14.2 Plate Tectonics: Seafloor Spreading, Convection & Plate Boundaries

Plate Tectonics: Seafloor Spreading, Convection & Plate Boundaries

Quick Answer: Plate tectonics is the unifying geological theory stating that Earth's rigid lithosphere is fractured into plates moving across the ductile asthenosphere. Alfred Wegener proposed continental drift (1912) using fossil, rock, and climatic correlations, but his hypothesis was rejected due to lack of a physical mechanism. In the 1960s, the discovery of seafloor spreading and symmetrical paleomagnetic reversal stripes at mid-ocean ridges confirmed plate motion. Plates are propelled primarily by slab pull (gravitational sinking of cold, dense oceanic slabs) alongside ridge push and mantle convection. The three boundary types—divergent, convergent, and transform—produce rift valleys, ocean trenches, volcanic arcs, collision mountains, and strike-slip faults.

Plate tectonics governs the distribution of earthquakes, volcanoes, and mountain belts. Exam questions test your ability to link boundary types with landforms and analyze historical evidence.

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Wegener's Continental Drift Hypothesis

In 1912, German meteorologist Alfred Wegener proposed that 300 to 200 million years ago, all landmasses formed a supercontinent named Pangaea that subsequently rifted apart. Wegener presented four lines of empirical evidence:

1. Continental Jigsaw Fit: Coastlines match closely, notably eastern South America and western Africa.
2. Fossil Correlation: Identical fossils occur on separated landmasses:
   • Mesosaurus: Small freshwater reptile found only in eastern South America and southern Africa.
   • Glossopteris: Heavy-seeded fern found across South America, Africa, India, Australia, and Antarctica.
   • Cynognathus and Lystrosaurus: Land reptiles unable to swim across ocean basins.
3. Matching Rock Sequences: The Appalachian Mountains in North America align in age and stratigraphy with the Caledonian Mountains in the British Isles and Scandinavia.
4. Paleoclimatic Evidence: Late Paleozoic glacial deposits (tillites) and scratches (striations) occur in modern tropical India, Africa, and South America, proving these landmasses once clustered near the South Pole.

Why Wegener Was Rejected

Wegener suggested continents plowed through solid seafloor driven by tidal and rotational forces, which physicists proved were vastly inadequate. Lacking a viable mechanism to propel continents, the scientific community rejected continental drift for decades.

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Seafloor Spreading & Paleomagnetism

In the 1960s, Harry Hess used sonar bathymetry to propose seafloor spreading:

• Magma wells up along mid-ocean ridges, cools into new basaltic crust, and spreads laterally.
• The seafloor recycles into the mantle at subduction trenches.
• Ocean drilling confirmed oceanic crust is youngest at ridges, progressively older farther away, and nowhere older than ~200 million years.

Paleomagnetism & Magnetic Reversals

As basaltic lava cools below the Curie point (~$580^\circ\text{C}$), iron-bearing magnetite crystals align with Earth's magnetic field. Earth's magnetic polarity periodically reverses. Magnetometer surveys revealed alternating normal and reversed polarity stripes forming a symmetrical, mirror-image pattern centered on mid-ocean ridges (Vine-Matthews-Morley hypothesis), verifying seafloor spreading.

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Driving Forces of Plate Motion

Plates are propelled by coupled convective and gravitational mechanisms:

1. Mantle Convection: Heat from core cooling and radioactive decay generates plastic circulation in the asthenosphere.
2. Slab Pull (Primary Driver): As oceanic lithosphere cools and moves away from ridges, it becomes denser than the asthenosphere. At subduction zones, this cold, dense slab sinks under gravity, dragging the plate along. Slab pull provides the vast majority (>90%) of driving force.
3. Ridge Push: Mid-ocean ridges stand 2 to 3 km higher than abyssal plains due to thermal buoyancy. Gravity causes this elevated crust to slide down the asthenospheric slope.

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The Three Major Plate Boundary Types

Lithospheric plates interact along three distinct boundary classes:

Divergent Boundaries

Tensional stress pulls plates apart. At oceanic ridges, decompression melting produces basaltic pillow lavas and hydrothermal vents. On continents (e.g., East African Rift), crust thins into rift valleys, eventually opening new ocean basins.

Convergent Boundaries

• Oceanic-Continental: Dense oceanic lithosphere subducts under buoyant continental crust, carving a trench and melting mantle rock via flux melting to build an inland continental volcanic arc (e.g., the Andes).
• Oceanic-Oceanic: The older, denser oceanic slab subducts, forming a trench and a curved volcanic island arc (e.g., the Aleutians).
• Continental-Continental: Buoyant continental crust ($2.7\text{ g/cm}^3$) resists subduction into dense mantle ($3.3\text{ g/cm}^3$). The collision folds crust into massive mountain ranges like the Himalayas. Volcanism is absent because no slab subducts.

Transform Boundaries

Plates slide horizontally past one another along strike-slip faults. Crust is conserved. Friction locks the fault until stress triggers violent, shallow earthquakes (e.g., California's San Andreas Fault).

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HiSET Exam Traps & Strategic Takeaways

• Trap: Why Wegener Was Rejected: Wegener was rejected due to lack of a driving mechanism, not lack of fossil or rock evidence.
• Trap: Dominant Force: Slab pull is the primary driver of plate tectonics, exerting far more force than convection drag or ridge push.
• Trap: Collision Volcanism: Continental collisions (like the Himalayas) produce towering mountains and earthquakes, but ZERO active subduction volcanism.