Convergent Boundary

🌋 Ocean–Ocean Convergence (O–O Convergence)

aka Island Arc Convergence

We have already studied in earlier sections that at an O–O convergent boundary, two oceanic plates collide. Since both are made of oceanic crust, the denser of the two (usually older, colder, and heavier) subducts beneath the other.

🧠 Just like continental convergence forms Himalayas, oceanic convergence forms Island Arcs.

🧭 Where does this happen?

O–O convergence is responsible for the formation of:

• 🇯🇵 Japanese Island Arc
• 🇮🇩 Indonesian Archipelago
• 🇵🇭 Philippine Island Arc
• 🌴 Caribbean Islands

These are regions with:

• Deep oceanic trenches
• Frequent earthquakes
• Chain of volcanic islands (called Island Arcs)

🔷 Step-by-Step Process: How Are Island Arcs Formed?

Let’s break it into simple steps:

🌀 1. Subduction Initiation

• A denser oceanic plate begins to subduct below a less dense oceanic plate.
• This forms a deep oceanic trench (e.g., Japan Trench, Philippine Trench, Sunda Trench).

🔥 2. Metamorphism & Magma Generation

• At ~100 km depth, high pressure and temperature cause:
  • Metamorphism of rocks
  • Melting of sediments + plate material
• This generates andesitic magma (intermediate in silica content).

🎈 3. Magma Ascent & Volcanism

• The magma is hot and less dense, so it rises through the crust.
• It erupts on the ocean floor → layer upon layer → volcanic mountains.

🏝️ 4. Formation of Island Arc

• These volcanic peaks emerge above sea level, forming a chain of volcanic islands.
• This curved chain is called an Island Arc.

🧱 5. Orogenesis (Mountain Building)

• Over time, the islands merge, and oceanic crust slowly transforms into continental crust.
• This begins the process of continent building in the long run.

📌 UPSC Note:
Island Arcs are the oceanic equivalents of Fold Mountains in continent–continent convergence.

🌍 Key Island Arcs Formed by O–O Convergence

Let’s understand the important cases with involved tectonic plates and trenches:

🇵🇭 Philippine Island Arc

Feature	Description
Converging Plates	Philippine Sea Plate subducts beneath Sunda Plate (Eurasian)
Trench	Philippine Trench
Type	Classic Island Arc with volcanic origin

📝 Sunda Shelf = A shallow, submerged part of the Eurasian Plate covering Java, Sumatra, Borneo, etc.

🇮🇩 Indonesian Archipelago

Feature	Description
Converging Plates	Indo-Australian Plate subducts beneath Sunda Plate
Trench	Sunda Trench (includes Java Trench)

🇯🇵 Japanese Island Arc

Japan is formed by three arcs; each linked to a different plate boundary:

Arc	Subduction Interaction	Trench
Northern	Pacific Plate under Eurasian Plate	Japan Trench
Central	Pacific Plate under Philippine Plate	Izu Trench
Southern	Philippine Plate under Eurasian Plate	Ryukyu Trench

• These arcs meet at a triple junction on Honshu Island.
• The eastward tilt of the arc system created the Sea of Japan.

🌴 Caribbean Islands

Feature	Description
Converging Plates	South American Plate subducts under Caribbean Plate (eastern margin)
Result	Volcanic islands of the Lesser Antilles (e.g., Mount Pelée)
Mount Pelée	Deadliest Caribbean volcano (killed ~30,000 in 1902)

🌉 Formation of the Isthmus of Panama

• Caused by subduction of the Pacific–Farallon Plate beneath Caribbean and South American Plates
• Led to:
  • Panama Arc (volcanic origin)
  • Collision with South America → uplift → Isthmus of Panama
• Result: Connection between North and South America formed; major biogeographic event.

🧠 Interesting: The Farallon Plate has since broken into smaller plates:

• Cocos Plate
• Nazca Plate
• Juan de Fuca Plate

The Mariana Trench

Feature	Description
Plates Involved	Pacific Plate subducting beneath Mariana Plate
Depth	~11,000 metres (deepest known point: Challenger Deep)
Fun Fact	Not the point closest to Earth’s center — due to Earth’s geoid shape!

Difference Between Island Arc and Archipelago

Term	Meaning	Example
Island Arc	Curved chain of volcanic islands along O–O subduction zone	Japanese Islands, Philippines
Archipelago	Large group of islands (not always volcanic)	Malay Archipelago, Indonesia

🌄 Continent–Ocean Convergence (C–O Convergence)

Also called Cordilleran Convergence

Why is it called Cordilleran?

The word Cordillera refers to a chain of mountains or a system of parallel mountain ranges near continental margins.

So, whenever a continent and an oceanic plate converge, and this collision creates massive coastal mountain systems, we call it Cordilleran Convergence.

🗻 Examples of such mountains:

• Andes in South America
• Rockies in North America

What Happens During C–O Convergence?

🌊 1. Subduction Begins

• When an oceanic plate collides with a continental plate, the denser oceanic plate subducts beneath the lighter continental plate.
• This creates a subduction zone, often marked by a trench near the continental margin.

🧠 Example: Nazca Plate subducts below South American Plate → Peru-Chile Trench

🔥 2. Formation of Magma and Volcanism

• As the subducting oceanic plate sinks, it heats up and undergoes partial melting.
• This produces andesitic magma (silica-rich), which rises through the continental crust.
• The magma erupts, forming volcanic mountains on the continent (not in the ocean like O–O convergence).

This linear chain of volcanoes formed on the continental edge is called a Continental Arc.

🧨 Examples:

• Cascade Range (USA, west of the Rockies)
• Western Chile Range (parallel to Andes)

🪨 3. Formation of Accretionary Wedge

• The subducting oceanic plate carries sediments scraped from the ocean floor.
• These sediments accumulate at the trench and compress into a wedge-shaped mass called the Accretionary Wedge/Prism.
• This wedge adds compressive force and thickens the continental margin.

📌 Concept:
👉 Crustal Shortening at convergence = Crustal Widening at divergence (a tectonic balance)

🏔️ 4. Formation of Fold Mountains — Orogeny

Here’s where things get really interesting from a UPSC Mains perspective:

• Orogeny = Mountain building process due to lateral compression.
• As the accretionary wedge and crustal sediments are compressed, they fold, fault, and metamorphose.
• The continental edge rises, creating Fold Mountains (Orogenic belts).

🧠 Why near the coast? Because that’s where the continental plate meets the subducting oceanic plate.

✳️ Key Landforms from C–O Convergence

🇨🇱 The Andes Mountains

Feature	Description
Plates Involved	Nazca Plate (oceanic) + South American Plate (continental)
Trench	Peru-Chile Trench
Result	Continental Arc + Fold Mountains
Notable Volcano	Ojos del Salado (6,893 m, highest active volcano on Earth)
Highest Peak	Mount Aconcagua (6,960 m, extinct volcano)

• Western Chile Range was separated from the Andes due to a depression called Intermediate Depression.
• Volcanism is still active, indicating that orogeny is ongoing.

🇺🇸 The Rocky Mountains

Feature	Description
Plates Involved	Pacific & Juan de Fuca Plates (oceanic) + North American Plate
Result	Fold mountains at a distance from coast (unlike Andes)
Reason	Shallow subduction angle + presence of fault zones like San Andreas

• Formed inland, not along the exact coastal edge.
• Subduction is not steep, hence magma doesn’t erupt easily, and trenching is less obvious.

🔁 Final Stage: Isostatic Adjustment

After orogeny, what happens?

• With time, erosion wears down mountain peaks.
• The earth’s crust undergoes isostatic rebound:
  • Heavier/denser regions sink
  • Lighter ones rise

This exposes the roots of the mountain belt, as seen in ancient ranges like the Rockies.

🧠 Important UPSC Concepts

✅ Arc vs Fold Mountain

Term	Meaning	Example
Island Arc	Volcanic islands in oceanic region	Japan, Indonesia
Continental Arc	Volcanic mountains on continents	Andes, Cascade Range
Fold Mountains	Compressed, folded sedimentary layers	Himalayas, Rockies, Andes

✅ Terminologies

Term	Meaning
Accretionary Wedge	Accumulated oceanic sediments at trench compressed onto continental plate
Crustal Shortening	Horizontal compression at convergence zones
Isostasy	Vertical adjustment of crust based on load balance (e.g., erosion, uplift)

🏔️ Continent–Continent Convergence (C–C Convergence)

Also known as the Himalayan Convergence type

Why subduction fails in C–C convergence?

• Continental crust is too buoyant (lower density) to be subducted into the mantle like oceanic crust.
• Even if one of the plates starts to descend, subduction stops around 40–50 km depth.
• Instead of deep trenches, the crust buckles, folds, and faults, creating massive fold mountain systems.

🧠 Key Concept: Instead of “subduction → volcanism” (as in O–O or C–O), here we have “compression → uplift → folding”.

🪨 What happens during C–C convergence?

🧱 1. Collision and Suture Formation

• The two continental plates meet and collide.
• Any oceanic crust between them (e.g., Tethys Sea) is consumed.
• Fragments of this oceanic crust are welded between the plates — this weld is called the Suture Zone.

🧠 Example:
Indus–Tsangpo Suture Zone in the Himalayas.

🌊 2. Squeezing of Sedimentary Basin

• Geosynclinal sediments (accumulated over millions of years in the Tethys) are trapped between the converging plates.
• They get compressed, folded, and faulted, forming a towering fold mountain belt.

📌 Important: These sediments include marine fossils — for example, marine limestone found at the summit of Mount Everest!

🏞️ 3. Formation of Fold Mountains and Uplifted Plateau

• Massive compression causes uplift of crust, creating:
  • A fold mountain chain (Himalayas)
  • An uplifted plateau behind the collision zone (Tibetan Plateau)

🧠 Why is the Tibetan Plateau higher than the Indian side?
Because India’s crust was more firmly attached to the oceanic base, while Asia’s block was more flexible and got pushed upward.

⛰️ 4. Crustal Shortening & Isostatic Adjustment

• Collision causes crustal shortening (500 km in Himalayas!).
• This shortening is compensated by crustal widening elsewhere (e.g., sea-floor spreading in the Indian Ocean).
• Erosion of mountains leads to isostatic uplift (like floating icebergs — lighter areas rise).

🌏 Case Study: Formation of the Himalayas

📚 Geological Timeline:

Time Period	Event
~250 mya	Existence of Pangaea with Laurasia in the north and Gondwanaland in the south
200–140 mya	Indian Plate breaks from Gondwanaland, drifts north
60 mya	Deccan Traps erupt (volcanic activity due to mantle plumes)
50–40 mya	India collides with Asia → Initial formation of the Himalayas
30 mya	Major uplift of Greater Himalayas
25–20 mya	Formation of Middle Himalayas
2–20 mya	Formation of Shiwalik Hills (youngest part of Himalayas)

🧠 Additional Notes:

• India’s speed: Moving northward at ~5–6 cm/year (still ongoing).
• Earthquakes in Himalayan and North Indian regions are proof that orogeny is active.
• Himalayan rivers are rejuvenated and in their youthful stage because uplift is ongoing.

🧭 What is the Suture Zone?

• A suture is where two continental plates have fused.
• It includes remnants of oceanic crust, trapped sediments, and metamorphosed rocks.
• E.g., the Indus–Tsangpo Suture Zone is a crucial marker of the India–Asia collision.

🧬 Tibetan Plateau: The Roof of the World

• Formed due to upthrusting of the Eurasian Plate.
• Today, it’s the highest and largest plateau in the world.
• Evidence like terraced lakes and gravel deposits shows it is still rising.

📈 Evidence of Ongoing Himalayan Uplift

Evidence	Explanation
Satellite measurements	Himalayas rising 5–10 cm/year
Lake desiccation in Tibet	Raised lakebeds above current water levels
Youthful river profiles	Rejuvenation due to land uplift
Frequent earthquakes	Indicates continuing crustal compression

🌍 Other Fold Mountains Formed by C–C Convergence

Mountain Range	Plates Involved	Type	Notes
Himalayas	Indian Plate ↗ Eurasian Plate	Young Fold	Active, highest mountain range
Alps	African Plate ↗ Eurasian Plate	Young Fold	Active, in Europe
Atlas	African Plate ↗ Eurasian Plate	Young Fold	In North Africa, still rising
Urals	Europe ↗ Asia (ancient plates)	Old Fold	~300 mya, very weathered
Appalachians	North America ↔ Europe (before Pangaea breakup)	Old Fold	Ancient, heavily eroded

🌋 Why No Volcanoes in C–C Convergence?

Reason	Explanation
Thick continental crust (50–70 km)	Magma cannot pierce this thick, buoyant crust
Lack of subduction beyond 40–50 km	No melting zone like in O–O or C–O convergence
Result	No active volcanoes, but frequent earthquakes
Common Rock Type	Metamorphic rocks (formed under pressure)