Convergent boundaries: subduction, island arcs and fold mountains

Detail

What a convergent boundary is

• Convergent boundary: a boundary where two plates move towards each other.
• Crust is destroyed here, because one plate dives under the other.

• This is the opposite of a divergent boundary, where new crust is made at mid-ocean ridges.

• Subduction zone: the place where the denser plate, usually oceanic, sinks into the mantle.

• Oceanic crust is made of basalt, so it is heavier. Continental crust is made of granite-type rock, so it is lighter.
• When the two meet, the heavier plate goes down.

• Results: deep trenches, deep earthquakes and chains of volcanoes.

• Class 9 NCERT puts it simply:

• When an oceanic plate meets a continental plate, the oceanic plate sinks. This causes volcanoes and earthquakes.

• When two continents collide, fold mountains such as the Himalaya form.

• Three types of convergence:

Type	Which plate sinks	Main landforms	Examples
Ocean-continent	Oceanic plate	Trench offshore, plus volcanic fold mountains on land	Andes (Nazca plate under the South American plate), Cascades
Ocean-ocean	The older, denser oceanic plate	Deep trench, plus an island arc	Japan, Aleutians, Marianas, Indonesia, Andaman-Nicobar arc
Continent-continent	Neither plate sinks easily	Crust crumples and thickens into great fold mountains	Himalayas (India-Eurasia), Alps (Africa-Europe)

How subduction works (cause → process → result)

• Cause: two plates meet, and one is denser than the other.
• Process:
• The dense plate bends and slides down into the mantle at an angle.
• Where it bends down, the sea floor sinks deep and forms a trench (a long, narrow and very deep valley in the ocean floor).
• As the plate sinks, it grinds against the plate above it. This causes earthquakes at shallow, middle and deep levels.

• 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.

Ocean-continent convergence

• Process: heavy oceanic crust sinks under lighter continental crust.
• A trench forms just offshore.
• Deep earthquakes occur below the land.

• The edge of the continent is squeezed and lifted, and volcanoes grow on it. Together these form volcanic fold mountains.

• Andes (South America):

• 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].

• Example of a 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 was very strong but caused no major damage, because it was so deep [2].

• Cascade Range (Pacific north-west of the USA): the Cascades are also active volcanoes made by ocean-continent convergence, like the Andes [2].

Ocean-ocean convergence and island arcs

• Process: one oceanic plate sinks under another. Usually the older, colder and denser plate is the one that sinks.
• A deep trench forms.
• Magma rises through the upper plate.

• Lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level [2].

• Island arc: a curved chain of volcanic islands that runs parallel to the trench, on the side of the upper plate.

• It is curved because the Earth's surface is curved, so a sinking plate bends along an arc.

• Mariana Trench (western Pacific):

• It runs alongside the Mariana Islands.
• It marks 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, which is deeper than Mount Everest is high above sea level [2]. (NCERT-based figure: about 11 km.)

• Other island arcs:

• Japan
• The Aleutian Islands, a volcanic island arc formed like the Marianas [2]

• Indonesia

• India's island arc: the Andaman-Nicobar arc (Bay of Bengal / Andaman Sea)

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

Wadati-Benioff zone (beyond NCERT)

• 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), who studied these earthquakes.

• What it shows:

• Earthquakes near the trench are shallow.
• Moving away from the trench, under the land or the island arc, the earthquakes get deeper and deeper.

• Plotted on a cross-section, they form a slanting line. That line is the sinking plate (slab) itself.

• Depth: these earthquakes go as deep as about 700 km.

• Example: the 636 km deep Bolivia earthquake of 1994 [2].

• Below about 700 km, the slab is too hot and soft to break, so earthquakes stop.

• Why it matters: it explains why trenches have deep-focus earthquakes (earthquakes that start deep inside the Earth). This is direct proof that plates really do sink into the mantle.

Continent-continent convergence

• Process: both plates are light continental crust, so neither plate sinks easily.
• The ocean between them closes first, as its floor subducts.
• Then the two continents crash into each other.

• Rock layers crumple, fold, break and stack on top of each other. The crust becomes very thick.

• Himalayas (India-Eurasia):

• About 225 million years ago, India lay off the coast of Australia. A vast ocean, the Tethys Sea, separated it from Asia [3].
• About 80 million years ago, India was roughly 6,400 km south of Asia. It moved north at about 9 m per century [3].
• India hit Asia about 40-50 million years ago [3].
  • After the collision, its northward speed fell by about half [3].
  • The Indian and Eurasian plates crumpled up into the Himalayas [2].
• Tibetan Plateau: average height about 4,600 m [2].
• Still rising: the Himalayas rise more than 1 cm a year, which adds up to 10 km in a million years [3].
• Fossil proof: sandstone layers in the Himalayas contain plant and animal fossils. These were laid down when the Tethys Sea flooded the area [3].

• Class 7 NCERT: the Himalayas formed when the Indian plate pushed into the Eurasian plate. Sediments of the Tethys Sea were squeezed and lifted up.

• Alps (Africa-Europe): the African plate pushes north into Europe and folds the rock between them.

Fold mountains: young and old

• Fold mountain: a mountain made when rock layers are squeezed sideways and buckled into up-folds and down-folds where plates meet.

Feature	Young fold mountains	Old fold mountains
Examples	Himalayas, Alps, Andes, Rockies	Aravallis (India)
Age	Tens of millions of years	About 2.5 billion years (NCERT Class 7)
Height and shape	High, with sharp peaks	Worn down, low and rounded
Activity	Still rising; frequent earthquakes	Stable; worn down by erosion over a long time

• Aravallis: they run north-east to south-west, mainly through Rajasthan, from near Delhi to Gujarat. They are among the oldest fold mountains in the world.
• How folding works (anticlines, synclines, faults) is covered in the geomorphic-processes note.

Prelims Hooks

• Peru-Chile trench and Andes: formed where the Nazca plate sinks under the South American plate. This is ocean-continent convergence.
• 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.
• Island arc examples: Japan, Aleutians, Marianas, Indonesia and the Andaman-Nicobar arc. Trap: the Himalayas and Alps are not island arcs. They come from continent-continent collision.
• Barren Island (Andaman Sea): India's only active volcano. It sits on the subduction zone where the Indian plate sinks under the Burma plate.
• Wadati-Benioff zone: a sloping band of earthquakes that traces the sinking slab, reaching about 700 km deep. Earthquakes are shallow near the trench and deeper farther from it.
• Continent-continent convergence: no true subduction and no major volcanoes. Instead, crust thickens into fold mountains. Trap: the Himalayas have no active volcanoes.
• Young vs old fold mountains: the Himalayas, Alps, Andes and Rockies are young. The Aravallis are old (about 2.5 billion years).
• Order of plate collision dates: Tethys separates India from Asia (about 225 million years ago) → India about 6,400 km south of Asia (about 80 million years ago) → collision (about 40-50 million years ago).

Mains Points

• Distribution pattern:
• Convergent boundaries explain the Pacific "Ring of Fire": a belt of trenches, island arcs and volcanic mountains (the Andes, Cascades, Aleutians, Japan, Marianas and Indonesia) that has most of the world's volcanoes and deep earthquakes.

• The Andaman-Nicobar arc brings this risk into Indian territory.

• Hazard link (GS-III, disasters):

• Subduction zones produce the biggest earthquakes and tsunamis, for example on the Sumatra-Andaman zone in 2004.

• 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 for earthquakes.

• Himalayan dynamics (GS-I and GS-III):

• India is still pushing into Eurasia, and the Himalayas are still rising more than 1 cm a year [3].
• So the Himalayan belt is young, unstable and prone to earthquakes and landslides.

• This affects dams, roads, tunnels and towns in Uttarakhand, Himachal Pradesh, Jammu and Kashmir and the north-east.

• Value of the mountains:

• Young fold mountains act as a climate barrier: the Himalayas block cold Central Asian winds and trap monsoon rain.
• They are water towers that feed perennial rivers.
• Old fold mountains like the Aravallis are worn down, but they hold minerals and act as a barrier that slows the spread of the Thar Desert.