1.5 - Geography Notes 13

Elaborate Notes

Ocean-Ocean Convergence

• Fundamental Principle: This type of convergent boundary occurs when two oceanic plates collide. As oceanic lithosphere cools and moves away from a mid-ocean ridge, it becomes denser. Consequently, when two oceanic plates converge, the older, colder, and therefore denser plate subducts, or sinks, into the asthenosphere beneath the younger, less dense plate. This zone of descent is known as a subduction zone. This concept was solidified with the development of plate tectonic theory in the 1960s, building on earlier ideas of seafloor spreading proposed by Harry Hess (1962).
• Formation of Ocean Trenches: The process of subduction causes the seafloor to bend and form a deep, linear depression known as an oceanic trench. These are the deepest parts of the Earth’s oceans. A prime example is the Mariana Trench, formed by the subduction of the Pacific Plate beneath the smaller Mariana Plate. Its deepest point, the Challenger Deep, is approximately 11,000 meters below sea level. Similarly, the Japan Trench is formed where the Pacific Plate subducts under the Okhotsk Plate (part of the larger Eurasian Plate system).
• Process of Magma Generation: As the subducting slab descends, it is subjected to increasing temperature and pressure. Water trapped in the minerals of the oceanic crust is released at depth (a process called dewatering). This water lowers the melting point of the overlying mantle wedge, causing it to melt and form magma. This process is known as flux melting.
• Formation of Volcanic Island Arcs: The generated magma, being less dense than the surrounding mantle material, rises upwards. It may accumulate in magma chambers and eventually erupt on the ocean floor. Over millions of years, successive eruptions build up volcanic mountains. When these submarine volcanoes grow tall enough to emerge above sea level, they form a chain of volcanic islands. This chain is typically curved, paralleling the trench, and is hence called a volcanic island arc.
  • Examples: The Japanese archipelago is a classic example of an island arc. The Aleutian Islands (formed by the Pacific Plate subducting under the North American Plate) and the Lesser Antilles in the Caribbean are other prominent examples.
• Archipelago: This is a broader geographical term for a group or chain of islands. While many archipelagos are volcanic island arcs formed by ocean-ocean convergence (e.g., Indonesia, Philippines), the term can also apply to islands formed by other geological processes. The archipelagos of Indonesia and the Philippines are particularly complex, resulting from the interaction of multiple tectonic plates (Eurasian, Pacific, Philippine, and Indo-Australian plates).
• Seismicity and the Wadati-Benioff Zone: Subduction zones are seismically very active. The friction and stress between the descending and overriding plates generate frequent earthquakes. These earthquakes occur at a range of depths.
  • Shallow-focus earthquakes: Occur near the trench where the plates first make contact.
  • Intermediate and Deep-focus earthquakes: Occur deeper within the descending slab as it plunges into the mantle.
  • The distribution of these earthquakes is not random; they are concentrated in a dipping planar zone that traces the path of the subducting slab. This zone is named the Wadati-Benioff Zone, after the seismologists Kiyoo Wadati (1935) and Hugo Benioff (1949), who independently identified its existence.

Ocean-Continent Convergence

• Fundamental Principle: This boundary involves the collision of a dense oceanic plate with a less dense, more buoyant continental plate. The oceanic plate invariably subducts beneath the continental plate. The subduction of the Nazca Plate beneath the South American Plate is a textbook example of this process.
• Formation of Continental Volcanic Arcs and Fold Mountains:
  • As the oceanic plate subducts, similar to ocean-ocean convergence, it heats up and releases water, causing flux melting in the overlying mantle wedge. The resulting magma rises.
  • Some of this magma erupts on the surface of the continent, forming a chain of volcanoes known as a continental volcanic arc. For instance, the Andes mountain range is dotted with active volcanoes like Ojos del Salado (the world’s highest active volcano) and Cotopaxi. Mount St. Helens in the Cascade Range of North America is another example, formed by the subduction of the Juan de Fuca Plate under the North American Plate.
  • Simultaneously, the immense compressional forces exerted by the converging plates deform the continental margin. Sediments scraped off the subducting oceanic plate (forming an accretionary wedge) and the rocks of the continental edge are folded, faulted, and uplifted to create a linear chain of fold-and-thrust mountains, like the Andes and the Rocky Mountains.
• Trenches and Seismicity: An oceanic trench, such as the Peru-Chile Trench, forms offshore, parallel to the mountain range. This boundary is also characterized by a Wadati-Benioff zone with shallow, intermediate, and deep-focus earthquakes. The 1960 Valdivia earthquake in Chile, the most powerful earthquake ever recorded (magnitude 9.5), occurred at this type of boundary.
• The Pacific Ring of Fire: The Pacific Ocean is almost entirely rimmed by convergent plate boundaries (both O-O and O-C types). This has resulted in a continuous belt of trenches, volcanic arcs, and intense seismic activity. This zone, stretching from the coast of the Americas to Japan, the Philippines, and New Zealand, is famously known as the Pacific Ring of Fire. It is home to approximately 75% of the world’s active volcanoes and about 90% of its earthquakes.

Continent-Continent Convergence

• Pre-collision Stage: This type of collision is preceded by the convergence of an oceanic plate and a continental plate. The intervening ocean basin (a geosyncline, in older terminology) between two continental landmasses gradually closes as the oceanic lithosphere is subducted. The Tethys Sea, which once separated the supercontinents of Laurasia and Gondwana, is a prime example of such an ocean basin.
• Collision and Suture Zone: Once the oceanic lithosphere is completely subducted, the two continental plates collide. Because continental crust is thick and buoyant (composed largely of granitic rocks), it resists subduction. Instead, the continental masses crumple, fold, and are thrust upon each other. The zone where the two continents are joined or “stitched” together is called a suture zone. The Indus-Tsangpo Suture Zone in the Himalayas marks the boundary where the Indian Plate was welded to the Eurasian Plate.
• Cessation of Volcanism: Volcanic activity is common during the subduction phase when the oceanic plate is being consumed. However, once the continents collide, the source of magma (flux melting from a wet, subducting oceanic slab) is cut off. Therefore, continent-continent collision zones are characterized by a lack of active volcanism.
• Mountain Building (Orogeny): The continued compression leads to extreme crustal shortening and thickening. The rocks are intensely folded and faulted. Large sections of crust can be thrust over others for many kilometers, forming complex structures called nappes. This process results in the formation of the world’s highest mountain ranges. The collision between the Indian and Eurasian plates, which began around 50 million years ago, created the Himalayas, the Tibetan Plateau, and associated ranges. The formation of the Alps is another example, resulting from the collision of the African and Eurasian plates.
• Seismicity: Due to the immense stress, these collision zones experience frequent and powerful earthquakes. However, because there is no deep subducting slab, the earthquakes are typically confined to shallow and intermediate depths.

Parallel / Conservative Plate Margin

• Fundamental Principle: At these margins, also known as transform boundaries, two plates slide horizontally past one another. Lithosphere is neither created (as at divergent boundaries) nor destroyed (as at convergent boundaries). The concept of transform faults was proposed by Canadian geophysicist J. Tuzo Wilson in 1965.
• Transform Faults: The line along which the movement occurs is a transform fault. These faults connect segments of other plate boundaries, most commonly offsetting segments of mid-oceanic ridges. The motion along the fault is parallel to the direction of plate movement.
• Characteristics and Examples:
  • Volcanism: There is no subduction or upwelling of magma from the mantle, so these boundaries are devoid of volcanic activity.
  • Seismicity: As the plates grind past each other, immense stress builds up due to friction, which is released periodically as earthquakes. These earthquakes are exclusively shallow-focus but can be extremely powerful and destructive due to their proximity to the surface.
  • The most famous example is the San Andreas Fault in California, which marks the boundary between the Pacific Plate and the North American Plate. The Pacific Plate is moving northwestward relative to the North American Plate. Other examples include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey.

Table Summarizing Plate Boundaries

Feature	Divergent (O-O)	Divergent (C-C)	Convergent (O-O)	Convergent (O-C)	Convergent (C-C)	Conservative/Parallel
Topography/Structures	Mid-Oceanic Ridges (MOR), transform faults	Rift Valleys, volcanoes, shallow seas (future ocean)	Trenches, Subduction Zones, Volcanic Island Arcs, Archipelagoes	Trenches, Subduction Zones, Continental Volcanic Arcs, Fold Mountains	High Fold Mountains, Suture Zones, Nappes, thickened crust	Transform Faults
Seismicity (Earthquakes)	Shallow-focus, low magnitude	Shallow-focus	Shallow, Intermediate, and Deep-focus (Wadati-Benioff Zone)	Shallow, Intermediate, and Deep-focus (Wadati-Benioff Zone)	Shallow and Intermediate-focus	Shallow-focus, often high magnitude
Volcanism	Yes (Basaltic, effusive)	Yes (Basaltic and Rhyolitic)	Yes (Andesitic, explosive)	Yes (Andesitic, explosive)	No (ceases after collision)	No
Geological Examples	Mid-Atlantic Ridge, Carlsberg Ridge	East African Rift Valley, Red Sea	Japan, Aleutian Islands, Philippines, Mariana Trench	Andes Mountains, Rocky Mountains, Peru-Chile Trench	Himalayas, Alps, Urals	San Andreas Fault, Alpine Fault (NZ)

Causes of Plate Movement

The movement of tectonic plates is a complex process driven by multiple forces originating from Earth’s interior heat. No single force is solely responsible; they act in concert.

• Mantle Convection:
  • Proposed by British geologist Arthur Holmes in 1929, this theory suggests that the Earth’s mantle behaves like a viscous fluid that convects.
  • Intense heat from the core heats the lower mantle material, causing it to become less dense and rise. Upon reaching the base of the lithosphere, it cools, becomes denser, and sinks back down.
  • This continuous circulation creates large convection cells within the mantle, which exert a drag force on the overlying lithospheric plates, acting like a giant conveyor belt. Ascending currents are thought to cause divergence (rifts), while descending currents cause convergence (subduction).
• Mantle Plumes and Hotspots:
  • Developed by W. Jason Morgan in 1971, this hypothesis posits the existence of narrow, jet-like upwellings of abnormally hot rock called mantle plumes. These are thought to originate from the core-mantle boundary.
  • When a plume head reaches the base of the lithosphere, it can cause massive volcanic eruptions and contribute to the rifting of continents. As a tectonic plate moves over a stationary plume, it creates a chain of volcanoes known as a hotspot track. The Hawaiian Islands and the Reunion hotspot are classic examples. While primarily associated with intra-plate volcanism, plumes can also influence plate motion.
• Ridge Push (Gravitational Sliding):
  • Mid-oceanic ridges are elevated areas on the seafloor due to the upwelling of hot, buoyant mantle material. The newly formed oceanic lithosphere is hot and less dense, sitting at a higher elevation than the older, colder lithosphere further away.
  • Gravity acts on this elevated ridge, causing the lithosphere to effectively “slide down” the gentle slope away from the ridge crest. This force, which pushes the plates apart, is called ridge push.
• Slab Pull:
  • This is now considered by many geophysicists to be the most significant driving force of plate motion.
  • At a subduction zone, the older, colder edge of an oceanic plate is denser than the hot asthenosphere it is sinking into.
  • This density difference causes the subducting slab to sink under its own weight, pulling the rest of the plate along with it. This gravitational pulling force is known as slab pull. It is a much stronger force than ridge push because the subducting slab is a large, dense mass. Research by scientists like Forsyth and Uyeda (1975) provided strong evidence for the dominance of slab pull by correlating plate velocities with the length of subducting slabs.

---

Prelims Pointers

• Subduction Zone: A region where one tectonic plate descends beneath another.
• Oceanic Trenches: Deepest parts of the ocean formed at subduction zones. Examples: Mariana Trench, Japan Trench, Peru-Chile Trench.
• Volcanic Island Arc: A curved chain of volcanic islands parallel to an ocean trench, formed at an ocean-ocean convergent boundary. Examples: Japan, Aleutian Islands, Philippines.
• Continental Volcanic Arc: A chain of volcanoes on a continent parallel to a subduction zone. Examples: Andes Mountains, Cascade Range (USA).
• Archipelago: A group or chain of islands. Can be formed by volcanism at plate boundaries (e.g., Indonesia).
• Wadati-Benioff Zone: A dipping planar zone of earthquakes produced by the interaction of a down-going oceanic crustal plate with an overriding plate. Named after Kiyoo Wadati and Hugo Benioff.
• Pacific Ring of Fire: A major area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur.
• Suture Zone: The zone where two continental plates have collided and are welded together. Example: Indus-Tsangpo Suture Zone in the Himalayas.
• Nappe: A large sheetlike body of rock that has been moved a long distance from its original position by faulting or folding. Common in continent-continent collision zones.
• Conservative/Transform Boundary: Plate boundary where plates slide past each other. Lithosphere is neither created nor destroyed.
• Transform Fault: The fault along which movement occurs at a conservative boundary. Example: San Andreas Fault (California), Alpine Fault (New Zealand).
• Mantle Convection: The slow creeping motion of Earth’s solid silicate mantle caused by convection currents carrying heat from the interior to the planet’s surface. Proposed by Arthur Holmes.
• Mantle Plume: An upwelling of abnormally hot rock within the Earth’s mantle. Associated with hotspots.
• Hotspot: A volcanic region thought to be fed by an underlying mantle plume. Examples: Hawaii, Reunion Island.
• Ridge Push: A proposed driving force for plate motion that results from the elevated position of the mid-ocean ridge.
• Slab Pull: A proposed driving force for plate motion caused by the downward pull of a cold, dense subducting slab of lithosphere. It is considered the dominant driving force.
• Oceanic crust is denser than continental crust.
• Volcanism is absent at continent-continent convergent and conservative plate boundaries.
• Deep-focus earthquakes are characteristic of convergent boundaries involving subduction (O-O and O-C).

---

Mains Insights

GS Paper I (Geomorphology and Global Distribution of Resources)

• Plate Tectonics as a Unifying Theory: The theory of plate tectonics provides a comprehensive framework that explains diverse geological phenomena: the distribution of continents and oceans, the formation of mountain ranges (orogeny), the global patterns of volcanism and seismicity, and the creation of valuable mineral deposits (e.g., metallic ores like copper and gold are often concentrated in zones of subduction-related volcanism).
• Cause-Effect Relationship in Landform Development:
  1. Convergence as the Cause: The collision of plates acts as the primary driver.
  2. Subduction/Collision as the Process: The nature of the crust (oceanic vs. continental) determines the subsequent process—subduction for dense oceanic plates, and collision/folding for buoyant continental plates.
  3. Landforms as the Effect: This process directly results in distinct landform assemblages:
     • O-C Convergence: Trench + Fold Mountains + Continental Volcanic Arc (e.g., Peru-Chile Trench + Andes).
     • O-O Convergence: Trench + Volcanic Island Arc (e.g., Mariana Trench + Mariana Islands).
     • C-C Convergence: Suture Zone + High Fold Mountains (e.g., Himalayas).
• Debate on Driving Mechanisms: While mantle convection was the earliest proposed mechanism, the modern consensus leans towards a model dominated by gravity-driven forces. The debate centers on the relative importance of slab pull versus ridge push. Most evidence suggests slab pull is the primary driver, being several times stronger than ridge push. Mantle convection is now seen more as a facilitating process that allows the plates to move, rather than the primary engine dragging them along.

GS Paper III (Disaster Management)

• Linking Plate Boundaries to Specific Hazards: Understanding plate tectonics is fundamental to hazard zonation and disaster preparedness.
  • Convergent Boundaries (Subduction Zones): These are the most hazardous zones. They produce the world’s largest earthquakes (megathrust earthquakes), which can generate devastating tsunamis (e.g., the 2004 Indian Ocean Tsunami caused by subduction at the Sunda Trench; the 2011 Tohoku Tsunami in Japan). They are also sites of explosive andesitic volcanism, which poses risks like pyroclastic flows and lahars.
  • Conservative (Transform) Boundaries: These produce powerful, shallow earthquakes that can cause extreme ground shaking and destruction in populated areas. The seismic risk to cities like San Francisco and Los Angeles from the San Andreas Fault is a prime example of this threat. The lack of vertical displacement means the tsunami risk is low, but the potential for urban devastation is very high.
  • Divergent Boundaries: Hazard is relatively low. Earthquakes are frequent but typically of low magnitude. Volcanism is effusive (gentle lava flows) and usually occurs underwater at MORs or in sparsely populated areas like Iceland and the East African Rift.
• Role in Prediction and Mitigation:
  • Mapping the Wadati-Benioff zone helps seismologists understand the geometry of a subducting slab and the potential depth and magnitude of earthquakes.
  • Monitoring crustal deformation using GPS along boundaries like the San Andreas Fault helps in assessing strain accumulation and forecasting long-term earthquake probability.
  • The concept of the “seismic gap”—a segment of an active fault that has not experienced a major earthquake for a long time—is derived from plate tectonic theory and helps identify areas at high risk for future large earthquakes. This knowledge is crucial for implementing building codes, creating early warning systems (like tsunami warning centers), and planning evacuation routes.