Thermal Convection Current Theory by Sir Arthur Holmes

Introduction

• Lecturer: Dr. Krishnan
• Topic: Geography and environment
• Focus: Thermal convection current theory
• Relevance: Critical for understanding the internal structure of Earth and mountain building.

Historical Background

• Before 1928-29: Various scholars proposed different theories
  • Theories of Isostasy
  • Airy and Pratt
  • Tetrahedral Hypothesis (Lithium Greens)
  • Cycle of Erosion (Davis)
  • Continental Drift Theory (Taylor and Wegener)
• Wegener's Theory: Criticized due to weak explanations (Tidal Forces, Pole Fleeing Force)
• Need for a better theory: Convection Current Theory filled the knowledge void.

Thermal Convection Current Theory

• Proponent: Sir Arthur Holmes (1928-29)
• Importance: First to grasp mantle convection
• Key Idea: Thermal convection currents in Earth's mantle drive mountain building.
• Origin of Convection Currents: Excessive heat in the asthenosphere
• Earth's Layers according to Holmes:
  1. Granodiorite Layer (Upper Layer): 10-12 km
  2. Amphibolite Layer (Intermediate Layer): 20-25 km
  3. Eclogite Layer (Lower Layer)

Characterization of Layers

• Granodiorite vs. Granite: Potassium feldspar and dark minerals
  1. Granodiorite: More plagioclase feldspar, darker minerals
  2. Granite: Potassium feldspar, lighter minerals
• Amphibolite: Formed at convergent plate boundaries due to regional metamorphism
• Eclogite: Formed under high pressure, key in driving mantle convection

Crust and Mantle Zones

• Crustal Zone: Upper and intermediate layers
• Mantle Zone: Lower layer
• Impact on Geomorphology: Divergent and convergent convection currents responsible for landforms like mountains, trenches, and volcanic ridges.

Mechanism of Convection Currents

• Two Factors: Thickness of crust and distribution of radioactive elements
• Equatorial vs. Polar Currents:
  • Ascending Currents (Equator): Greater thickness, upward motion
  • Descending Currents (Poles): Shallower depth, downward motion
• Examples of Geomorphological Impact:
  1. Rising currents cause stretching and thinning
  2. Converging currents cause subsidence and geosynclines

Validation and Further Theories

• 1960s Validation: Harry Hess's theory of seafloor spreading
• Sea-floor Spreading: Magma upwelling forms ridges and trenches
• Connection to Plate Tectonics: Convection currents explained tectonic activities

Conclusion

• Significance: Thermal convection current theory paved the way for modern understanding of Earth's internal dynamics and surface features.
• Further Studies: Upcoming sessions will cover more grand theories in geomorphology

Additional Notes

• Recommended Videos: Internal structure, isostasy
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Study Tip: Review diagrams and videos mentioned for better understanding of complex processes.