Ocean-Ocean Convergence and Volcanic Island Arc Formation

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Convergent Boundary

  • Along a convergent boundary two lithospheric plates collide against each other.
  • When one of the plates is an oceanic plate, it gets embedded in the softer asthenosphere of the continental plate, and as a result, trenches are formed at the zone of subduction.

Convergent Boundary

In convergence there are subtypes namely:

  1. Collision of oceanic plates or ocean-ocean convergence (formation of volcanic island arcs).
  2. Collision of continental and oceanic plates or ocean-continent convergence (formation of continental arcs and fold mountains).
  3. Collision of continental plates or continent-continent convergence (formation of fold mountains)
  4. Collision of continent and arc, or continent-arc convergence.

Ocean-Ocean Convergence or The Island-Arc Convergence

  • The concept of Ocean-Ocean Convergence helps us understand the formation of Japanese Island Arc, Indonesian Archipelago, Philippine Island Arc and Caribbean Islands.
  • Archipelago: an extensive group of islands.
  • Island arc: narrow chain of islands which are volcanic in origin. An island arc is usually curved.

Basics

In all types of convergence, the denser plate subducts, and the less dense plate is either up thrust or folded or both (upthrust and folded).

  • In Ocean-Ocean Convergence, a denser oceanic plate subducts below a less dense oceanic plate forming a trench along the boundary.

Ocean – Ocean Convergence (via Wikimedia Commons)

  • As the ocean floor crust (oceanic plate) loaded with sediments subducts into the softer asthenosphere, the rocks in the subduction zone become metamorphosed (alteration of the composition or structure of a rock) under high pressure and temperature.
  • After reaching a depth of about 100 km, the plates melt. Magma (metamorphosed sediments and the melted part of the subducting plate) has lower density and is at high pressure.
  • It rises upwards due to the buoyant force offered by surrounding denser medium.
  • The magma flows out to the surface. A continuous upward movement of magma creates constant volcanic eruptions at the ocean floor.

Subduction Zone Illustration (Eround1, via Wikimedia Commons)

  • Constant volcanism above the subduction zone creates layers of rocks. As this process continues for millions of years, a volcanic landform is created which in some cases rises above the ocean waters.
  • Such volcanic landforms all along the boundary form a chain of volcanic islands which are collectively called as Island Arcs (Indonesian Island Arc or Indonesian Archipelago, Philippine Island Arc, Japanese Island Arc etc.).
  • Orogenesis (mountain building) sets in motion the process of building continental crust by replacing the oceanic crust (this happens at a much later stage. For example, new islands are born around Japan in every few years. After some million years, Japan will be a single landmass because continental crust formation is constantly replacing the oceanic crust).

This explanation is common for all the island arc formations (ocean-ocean convergence). We only need to know the plates involved with respect to each island formation.

Formation of the Philippine Island Arc System

  • Philippine Island Arc system is formed due to subduction of Philippine Sea plate under the Sunda Plate (major continental shelf of the Eurasian plate). The trench formed here is called Philippine Trench.
  • Sunda Shelf: The extreme south-eastern portion of the Eurasian plate, which is a part of Southeast Asia, is a continental shelf. The region is called the Sunda Shelf. The Sunda Shelf and its islands is known as the Sundaland block of the Eurasian plate.

Minor plates in the Southeast Asia

Formation of the Indonesian Archipelago

  • In the case of Indonesian Archipelago, the Indo-Australian plate is subducting below Sunda Plate (part of Eurasian Plate). The trench formed here is called Sunda trench (Java Trench is a major section of Sunda trench).

  • Anak Krakatau (child of Krakatau) volcano lies close to the Java Trench. It is situated in the Sunda Strait between the Indonesians Islands of Java and Sumatra.
  • Underwater land shifting on the Anak Krakatau volcano in December 2018 triggered a Tsunami that killed more than 400 people.

Anak Krakatau volcano situated between the Indonesians Islands of Java and Sumatra (Google Maps)

Formation of the Caribbean Islands

  • The Caribbean Plate is a mostly oceanic tectonic plate. The northern boundary with the North American plate is a transform or strike-slip boundary (more about this in the subsequent chapters).
  • The Caribbean Plate is moving to the east while the North American Plate is moving to the west.
  • The Puerto Rico Trench is located at a boundary between the two plates that pass each other along a transform boundary with only a small component of subduction.
  • The boundary between the two plates in the past has been convergent, and most of the Greater Antilles group of islands are formed due to the complex interaction between the two plates.
  • The eastern boundary of the Caribbean Plate is a subduction zone, the Lesser Antilles subduction zone, where oceanic crust of the South American Plate is being subducted under the Caribbean Plate.
  • This subduction zone explains the presence of active volcanoes along the Lesser Antilles.
  • Mount Pelée is an active volcano at the northern end of Martinique Island (French overseas department) in the Lesser Antilles island arc of the Caribbean.
  • The volcano is famous for its eruption in 1902. The eruption killed about 30,000 people. Most deaths were caused by pyroclastic flows which destroyed the city of Saint-Pierre.

3D view of the Caribbean Islands (Wikipedia Commons)

The Greater Antilles and Lesser Antilles Island Arcs (Map from Google Earth)

The island groups in the Caribbean Sea

  • The Greater Antilles is a grouping of the larger islands in the Caribbean Sea: Cuba, Hispaniola (containing Haiti and the Dominican Republic), Puerto Rico, Jamaica, and the Cayman Islands.
  • Together, the Lesser Antilles and the Greater Antilles compose the Antilles (or the Caribbean islands).
  • When combined with the Lucayan Archipelago (Bahama Archipelago), all three are known as the West Indies.
  • Lucayan Archipelago is an island group comprising the Commonwealth of The Bahamas and the British Overseas Territory of the Turks and Caicos Islands.

Islands in the Lesser Antilles Island Arcs (Map from Google Earth)

Formation of Isthmus of Panama

  • Formation of the Isthmus of Panama involved subduction of the Pacific-Farallon Plate beneath the Caribbean and South American plates, forming a volcanic arc on the edge of the Caribbean Plate.
  • The remains of the ancient Farallon Oceanic Plate are the Juan de Fuca Plate, parts of the North American Plate and the South American Plate, the Cocos Plate and the Nazca Plate.
  • This initial Panama Arc began to form as the Caribbean Plate moved eastward.
  • The North and South American plates continued to move westward past the Caribbean Plate.
  • In addition to their east-west (strike-slip) motion, the plates also acquired a north-south component of convergence, leading to the collision of the Panama Arc with South America.
  • This collision drove uplift in both the Northern Andes and the Panama Arc, forming the Isthmus of Panama.

Plates in the region of Isthmus of Panama

Formation of the Japanese Island Arc

  • Japan’s volcanoes are part of three volcanic arcs.

Triple Junction of the plates

  • The arcs meet at a triple junction on the island of Honshu.
  • Northern arc is formed due to the subduction of the Pacific Plate under the Eurasian Plate. The trench formed is Japan Trench.
  • Central arc is formed due to the subduction of the Pacific Plate under the Philippine Plate (island formation is not significant along this arc). The trench formed is Izu Trench.
  • Southern Arc is formed due to the subduction of the Philippine Plate under the Eurasian Plate. The trench formed is Ryukyu Trench.
  • Japanese island arc was very close to the mainland.
  • The force exerted by the Pacific plate and the Philippine plate tilted the arc towards its east giving rise to the Sea of Japan.

The Mariana Trench or Marianas Trench

  • The Mariana Trench or Marianas Trench, the deepest trench, is located in the western Pacific Ocean.
  • The Mariana Trench is formed due to the subduction of the Pacific Plate below the Mariana Plate.
  • The maximum known depth is between 10,994 & 11,034 metres in its floor known as the Challenger Deep.

The Mariana trench is not the part of the seafloor closest to the centre of the Earth. This is because the Earth is not a perfect sphere (its Geoid); its radius is about 25 kilometres smaller at the poles than at the equator.

As a result, parts of the Arctic Ocean seabed are at least 13 kilometres closer to the Earth’s centre than the Challenger Deep seafloor.

Explain the formation of thousands of islands in Indonesian and Philippines archipelagos (20 marks – UPSC CSE Mains 2014)

20 marks = 200 words

  • Indonesian archipelago and Philippine archipelago are located along the plate margins. Both the archipelagos were formed due to ocean-ocean convergence.
  • Indonesian archipelago was formed due to convergence between Sunda oceanic plate (part of the Eurasian plate) and Indo-Australian plate whereas Philippine archipelago was formed due to convergence between Sunda oceanic plate and Philippine Sea plate.
  • In ocean-ocean convergence, two oceanic plates converge or collide. The denser plate subducts into the asthenosphere below the convergence zone and forms a trench at the surface. This region below the convergence zone is called the zone of subduction.
  • In the zone of subduction, due to high temperature and pressure, the rocks undergo metamorphosis and the sediments in the oceanic plate melt to form magma.
  • The magma being lighter moves upwards due to the buoyant force offered by the surrounding denser medium. At the surface magma at high pressure escapes in the form of volcanic eruptions.
  • The magma solidifies creating a volcanic layer. Subsequent volcanism builds a layer over layer and a volcanic mountain if formed. Such mountains are formed all along the converging edge above the less dense plate.
  • Over time the mountains merge, and the oceanic crust gets transformed into continental crust.
  • This is how Indonesian archipelago and Philippine archipelago were formed.

If asked for 10 marks = 100 words

  • Indonesian and Philippine archipelagos are formed due to ocean-ocean convergence.
  • In ocean-ocean convergence, the denser plate subducts into the asthenosphere. This region below the convergence zone is called the zone of subduction.
  • In the zone of subduction, the rocks undergo metamorphosis and the sediments in the oceanic plate melt to form magma.
  • At the surface magma escapes in the form of volcanic eruptions.
  • Constant volcanism builds a layer over layer and a volcanic mountain if formed.
  • Such mountains are formed all along the converging edge.
  • Over time the mountains merge, and the oceanic crust gets transformed into continental crust.
  • This is how Indonesian archipelago and Philippine archipelago were formed.

Related question

In spite of extensive volcanism, there is no island formation along the divergent boundary (mid-ocean ridge)

  • Basaltic magma flows out along the divergent edge (fissure type or shield type volcano).
  • Basaltic magma has less silica, and hence it is less viscous. It flows over a long-distance causing seafloor spreading but not volcanic islands.
  • On the other hand, along the convergent boundary, andesitic or acidic magma flows out.
  • Andesitic or acidic magma has more silica content, and hence it has higher viscosity. It doesn’t move quickly and also solidifies faster. This helps in building a layer over layer on a confined region giving rise to a volcanic mountain.
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