Elaborate Notes
Causes of Plate Movements
The theory of Plate Tectonics posits that the Earth’s lithosphere is divided into several large and small plates that move relative to one another. The driving forces behind this movement are complex and originate from the Earth’s internal heat.
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The Convection Currents:
- Concept and Proponent: This theory was first proposed by the British geologist Arthur Holmes in 1929 as a mechanism to explain Alfred Wegener’s theory of Continental Drift. Holmes suggested that thermal convection in the Earth’s mantle could be the engine driving continental movement.
- Mechanism: The Earth’s deep interior contains radioactive elements (like Uranium, Thorium, and Potassium) whose decay generates immense heat. This heat causes material in the mantle to become less dense and rise. Upon reaching the base of the lithosphere, this rising material spreads laterally, cools, becomes denser, and then sinks back down. This continuous circulatory motion forms convection cells.
- Models of Convection:
- Whole-Mantle Convection: This model suggests that the convection cells extend throughout the entire mantle, from the core-mantle boundary to the base of the lithosphere.
- Layered or Stratified Convection: This model proposes two separate convection layers—one in the upper mantle (asthenosphere) and another in the lower mantle, separated by a thermal boundary layer.
- Impact on Plates:
- Divergence: Where the rising limbs of two adjacent convection cells diverge, they exert tensional forces on the overlying lithosphere, causing it to stretch, fracture, and create rift valleys or mid-oceanic ridges (MORs).
- Convergence: Where the cool, dense material descends, it drags the attached lithospheric plate downwards, leading to subduction at convergent boundaries.
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Mantle Plumes:
- Concept and Proponent: The mantle plume hypothesis was advanced by American geophysicist W. Jason Morgan in 1971. It proposes that narrow, jet-like upwellings of abnormally hot rock originate from the deep mantle, possibly the D” layer at the core-mantle boundary.
- Mechanism: These plumes are stationary relative to the moving tectonic plates above them. As a plume rises and reaches the base of the lithosphere, it spreads out radially, creating a “hotspot.” This process can dome the surface, cause rifting, and lead to extensive volcanism.
- Examples:
- The Hawaiian-Emperor seamount chain is a classic example. The Pacific Plate has moved over the stationary Hawaiian hotspot, creating a chain of volcanic islands and seamounts. The age of the volcanoes increases progressively away from the current hotspot location.
- The Réunion hotspot in the Indian Ocean is believed to be responsible for the massive flood basalt eruption that formed the Deccan Traps in India around 66 million years ago.
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Ridge Push (Gravitational Sliding):
- Mechanism: This is a secondary force resulting from the topography of mid-oceanic ridges. At MORs, the upwelling of hot mantle material heats the lithosphere, making it less dense and more buoyant. This causes the ridge to stand several kilometers higher than the abyssal plains.
- Process: Due to this elevation, gravity causes the elevated lithosphere at the ridge crest to “push” or slide downhill, away from the ridge axis. It is more accurately described as “ridge slide” or “gravity sliding.” This force acts as a continuous push on the entire plate.
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Slab Pull:
- Mechanism: This force is widely considered the most significant driver of plate motion. It occurs at subduction zones, where an oceanic plate collides with another plate and is forced to sink into the mantle.
- Process: As the oceanic lithosphere moves away from the MOR, it cools and becomes progressively denser. When it reaches a subduction zone, its high density (greater than the underlying asthenosphere) causes it to sink under its own weight into the mantle. This sinking slab acts like a heavy weight, pulling the rest of the plate along with it.
- Significance: Studies, such as those by D. Forsyth and S. Uyeda (1975), have shown a strong correlation between the velocity of a plate and the length of its subducting slab, suggesting that slab pull is the dominant driving force.
The Criticism of Plate Tectonic Theory
While Plate Tectonics is a powerful and widely accepted unifying theory in geology, it has certain limitations and unanswered questions.
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Plate Motion without Subduction: The theory struggles to fully explain the motion of plates that are almost entirely surrounded by divergent boundaries.
- Case of Africa and Antarctica: The African and Antarctic plates are nearly encircled by mid-oceanic ridges (divergent boundaries), where new crust is being formed and pushing them. However, they lack significant zones of subduction along their margins to accommodate this growth. The “slab pull” mechanism, considered the primary driver, is largely absent for these plates. This raises questions about the balance of forces and the future trajectory of these continents.
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Precambrian Tectonics: The theory is based on the present-day Earth system, and its application to the deep geological past is challenging.
- Early Earth Conditions: The Archean and Proterozoic eons had a much hotter mantle due to greater radiogenic heat production. This may have resulted in a different style of tectonics, possibly involving smaller, faster-moving plates or a “stagnant lid” regime where the lithosphere did not break into distinct plates as it does today.
- Wilson Cycle: Reconstructing ancient plate configurations and supercontinents (like Rodinia or Columbia) relies on interpreting ancient mountain belts and rock formations, a process fraught with uncertainty. The precise mechanisms of plate movement in the early Earth remain a subject of active research and debate.
Volcanism
Volcanism encompasses all geological phenomena associated with the origin of magma, its ascent through the crust, and its extrusion onto the surface as lava, along with the associated gases and pyroclastic materials.
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Stages of Volcanism:
- Generation of Magma: Magma is generated in the upper mantle (asthenosphere) or lower crust through three primary mechanisms:
- Decompression Melting: Occurs when pressure decreases, lowering the melting point of rock. This is common at divergent boundaries (MORs, rift valleys) where the lithosphere is stretched and thinned.
- Flux Melting: The addition of volatiles (like water) to hot rock lowers its melting point. This happens at subduction zones, where water from the subducting oceanic slab is released into the overlying hot mantle wedge.
- Heat Transfer Melting: Hot magma rising from the mantle can transfer heat to the surrounding crustal rock, causing it to melt. This is common at continental hotspots and subduction zones.
- Intrusion: Being less dense than the surrounding solid rock, the generated magma rises and often accumulates in underground reservoirs known as magma chambers within the lithosphere.
- Extrusion: When the pressure within the magma chamber, largely from dissolved gases expanding out of the solution, exceeds the confining pressure of the overlying rock, the magma is forced to the surface in an eruption.
- Generation of Magma: Magma is generated in the upper mantle (asthenosphere) or lower crust through three primary mechanisms:
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Magma and Lava:
- Magma: It is the molten or semi-molten silicate material found beneath the Earth’s surface. It is a complex mixture of molten rock (liquid), suspended crystals (solid), and dissolved gases (e.g., water vapor, carbon dioxide, sulfur dioxide).
- Lava: It is the term for magma once it erupts onto the Earth’s surface. During eruption, most of the dissolved gases escape, so lava is compositionally different from the parent magma. Lava can be hotter and more fluid initially but cools and solidifies to form extrusive igneous rocks.
Types of Magma
The composition of magma, particularly its silica (SiO₂) content, determines its physical properties and eruptive behavior.
| Property | Basaltic (Mafic) Magma | Granitic (Felsic/Rhyolitic) Magma |
|---|---|---|
| Silica Content | Low (45-52%) | High (>66%) |
| Temperature | High (1000-1200°C) | Low (650-800°C) |
| Viscosity | Low (fluid, like honey) | High (viscous, like toothpaste) |
| Gas Content | Low, gases escape easily | High, gases are trapped |
| Density | High | Low |
| Composition | Rich in iron (Fe) and magnesium (Mg) | Rich in silica (Si), potassium (K), sodium (Na) |
| Eruptive Style | Effusive, quiet eruptions (lava flows) | Explosive, violent eruptions (pyroclastic flows) |
| Associated Rock | Basalt, Gabbro | Rhyolite, Granite |
| Tectonic Setting | Divergent boundaries (MORs, rifts), hotspots | Convergent boundaries (O-C), continental hotspots |
| Resulting Crust | Oceanic Crust | Continental Crust |
*Note: An intermediate type, Andesitic Magma, with 52-66% silica, is characteristic of subduction zones and forms composite volcanoes. Its properties are intermediate between basaltic and granitic magma.
Magma Generation and Plate Tectonics
The tectonic setting directly controls the mechanism of magma generation and thus the type of volcanism.
| Tectonic Setting | Location | Cause of Melting | Type of Magma | Nature of Eruption |
|---|---|---|---|---|
| O-O Divergence | Mid-Oceanic Ridge | Decompression Melting | Basaltic | Fissure type; effusive, slow, and continuous. Mostly submarine. |
| C-C Divergence | Rift Valley (e.g., East African Rift) | Decompression Melting | Basaltic | Along volcanoes in the rift; can be slightly explosive due to interaction with continental crust. |
| O-O Convergence | Volcanic Island Arcs (e.g., Japan) | Flux Melting of mantle wedge | Andesitic | Highly explosive due to higher viscosity and gas content. |
| O-C Convergence | Continental Volcanic Arcs (e.g., Andes) | Flux Melting & Heat Transfer (melting of continental crust) | Andesitic to Rhyolitic | Very violent and explosive due to high silica, high viscosity, and trapped gases. |
| Hotspots | Intra-plate (e.g., Hawaii, Réunion) | Mantle Plume (Decompression Melting) | Basaltic (if under ocean) or mixed (if under continent) | Effusive and smooth lava flows forming shield volcanoes. |
Distribution of Volcanoes
The global distribution of volcanoes is not random; it is predominantly concentrated along plate boundaries and at hotspots.
- Ridge Volcanism: Occurs along mid-oceanic ridges where plates diverge. This is the most voluminous type of volcanism on Earth, but it is mostly submarine and goes unnoticed. Examples include the Mid-Atlantic Ridge, East Pacific Rise, and Carlsberg Ridge.
- Arc Volcanism: Found at ocean-ocean convergent boundaries. The subduction of one oceanic plate creates a curved chain of volcanic islands known as an island arc. Examples: Japanese Archipelago, Aleutian Islands, Mariana Islands. This zone is a major part of the Pacific Ring of Fire.
- Volcanic Chains: Located along ocean-continent convergent boundaries. Subduction of an oceanic plate beneath a continent creates a linear chain of volcanoes on the continental margin, often associated with fold mountains. Examples: The Andes in South America and the Cascade Range in North America.
- Volcanic Clusters: Found along continent-continent divergent boundaries (rift zones). Continental rifting leads to decompression melting and the formation of volcanoes, often in clusters. Example: Volcanoes like Mount Kilimanjaro and Nyiragongo in the East African Rift Valley.
- Volcanic Lines (Hotspot Tracks): A linear arrangement of volcanoes created as a tectonic plate moves over a stationary mantle plume. The volcanoes are progressively older away from the active hotspot. Examples: The Hawaiian-Emperor Seamount Chain and the Réunion-Mascarene Plateau.
Volcanic Landforms
Volcanic activity creates distinct landforms both on the surface (extrusive) and within the crust (intrusive).
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Extrusive Landforms: Formed from the eruption of lava and pyroclastics onto the surface.
- Volcanic Cones: The classic conical hills built around a volcanic vent.
- Shield Volcano: Broad, gently sloping cone built from successive flows of low-viscosity basaltic lava. Example: Mauna Loa, Hawaii.
- Ash-Cinder Cone: Steep-sided, relatively small cone built from ejected pyroclastic fragments (cinders) that accumulate around the vent. Example: Parícutin, Mexico.
- Composite Volcano (Stratovolcano): Large, steep-sided, symmetrical cone built of alternating layers of viscous lava flows and pyroclastic deposits. Formed from andesitic magma and are highly explosive. Example: Mount Fuji (Japan), Mount St. Helens (USA).
- Flood Basalt Province (Plateau): Extensive, flat-topped plains formed by voluminous, low-viscosity basaltic lava erupting from fissures over a long period. Examples: Deccan Traps (India), Siberian Traps (Russia).
- Hydrothermal Features:
- Hot Springs: Areas where groundwater is heated by a nearby magma body and emerges at the surface.
- Geysers: A type of hot spring where water is superheated in underground channels, builds up pressure, and erupts periodically as a fountain of steam and hot water. Example: Old Faithful, Yellowstone National Park.
- Fumarole: A vent that emits only volcanic gases and steam.
- Mud Volcano: A landform created by the eruption of mud, water, and gases, not true magma. Often found in petroleum exploration regions or subduction zones.
- Volcanic Cones: The classic conical hills built around a volcanic vent.
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Intrusive Landforms (Plutons): Formed when magma cools and solidifies within the Earth’s crust. They are exposed at the surface only after significant erosion of the overlying rocks.
- Batholiths: The largest intrusive bodies; massive, discordant (cutting across existing rock layers) bodies of igneous rock. They form the core of major mountain ranges. Example: The Sierra Nevada Batholith in California.
- Laccoliths: Mushroom-shaped, concordant (parallel to existing rock layers) intrusions that have domed up the overlying strata.
- Lopoliths: Large, saucer-shaped, concordant intrusions that are sunken in the center.
- Sill: A tabular, concordant intrusion where magma has squeezed between layers of rock.
- Dykes: A tabular, discordant intrusion where magma has forced its way across rock layers, often vertically or at a steep angle.
Prelims Pointers
- Convection Current Theory: Proposed by Arthur Holmes (1929) to explain the movement of continents.
- Mantle Plume Theory: Proposed by W. Jason Morgan (1971) to explain hotspots.
- Primary Driving Force for Plate Tectonics: Slab Pull.
- Secondary Driving Force for Plate Tectonics: Ridge Push (Gravity Sliding).
- Magma: Molten silicate material with dissolved gases located beneath the Earth’s surface.
- Lava: Magma that has erupted onto the Earth’s surface and lost most of its gases.
- Basaltic Magma: Low silica (<52%), low viscosity (fluid), high temperature, associated with effusive eruptions at divergent boundaries and hotspots. Forms oceanic crust.
- Granitic (Felsic) Magma: High silica (>66%), high viscosity, low temperature, associated with explosive eruptions at convergent boundaries. Forms continental crust.
- Andesitic Magma: Intermediate silica content, associated with explosive eruptions at subduction zones (e.g., island arcs).
- Decompression Melting: Occurs due to a decrease in pressure at divergent boundaries and hotspots.
- Flux Melting: Occurs due to the addition of volatiles (water) at subduction zones.
- Pacific Ring of Fire: A zone of intense volcanic and seismic activity around the Pacific Ocean, associated with convergent plate boundaries.
- Intrusive Landforms (Plutons):
- Batholith: Largest, discordant intrusion.
- Laccolith: Mushroom-shaped, concordant.
- Lopolith: Saucer-shaped, concordant.
- Sill: Horizontal, sheet-like, concordant.
- Dyke: Vertical, sheet-like, discordant.
- Extrusive Landforms:
- Shield Volcano: Broad, gentle slopes, basaltic lava (e.g., Mauna Loa).
- Cinder Cone: Steep, small, made of pyroclasts (e.g., Parícutin).
- Composite Volcano: Steep, large, alternating layers of lava and ash (e.g., Mt. Fuji).
- Flood Basalt Provinces: Deccan Traps (India), Siberian Traps (Russia).
- Hotspot Examples: Hawaiian Islands, Réunion Island.
Mains Insights
Plate Tectonics: A Unifying Theory and Its Driving Mechanisms
- Synthesis of Concepts: Plate Tectonics is the cornerstone of modern geology as it elegantly synthesizes earlier ideas of Continental Drift (Wegener) and Seafloor Spreading (Hess, Dietz). It provides a single, coherent framework to explain a vast range of geophysical phenomena, including the distribution of earthquakes, volcanoes, formation of mountain ranges, ocean basins, and the global distribution of fossils and rock types. This makes it a crucial topic for GS Paper I.
- Debate on Driving Forces: While convection currents provide the thermal engine, there is a significant historiographical and scientific debate on the primary mechanical driver.
- Slab Pull vs. Ridge Push: Early models emphasized ridge push and mantle drag. However, modern consensus, supported by geophysical data (Forsyth & Uyeda, 1975), overwhelmingly favors slab pull as the dominant force. Plates with long subducting slabs (like the Pacific Plate) move fastest, while plates without significant slabs (like the African Plate) move slowest.
- Implications: This understanding has implications for predicting future plate movements and understanding the life cycle of ocean basins (the Wilson Cycle). The dominance of slab pull suggests that the initiation of subduction is the most critical event in changing the direction and speed of plate motion.
Linkages between Plate Tectonics and Volcanism
- Predictive Power: The theory of plate tectonics provides a powerful predictive model for volcanism. The type of plate boundary directly dictates the petrology (magma type) and morphology (volcano type) of volcanism. This cause-effect relationship is fundamental.
- Divergence (Cause) → Decompression Melting → Basaltic Magma → Effusive Eruptions → Shield Volcanoes/MORs (Effect).
- Convergence (Cause) → Flux Melting → Andesitic/Rhyolitic Magma → Explosive Eruptions → Composite Volcanoes/Island Arcs (Effect).
- Volcanism as a Hazard and a Resource (GS-III Disaster Management, GS-I Geography):
- Hazard: The most explosive and hazardous volcanoes (composite volcanoes) are located at convergent boundaries, often in densely populated regions (e.g., Japan, Indonesia, the Andes). Understanding this link is crucial for disaster risk reduction, monitoring, and land-use planning.
- Resource: Volcanic activity, while destructive, is also a constructive force. It creates new land (e.g., Hawaii), produces incredibly fertile soils (e.g., from volcanic ash), is a source for valuable mineral deposits (e.g., sulfur, copper), and provides a sustainable source of geothermal energy.
Unresolved Issues and the Evolving Nature of Science
- Limitations as Insight: The criticisms of plate tectonics (e.g., motion of Africa, Precambrian tectonics) are not failures of the theory but highlight that science is a progressive and self-correcting discipline. These unresolved questions drive future research into the complexities of mantle dynamics and Earth’s thermal history.
- Intra-plate Activity: The theory in its simplest form does not fully explain intra-plate earthquakes and volcanoes that are not associated with hotspots. This suggests that stresses can be transmitted far into the plate interiors and that the lithosphere is not perfectly rigid, leading to ongoing refinements of the model. For Mains answers, acknowledging these limitations demonstrates a deeper, more nuanced understanding of the scientific process.