1.5 - Geography Notes 12

Elaborate Notes

Seafloor Spreading

The theory of Seafloor Spreading, a pivotal precursor to the comprehensive theory of Plate Tectonics, emerged from extensive post-World War II oceanographic research. The development of technologies like sonar and magnetometers allowed for detailed mapping of the ocean floor, which was previously considered a flat, featureless plain.

• Evidence from Ocean Floor Mapping:

  • Mid-Oceanic Ridges (MORs): Surveys revealed a continuous, globe-encircling chain of underwater mountains, the Mid-Oceanic Ridges. A key example is the Mid-Atlantic Ridge. These ridges were found to have a central rift valley and were sites of significant geological activity.
  • Volcanic Activity: It was discovered that these ridges are volcanically active, with a continuous eruption of basaltic magma from the Earth’s mantle. This suggested that new crust was being formed at these locations.
  • Paleomagnetism and Rock Symmetry: In the 1960s, studies of the magnetic properties of the oceanic crust revealed a striped pattern of magnetic anomalies, symmetrical on either side of the MORs. This pattern corresponded to the Earth’s periodic magnetic field reversals. The Vine-Matthews-Morley hypothesis (1963) proposed that as new basaltic crust forms at the ridge, it records the Earth’s magnetic polarity at that time. This symmetrical pattern of “magnetic stripes” provided compelling evidence that the seafloor was spreading from the ridges.
  • Age of Oceanic Rocks: Radiometric dating of rock samples collected from the ocean floor showed that the rocks are youngest at the crest of the ridges and progressively increase in age away from the crest on both sides. For instance, rocks at the Mid-Atlantic Ridge are newly formed, while those near the coasts of North America and Africa are around 180-200 million years old.
  • Age Discrepancy with Continents: Crucially, no oceanic crust was found to be older than about 200 million years. This is significantly younger than the continental crust, which contains rocks as old as 4 billion years. This implied that oceanic crust is continuously being created and destroyed.
  • Crustal Thickness and Sediments: The oceanic crust is relatively thin (around 5-10 km) compared to the continental crust (30-70 km). Furthermore, the layer of sediments on the ocean floor was found to be thinnest near the ridges and thickest near the continental margins, corroborating the idea that the crust near the ridges is younger and has had less time to accumulate sediment.

• The Theory by Harry Hess:

  • Based on this mounting evidence, Harry Hess, in his seminal 1962 paper “History of Ocean Basins” (though the idea was formulated in 1960), proposed the theory of Seafloor Spreading.
  • Hess postulated that mantle convection currents carry hot magma up to the surface at the mid-oceanic ridges.
  • This upwelling magma causes the oceanic crust to rupture and move apart. The magma then cools and solidifies, forming new oceanic crust.
  • This process acts like a ‘conveyor belt’, pushing the older crust away from the ridge on both sides.
  • To accommodate the creation of new crust without the Earth expanding, Hess proposed that the older, denser oceanic crust eventually descends back into the mantle at deep oceanic trenches (e.g., the Mariana Trench). This process of sinking is known as subduction.

Plate Tectonics Theory

Plate Tectonics is the unifying theory of geology that explains the large-scale movements of the Earth’s lithosphere. It synthesized the earlier concepts of Continental Drift and Seafloor Spreading into a cohesive model.

• Foundations and Proponents:

  • While Alfred Wegener’s Continental Drift Theory (1912) proposed that continents move, it lacked a viable mechanism. Seafloor Spreading provided this mechanism for the oceans.
  • The term “plate” was first used in a geological context by Canadian geophysicist J. Tuzo Wilson in 1965. He also introduced the concept of transform faults.
  • The comprehensive theory of Plate Tectonics was independently developed and presented by D.P. McKenzie and R.L. Parker (1967), and W. Jason Morgan (1967). They mathematically modeled the movements of these rigid plates over the Earth’s surface.

• Core Concepts of Plate Tectonics:

  • Lithosphere and Asthenosphere: The theory posits that the Earth’s outer rigid layer, the lithosphere (comprising the crust and the solid uppermost part of the mantle, approx. 100 km thick), is broken into several large and small segments called plates.
  • These lithospheric plates “float” and move over a mechanically weaker, partially molten layer in the upper mantle called the asthenosphere. The semi-liquid, ductile nature of the asthenosphere allows the rigid plates above to move.
  • Plate Motion: Plates are in constant, slow motion relative to each other, typically at rates of a few centimeters per year (comparable to the rate of fingernail growth). This movement is driven primarily by convection currents in the mantle, along with forces like ridge push (gravitational force pushing plates away from elevated MORs) and slab pull (gravitational force pulling a subducting plate down into the mantle).
  • Major and Minor Plates: The Earth’s lithosphere is divided into seven major plates:
    1. Pacific Plate (the largest, entirely oceanic)
    2. North American Plate
    3. South American Plate
    4. Eurasian Plate
    5. African Plate
    6. Indo-Australian Plate
    7. Antarctic Plate
  • There are also numerous minor plates, such as the Nazca Plate, Cocos Plate, Arabian Plate, Philippine Plate, Juan de Fuca Plate, and Scotia Plate. The interactions at the boundaries of these plates are responsible for most of the Earth’s seismic and volcanic activity.

• Distinction from Continental Drift Theory (CDT): | Feature | Continental Drift Theory (Wegener) | Plate Tectonics Theory | | :--- | :--- | :--- | | Moving Units | Continents (composed of lighter Sial - Silicon & Aluminium) were thought to plow through the denser oceanic floor (Sima - Silicon & Magnesium). | The entire Lithosphere (both continental and oceanic crust plus the rigid upper mantle) moves as a coherent plate. | | Underlying Layer| Sial was thought to be floating on Sima. The concept of a rigid lithosphere and ductile asthenosphere was absent. | Rigid lithospheric plates move over the weak, ductile Asthenosphere. | | Resistance| Wegener’s model implied continents moved freely or with little resistance, which was geophysically implausible. The proposed forces (tidal pull, pole-fleeing force) were proven to be far too weak. | The movement of massive, rigid lithospheric plates is driven by powerful mantle convection forces (slab pull, ridge push) and meets significant resistance, leading to intense deformation at plate boundaries. |

Divergent / Constructive Plate Boundary

At a divergent boundary, two plates move apart from each other. This process creates new lithosphere, hence the term ‘constructive’.

• Ocean-Ocean Divergent Boundary:

  • This occurs where two oceanic plates are pulling away from each other. The classic example is the Mid-Atlantic Ridge, separating the North American and Eurasian plates, and the South American and African plates.
  • Process: Tensional forces cause the lithosphere to fracture. Hot magma from the asthenosphere rises to fill these fractures. It cools and solidifies to form new basaltic oceanic crust.
  • Features: This continuous process builds up a submarine mountain range, the mid-oceanic ridge. The valley at the center of the ridge is the rift valley. Volcanic activity here is typically non-explosive fissure eruptions. It is also characterized by shallow-focus earthquakes (focal depth < 70 km) due to the tensional stress and magma movement. Transform faults often cut across the ridge, offsetting segments of it.

• Continent-Continent Divergent Boundary:

  • This involves the rifting and breakup of a continental landmass. It is considered the initial stage of the formation of a new ocean basin. The process occurs in three main stages:
  1. Stage 1: Intra-continental Rifting:
     • An upwelling mantle plume heats and domes the overlying continental crust, causing it to stretch and thin.
     • Tensional forces lead to the development of a series of parallel normal faults, causing a central block of land to subside, forming a rift valley.
     • Example: The East African Rift Valley is the world’s most prominent active continental rift system, stretching from Ethiopia to Mozambique. It is characterized by deep valleys, large lakes (like Lake Tanganyika and Lake Malawi), and associated volcanism (e.g., Mount Kilimanjaro).
  2. Stage 2: Inter-plate Thinning (Incipient Ocean):
     • As the continental crust continues to stretch and thin, the rift valley widens and subsides further, eventually dropping below sea level.
     • Seawater floods the elongated depression, forming a narrow, linear sea.
     • Example: The Red Sea, which separates the Arabian Plate from the African Plate, is a prime example of this stage. A narrow strip of new oceanic crust is already forming along its central axis.
  3. Stage 3: Formation of a Mid-Oceanic Ridge (Mature Ocean):
     • With continued divergence, the narrow sea widens into a full-fledged ocean basin.
     • A new mid-oceanic ridge develops along the axis of spreading, and the continental fragments are pushed further apart.
     • Example: The Atlantic Ocean is the classic example of a mature ocean basin that formed from the rifting of the supercontinent Pangaea, with the Mid-Atlantic Ridge marking the zone of divergence.

Convergent / Destructive Plate Boundaries

At a convergent boundary, two plates move towards each other and collide. As one plate is typically forced beneath the other and destroyed in the mantle, these are also known as ‘destructive’ boundaries.

• Oceanic-Oceanic Convergence:
  • When two oceanic plates collide, the plate that is older, colder, and therefore denser, will bend and slide beneath the less dense plate.
  • Subduction and Trenches: This process of one plate sinking beneath another is called subduction. It creates a long, narrow, and extremely deep depression in the ocean floor known as an oceanic trench. The Mariana Trench, formed by the subduction of the Pacific Plate under the Philippine Plate, is the deepest point on Earth.
  • Earthquakes: The subducting slab generates immense friction and stress as it descends, causing intense and widespread earthquakes. These earthquakes can occur at shallow, intermediate, and deep levels along the descending slab, a zone known as the Wadati-Benioff zone. The deepest earthquakes (up to 700 km) are found only in these subduction zones.
  • Volcanic Island Arcs: As the subducting slab descends, heat and pressure cause water trapped in the oceanic crust to be released. This water lowers the melting point of the overlying mantle wedge, generating magma. This magma, being less dense, rises to the surface and erupts on the overriding oceanic plate, forming a chain of volcanic islands known as a volcanic island arc. Examples include the Japanese Archipelago, the Aleutian Islands, and the Philippines.

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Prelims Pointers

• Seafloor Spreading Theory: Proposed by Harry Hess (1961).
• Evidence for Seafloor Spreading:
  • Presence of Mid-Oceanic Ridges (MORs).
  • Symmetrical pattern of magnetic anomalies (paleomagnetism) on either side of MORs.
  • Rocks are youngest at the MOR crest and age increases away from it.
  • Oceanic crust is much younger (< 200 million years) than continental crust.
  • Sediment thickness increases away from the MOR.
• Plate Tectonics Terminology:
  • The term ‘Plate’ was coined by J. Tuzo Wilson (1965).
  • The theory was formulated by McKenzie, Parker, and Morgan (1967).
• Earth’s Layers (Mechanical):
  • Lithosphere: Rigid outer layer (crust + upper mantle).
  • Asthenosphere: Weaker, semi-molten layer below the lithosphere on which plates move.
• Major Tectonic Plates:
  1. Pacific Plate
  2. North American Plate
  3. South American Plate
  4. Eurasian Plate
  5. African Plate
  6. Indo-Australian Plate
  7. Antarctic Plate
• Minor Tectonic Plates: Nazca, Cocos, Arabian, Philippine, Juan de Fuca, Scotia.
• Plate Boundaries & Associated Features:
  • Divergent (Constructive): Plates move apart.
    • Ocean-Ocean: Mid-Oceanic Ridges (e.g., Mid-Atlantic Ridge), shallow earthquakes, fissure volcanoes.
    • Continent-Continent: Rift Valleys (e.g., East African Rift Valley), linear seas (e.g., Red Sea).
  • Convergent (Destructive): Plates collide.
    • Ocean-Ocean: Oceanic Trenches (e.g., Mariana Trench), Volcanic Island Arcs (e.g., Japan), deep earthquakes (Wadati-Benioff Zone).
• Subduction: The process where one tectonic plate moves under another and is forced to sink into the mantle. It occurs at convergent boundaries.

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Mains Insights

• Paradigm Shift in Earth Sciences:

  • The theory of Plate Tectonics represents a scientific revolution, providing a single, unifying framework to explain diverse geological phenomena like earthquakes, volcanism, mountain-building (orogeny), and the distribution of continents and oceans. It successfully replaced the earlier Continental Drift Theory by providing a robust physical mechanism.
  • Historiographical Perspective: The evolution from Wegener’s descriptive model (what is happening) to Hess’s explanatory model (how it happens in oceans) and finally to McKenzie and Morgan’s comprehensive PTT (a global, predictive model) illustrates the progressive nature of scientific inquiry, where theories are built upon, refined, and replaced by more powerful ones based on new evidence.

• Driving Mechanisms of Plate Tectonics (Cause-Effect):

  • Primary Driver: Mantle Convection. The slow circulation of rock in the Earth’s mantle, driven by heat from the core, is the fundamental engine behind plate tectonics. Hot, less-dense material rises, while cooler, denser material sinks.
  • Secondary Forces:
    1. Ridge Push: The elevated position of Mid-Oceanic Ridges creates a gravitational force that pushes the lithospheric plates away from the ridge. (Cause: thermal buoyancy at MOR; Effect: divergent motion).
    2. Slab Pull: As a dense oceanic plate subducts, its weight pulls the rest of the plate along with it. This is now considered the most significant force driving plate motion. (Cause: gravitational sinking of a cold, dense slab; Effect: powerful force for convergence).

• Geopolitical and Economic Significance:

  • The distribution of natural resources is directly linked to plate tectonics. For example, many metallic ore deposits (copper, gold, silver) are found in zones of past or present plate convergence (volcanic arcs). Hydrocarbon deposits are often formed in sedimentary basins created by continental rifting.
  • Understanding plate boundaries is crucial for hazard assessment and disaster management, particularly for seismically active regions like the Himalayan belt (India), Japan, and the Pacific Ring of Fire.

• Indian Subcontinent Context (GS Paper I):

  • Formation of the Himalayas: The Himalayas are a classic example of a continent-continent collision. The northward movement of the Indo-Australian plate and its collision with the stationary Eurasian plate, starting around 50 million years ago, led to the folding and uplift of sediments from the Tethys Sea, forming the world’s highest mountain range.
  • Seismic Activity: This ongoing collision makes the entire Himalayan region and the Indo-Gangetic plain highly seismically active. Understanding this tectonic setting is vital for infrastructure planning and earthquake preparedness in Northern India.
  • Deccan Traps: The formation of the Deccan Traps is linked to the Indian plate’s movement over the Réunion hotspot around 66 million years ago, which caused massive flood basalt eruptions. This demonstrates how plate motion over mantle plumes can lead to large igneous provinces.