Continental Drift Theory

  • B. Taylor (1908) first proposed this theory to reject the Contraction theory of tertiary fold mountains.
  • The Concept was elaborated by Prof. Alfred Wegener of Germany (1912).
  • As per the continental drift hypothesis, the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed.

Objective

  • To explain the global climate change.

 

Thinkers Views

Alfred Wegener:

  • Wegener proposed the Continental Drift Theory in the early 20th century.
  • He believed that continents were once part of a supercontinent called Pangaea, which later drifted apart.
  • Wegener's evidence included the fit of continents like South America and Africa, similar rock formations across continents, and fossil similarities.

Arthur Holmes:

  • Holmes supported Wegener's theory with his idea of convection currents in the mantle.
  • He suggested that these currents could cause the movement of continents, providing a mechanism for continental drift.
  • Holmes' insights contributed to the development of plate tectonics theory.

Marie Tharp:

  • Tharp's mapping of the ocean floor revealed mid-ocean ridges and deep-sea trenches.
  • Her work provided crucial evidence for seafloor spreading, a process linked to continental drift.

John Tuzo Wilson:

  • Wilson proposed the concept of transform faults, where tectonic plates slide past each other horizontally.
  • His work integrated continental drift with seafloor spreading into the theory of plate tectonics, explaining the movement of continents and ocean basins.

Harry Hammond Hess:

  • Hess introduced the idea of seafloor spreading as a mechanism for continental drift.
  • He suggested that new oceanic crust forms at mid-ocean ridges, pushing older crust away, which supports the movement of continents.

Basic Premise

1. Sial and Sima [Edward Suess]

2. United landmass ‘Pangaea’, Primitive Ocean ‘Panthalasa’ in mid-Mesozoic era.

  • All the continents formed a single continental mass- “Pangaea”. It means all earth.
  • This supercontinent was surrounded by a mega ocean called “Panthalassa”. It means all water.

3. Forces responsible for drift

  • Equatorward – gravitational forces, pole-fleeing force, and buoyancy forces.
  • Westward –The continental drift was westward due to tidal currents due to the earth’s rotation.

Note: The ‘pole-fleeing force’ is caused by an increase in centrifugal force from the poles towards the equator.

The Theory: Mechanism

  1. Drifting: Continents are floating and constantly drifting on the earth’s surface.
  • Carboniferous period: breaking of Pangaea into Laurasia and Gondwana.
    • “Flight from the pole”.
    • Tethys Sea was formed.
  • Cretaceous period: Disruption of Gondwana (Deccan Trap)
  1. Mountain building
  • Folding of frontal edge of drifting continents due to oceanic rock resistance.
  • g. Western Cordilleras by Westward drift.

  1. Island arcs
  • Trailing edge of drifting continent failed to keep pace with it.
  • Hence, islands and festoons of South east Asia and West Indies were formed.

  1. Carboniferous glaciation
  • Around south pole (Natal in South Africa).

Evidence in Support

  1. Jig Saw Fit:
  • Juxta Position of South America and Africa: When facing each other, the shorelines of South America and Africa form a similarity.
  • Similarly, when matched, Africa, Madagascar, and India’s east coast fit together.

  1. Geological structure (similarity of age of rocks):
  • Appalachian Mountains of North America and Hercynian Mountains of Europe.
  • Placer gold (Brazil and Nigeria);
  • There is a remarkable similarity in geological structure along the coasts of South America & West Africa.

  • Tillite: It is a sedimentary rock formed by the deposition of glaciers. The sediments from India are matched with Africa, Falklands Island, Madagascar, Antarctica, and Australia.
  1. Paleontological evidence:
  • Distribution of Fossils: Identical species and animals.
  • Mesosaurus, a freshwater crocodile-like reptile: It is only found in Southern Africa and Eastern South America.
  • Cretaceous period fossils and Glossopteris flora in Gondwanaland.
  1. Biological evidence:
  • The migratory pattern of some species hints towards the joined landmass.
  • g. Westward running tendency of Scandinavian Lemmings (mass suicide due to falls in the Atlantic).

  1. Paleoclimatic evidence: Glacial-Carboniferous.
  • Although coal deposits are formed in tropical regions, those are found in temperate and polar regions.

Criticism

  • Although now accepted, the theory of continental drift was rejected for many years. The evidence in its favor were considered insufficient.
  • One problem was that a plausible driving force was missing.
  • A second problem was that Wegener's estimate of the speed of continental motion, 250 cm/year, was very high. The currently accepted rate for the separation of the Americas from Europe and Africa is about 2.5 cm/year.
  • Furthermore, Wegener was treated less seriously because he was not a geologist.

The main criticisms are:

  • Not perfect jig saw fit.
  • The hypothesis was rejected for lack of any motive mechanism. However, the English geologist Arthur Holmes later proposed mantle convection for that mechanism.
  • The forces not enough to drift continent.
  • Directional chronological sequence of drift not explained.
  • Contrasting viewpoints on mountain building.
  • No explanation of intra-continent mountain.
  • Wrong postulation of Sial and Sima.
  • No account for Pre-Carboniferous era.

Evaluation

  • Wegener departed from the orthodox geological ideas like teleological theory.
  • Although the theory was mostly rejected, the central theme of horizontal displacement was retained. It became the basis of plate tectonic theory.
  • The idea of continental drift has been subsumed into the science of plate tectonics.