Plate Tectonics, Driving Forces, Exam Logistics, and Course Review

Midterm Examination Structure and Logistics


Empirical Evidence for Plate Tectonics

Plate tectonic theory is supported by foundational lines of evidence inherited from earlier continental drift hypotheses, in addition to modern oceanic and geophysical observations.

Continental Drift Corroboration

  • Continuous Geological Structures: Identical, connected mountain ranges terminate at continental margins and align continuously when continental landmasses are reassembled.

  • Fossil Distribution: Identical fossil flora and fauna occur across widely separated geographical landmasses currently divided by extensive ocean basins and arid zones.

Seafloor and Geophysical Evidence

  • Spatial Distribution of Earthquakes: Seismic activity is not randomly dispersed across the globe; earthquakes consistently cluster along narrow, discrete belts corresponding directly to active tectonic plate boundaries.

  • Age Distribution of Oceanic Crust: Drilling and dating of oceanic crust demonstrates that rock age increases progressively with distance away from spreading centers. The youngest crust is situated precisely along the crest of mid-ocean ridges, while the oldest crust is situated furthest away near continental margins.

  • Symmetrical Marine Magnetic Anomalies: Paleomagnetic polarity reversals create symmetrical, striped magnetic anomaly patterns parallel to both sides of mid-ocean spreading ridges. This alternating magnetic striping can only occur if newly formed molten crust solidifies at the ridge axis, records Earth's prevailing magnetic field, and subsequently moves symmetrically outward away from the ridge crest over geological time. If horizontal spreading did not occur, erupting magma would simply stack vertically upon itself, as observed in stationary subaerial volcanoes, failing to produce horizontal symmetrical polarity stripes.

Dynamics and Driving Forces of Plate Motion

Lithospheric plate movement is driven by three primary mechanical forces occurring within the mantle and lithosphere.

Mantle Convection Currents

Historically conceptualized as the primary mechanism of plate transport. Thermally driven convection currents within the ductile asthenosphere act as a conveyor belt or treadmill, exerting basal shear traction on the overlying lithosphere and transporting plates across the surface.

Slab Pull

Slab pull constitutes the single strongest mechanical driving force in plate tectonics. Global kinematic data shows that the plates exhibiting the highest surface velocities—specifically the Pacific Plate and the Nazca Plate (transcribed as the NASCAR Plate)—are bounded by extensive subduction zones.

  • Mechanical Analogy: Sinking anchor.

  • Operational Mechanism: As oceanic lithosphere cools, ages, and becomes denser than the underlying asthenosphere, its leading edge subducts into the mantle. The negative buoyancy of the cold, dense oceanic slab generates intense downward gravitational pull. This sinking gravitational force drags the trailing oceanic plate behind it into the subduction trench while simultaneously generating tensile stress that pulls apart the mid-ocean ridge to which the plate is attached.

Ridge Push

Ridge push is a buoyant, gravitationally directed force operating at divergent boundaries.

  • Operational Mechanism: High thermal gradients at mid-ocean ridges cause buoyant, elevated topography. As new magma ascends and cools, it moves away from the ridge axis and undergoes densification. The cooling, densifying lithosphere produces an outward, down-slope sliding force directed away from the elevated ridge axis toward adjacent ocean basins.

Fluvial Geomorphology

River Channel Patterns and Categorization

  • Braided Rivers: Fluvial networks characterized by multiple, highly dynamic, branching and anastomosing channels separated by temporary bars or islands. These systems form in environments with high sediment supply, variable discharge rates, and erodible banks.

  • Exemplary Systems: The Delaware River exemplifies a premier North American river system.

Questions & Discussion

Flat Earth Claims and Sociological Discourse.

  • Documentary Film Behind the Curve: Focuses on Flat Earth proponents attempting to empirically substantiate their models. In the documentary's culminating laser experiment, the researchers' own empirical measurements disprove their planar hypothesis right before the closing credits. Proponents rationalize contradictory planetary and lunar observations by asserting that extraterrestrial photographic imagery exhibits static or atmospheric bubbles.