Lecture on Plate Tectonic Theory

Early Theories of Continental Movement

• 16th Century Observations:

  • Abraham Ortelius noted similarities in Atlantic coastlines.
  • Suggested Americas were separated from Europe and Africa by earthquakes and floods.

• 17th-19th Century Theories:

  • Francois Paget: Proposed sinking land created the Atlantic Ocean.
  • Georges-Louis Leclerc: Reintroduced earthquake and flood theory.
  • Antonio Snyder Pellegrini: Shape of continents and fossil evidence suggested a single origin continent including Australia.

Challenges in Explaining Continental Movement

• Lack of understanding of how continents moved thwarted theory acceptance.

Geological Discoveries

• Bathymetric Surveys: Discovered a submarine mountain ridge in the Atlantic.
• Seismic Data: Revealed Earth's layers: core, crust-mantle boundary, lithosphere, asthenosphere.

Alfred Wegener and Continental Drift

• 1912: Wegener hypothesized continental drift.
  • Proposed Earth's rotation or axis change as mechanisms.
  • Suggested the mid-Atlantic ridge opened up for new material.
• Dismissed by contemporaries as improbable.

Developments in the 20th Century

• Radioactivity Discovery (1896): Indicated Earth's interior heat from radioactive decay.

• Arthur Holmes (1927):

  • Suggested mantle convection as the driving power for continental drift.
  • Concept faced skepticism.

• Nuclear Testing in the 1950s:

  • Led to a global seismograph network.
  • Revealed most earthquakes near trenches and mid-ocean ridges.

• Marie Tharp & Bruce Heason (1956):

  • Tharp theorized mid-ocean ridges as extensional rift valleys from plate motion.
  • Heason published her work, initially attributing it to expanding Earth theory.

Acceptance of Plate Tectonics

• Harry Hess (1962):

  • Proposed oceans grew from spreading ridges.
  • Ocean trenches as sites of floor destruction and recycling.

• 1963 Ocean Floor Magnetic Surveys:

  • Found symmetrical magnetic striping on ridges.

• Tuzo Wilson: Proved plates moved over hot spots, connecting ocean ridges with transform faults.

Modern Understanding of Plate Tectonics

• Three Main Plate Boundary Types:

  • Divergent Margins: Plates move apart.
  • Convergent Margins: Plates push together.
  • Transform Margins: Plates move past each other horizontally.

• Forces in Plate Tectonics:

  • Mantle Convection: Plays a role but not sole explanation.
  • Gravity: Integral force, involves ridge push and slab pull.

• Ongoing Research: Continues to explore friction and other forces impacting tectonic movement.

Conclusion

• Plate tectonics is an evolving field with continuous discoveries enhancing our understanding of Earth's geodynamics.