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UPSC MainsGEOLOGY-PAPER-I201920 Marks
Q12.

What are 'island arc' systems? What are its major characteristics? Draw a schematic section through an island arc system and describe all its components.

How to Approach

This question requires a detailed understanding of island arc systems, their formation, and characteristics. The answer should begin with a clear definition of island arcs, followed by a description of their major features – geological, volcanic, and seismic. A schematic diagram is crucial, with each component labelled and explained. The answer should demonstrate knowledge of plate tectonics and subduction zones. Structure the answer into introduction, body (with subheadings for clarity), and conclusion. Focus on explaining the processes involved and the resulting geological formations.

Model Answer

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Introduction

Island arcs are curved chains of volcanic islands located in the ocean, typically parallel to oceanic trenches. They represent a prominent feature of convergent plate boundaries, specifically those involving oceanic-oceanic or oceanic-continental convergence. These systems are formed through the process of subduction, where one tectonic plate slides beneath another, leading to magma generation and subsequent volcanic activity. Understanding island arcs is crucial for comprehending plate tectonics, earthquake and volcanic hazards, and the evolution of continental crust. Their study provides insights into the Earth’s dynamic processes and the formation of complex geological structures.

Formation of Island Arc Systems

Island arcs are primarily formed at subduction zones. When an oceanic plate converges with another oceanic plate, the denser plate subducts beneath the less dense one. As the subducting plate descends into the mantle, it releases water and other volatile compounds. These volatiles lower the melting point of the overlying mantle wedge, causing partial melting and the generation of magma. This magma rises through the overlying plate, erupting at the surface to form a chain of volcanic islands – the island arc.

Major Characteristics of Island Arc Systems

  • Volcanism: Characterized by frequent and often explosive volcanic eruptions. The magma is typically andesitic to rhyolitic in composition, leading to stratovolcanoes.
  • Seismicity: Island arcs are associated with intense seismic activity. Earthquakes occur along the subduction zone (Benioff zone) and within the overriding plate due to stress accumulation.
  • Accretionary Wedge: A wedge-shaped mass of sediment and oceanic crust accumulates between the subducting and overriding plates. This accretionary wedge contributes to the growth of the island arc.
  • Forearc Basin: A sedimentary basin develops between the volcanic arc and the accretionary wedge. This basin is filled with sediments eroded from the arc and the accretionary wedge.
  • Backarc Basin: A basin forms behind the volcanic arc, often characterized by extension and back-arc spreading. This can lead to the formation of marginal seas.
  • Oceanic Trench: A deep, narrow depression in the seafloor marks the location of the subduction zone.

Schematic Section Through an Island Arc System

Below is a schematic section illustrating the key components of an island arc system.

Island Arc System Schematic

Components:

  • 1. Oceanic Plate: The denser plate undergoing subduction.
  • 2. Subduction Zone: The area where the oceanic plate descends into the mantle.
  • 3. Benioff Zone: The plane of earthquakes within the subducting plate.
  • 4. Mantle Wedge: The wedge-shaped region of mantle above the subducting plate.
  • 5. Magma Generation: Partial melting of the mantle wedge due to the addition of volatiles.
  • 6. Volcanic Arc: The chain of volcanic islands formed by magma rising to the surface.
  • 7. Stratovolcanoes: Cone-shaped volcanoes built up by layers of lava and ash.
  • 8. Forearc Basin: The sedimentary basin between the volcanic arc and the accretionary wedge.
  • 9. Accretionary Wedge: The accumulation of sediment and oceanic crust.
  • 10. Backarc Basin: The basin behind the volcanic arc, often experiencing extension.
  • 11. Oceanic Crust: The crust forming the seafloor.
  • 12. Oceanic Trench: The deepest part of the ocean floor, marking the subduction zone.

Examples of Island Arc Systems

  • Japan: Formed by the subduction of the Pacific Plate beneath the Eurasian Plate.
  • Philippines: A complex arc system formed by multiple subduction zones.
  • Aleutian Islands (Alaska): Formed by the subduction of the Pacific Plate beneath the North American Plate.
  • Indonesia: A vast archipelago formed by the subduction of the Indo-Australian Plate beneath the Eurasian Plate.
  • Lesser Antilles: Formed by the subduction of the Atlantic Plate beneath the Caribbean Plate.

Evolution of Island Arcs

Over geological time, island arcs can evolve into continental crust. Continued accretion of sediments and volcanic material, coupled with the eventual collision of the arc with a continent, can lead to the formation of mountain ranges. The Himalayas, for example, are thought to have originated from the collision of the Indian Plate with the Eurasian Plate, which involved the accretion of island arcs.

Conclusion

Island arc systems are fundamental features of plate tectonics, representing zones of intense geological activity. Their formation through subduction processes results in characteristic volcanism, seismicity, and the development of unique geological structures. Studying these systems provides crucial insights into the Earth’s dynamic processes and the evolution of continents. Understanding the hazards associated with island arcs, such as volcanic eruptions and earthquakes, is vital for mitigating risks in these densely populated regions.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Benioff Zone
A dipping planar zone of earthquakes that occurs at subduction zones. The focal depths of earthquakes increase with distance from the trench, tracing the path of the subducting plate into the mantle.

Key Statistics

Approximately 68% of all active volcanoes are located within the Pacific Ring of Fire, which is largely defined by subduction zones and island arcs.

Source: US Geological Survey (USGS) - as of knowledge cutoff 2023

The Pacific Plate subducts under numerous other plates at a rate of approximately 8-10 cm per year, contributing to the high frequency of volcanic and seismic activity in the Pacific Ring of Fire.

Source: Plate Boundary Observatory (PBO) - as of knowledge cutoff 2023

Examples

Krakatoa (Indonesia)

The 1883 eruption of Krakatoa, an island arc volcano in Indonesia, was one of the most catastrophic volcanic events in recorded history. The eruption caused a massive tsunami that killed over 36,000 people and had global climatic effects.

Frequently Asked Questions

▶What is the difference between a volcanic island arc and a continental volcanic arc?

A volcanic island arc forms when an oceanic plate subducts beneath another oceanic plate, creating a chain of volcanic islands. A continental volcanic arc forms when an oceanic plate subducts beneath a continental plate, creating a chain of volcanoes on the continent (e.g., the Andes Mountains).

Topics Covered

GeologyEarth SciencePlate TectonicsVolcanologySubduction Zones
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