D.7] Important Earth Surface Theories

1. Continental Drift Theory (Alfred Wegener)

1.1 Introduction

• Proposed by: Alfred Wegener (German meteorologist and geophysicist)
• Year Proposed: 1912
• Main Idea: Continents were once joined together in a single landmass called Pangaea and have since drifted apart.

1.2 Key Evidence

• Fossil Evidence: Similar fossils of species like Glossopteris and Lystrosaurus found on continents now separated by oceans.
• Rock Formations: Identical rock layers and mountain ranges (e.g., Appalachian Mountains in North America and Caledonian Mountains in Scotland and Scandinavia).
• Climatic Evidence: Evidence of ancient glacial deposits found in regions that are now tropical (e.g., India and Africa).
• Continental Fit: The coastlines of South America and Africa fit together like puzzle pieces.

1.3 Criticisms

• Lack of Mechanism: Wegener could not explain the mechanism of how continents moved.
• Insufficient Data: Insufficient evidence to support the movement of massive landmasses.

1.4 Key Terms

• Pangaea: A hypothetical supercontinent that existed during the late Paleozoic and early Mesozoic eras.
• Continental Drift: The hypothesis that continents move relative to each other through time.

1.5 Important Facts for Exams

• SSC/RRB Common Questions:
  • Who proposed the theory of continental drift?
  • What is the name of the supercontinent?
  • What are the key evidences supporting continental drift?
• Example: The fit of South America and Africa is a classic example of continental drift.

2. Plate Tectonic Theory (Modern Theory)

2.1 Introduction

• Proposed by: Developed by Harry Hess and Robert Dietz in the 1960s, building on the work of Alfred Wegener.
• Year of Formalization: 1960s
• Main Idea: The Earth’s lithosphere is divided into plates that move relative to each other due to convection currents in the asthenosphere.

2.2 Key Concepts

• Lithosphere: The rigid outer layer of the Earth, composed of the crust and uppermost mantle.
• Asthenosphere: The plastic layer beneath the lithosphere, where convection currents occur.
• Plate Boundaries: Three types:
  • Divergent Boundaries: Plates move apart (e.g., Mid-Atlantic Ridge).
  • Convergent Boundaries: Plates move towards each other (e.g., Himalayas from India and Eurasia).
  • Transform Boundaries: Plates slide past each other (e.g., San Andreas Fault).

2.3 Mechanism of Plate Movement

• Convection Currents: Heat from the Earth’s interior causes the movement of material in the mantle, driving plate tectonics.
• Mantle Plumes: Hot upwellings from the mantle that can cause volcanic activity and the formation of new crust.

2.4 Key Evidence

• Seafloor Spreading: Discovery of magnetic striping on the ocean floor, supporting the idea of new crust forming at mid-ocean ridges.
• Earthquake Distribution: Earthquakes are concentrated along plate boundaries.
• Volcanic Activity: Volcanoes are commonly found at plate boundaries.

2.5 Key Terms

• Seafloor Spreading: The process by which new oceanic crust is formed at mid-ocean ridges.
• Subduction Zone: A convergent boundary where one plate is forced beneath another.
• Hot Spot: A region of the Earth’s crust where magma rises to the surface, forming volcanic islands or mountains (e.g., Hawaiian Islands).

2.6 Important Facts for Exams

• SSC/RRB Common Questions:
  • What is the modern theory of earth’s surface movement?
  • What are the three types of plate boundaries?
  • What is seafloor spreading?
  • What is the role of convection currents in plate tectonics?
• Example: The Mid-Atlantic Ridge is an example of a divergent boundary, while the Himalayas are formed at a convergent boundary.

3. Comparison of Continental Drift and Plate Tectonic Theories

4. Summary Table