" (primary)
2USGS, "This Dynamic Earth: Understanding plate motions"pubs.usgs.gov · tier 2