What do you understand by continental drift ? Discuss various geological evidences in favour of continental drift.

Understanding Continental Drift

Continental drift describes the movement of Earth's continents relative to each other, leading to their changing positions over geological timescales. This movement is not merely a superficial displacement but involves the entire lithospheric plates upon which the continents rest. Wegener's theory fundamentally altered scientific understanding of Earth's dynamic nature, proposing that the planet's surface is in constant, albeit slow, motion.

Geological Evidences in Favour of Continental Drift

Alfred Wegener presented a compelling array of evidence from various geological disciplines to support his hypothesis. These evidences, while initially insufficient to convince the scientific community about the driving force, were remarkably accurate in pointing towards past continental configurations.

1. The Jigsaw Fit of Continents

• Observation: The most striking and earliest observation was the remarkable "jigsaw fit" of the continental coastlines, particularly evident between South America and Africa. Abraham Ortelius first noted this in 1596, but Wegener emphasized it as a primary line of evidence.
• Implication: This congruence suggests that these landmasses were once connected and have since pulled apart. When considering the continental shelf margins, the fit becomes even more precise, indicating a shared boundary before separation.

2. Paleontological Evidence (Fossil Distribution)

• Observation: Identical species of fossil plants and animals are found on continents now widely separated by vast oceans. These organisms, often freshwater or land-dwelling, could not have traversed such large bodies of saltwater.
• Specific Examples:
  • Mesosaurus: Fossils of this freshwater reptile are found only in Early Permian rocks of Brazil and South Africa. Its inability to survive in saltwater strongly suggests these continents were once joined.
  • Lystrosaurus: This land-dwelling reptile's fossils are present in rocks of the same age in Africa, India, and Antarctica, indicating a continuous landmass.
  • Cynognathus: Another land reptile, its fossils are found in South Africa and South America.
  • Glossopteris Flora: Fossils of this ancient seed fern are widely distributed across Permian-age rocks in Australia, India, South Africa, South America, and Antarctica. The heavy seeds of this plant make wind or water dispersal across oceans highly improbable.
• Implication: The distribution of these specific fossils across currently disparate continents provides strong evidence for their past connection within the supercontinent Gondwana.

3. Geological Similarities and Rock Formations

• Observation: Striking similarities in rock types, geological structures, and mountain ranges are observed on continents that are now far apart.
• Specific Examples:
  • Appalachian Mountains and Caledonian Orogeny: The Appalachian mountain ranges in eastern North America show remarkable geological continuity with mountain belts found in eastern Greenland, Ireland, Great Britain, and Scandinavia. These ancient mountain chains, formed during the Caledonian Orogeny, align perfectly when the continents are reassembled.
  • Ancient Cratons: Precambrian rocks of similar age and composition are found in Brazil and West Africa, suggesting they were once part of a single ancient craton that subsequently rifted apart. The rocks on both sides of the Atlantic Ocean, particularly between South America and Africa, are virtually identical in type, structure, and age.
• Implication: These geological correlations imply that these landmasses experienced the same tectonic and deformational events, which would only be possible if they were once contiguous.

4. Glacial Evidence (Paleoclimatic Evidence)

• Observation: Evidence of widespread glaciation from the Late Paleozoic (around 300 to 250 million years ago) is found in regions that are currently tropical or subtropical, including parts of South America, Africa, India, Australia, and Antarctica. These glacial deposits, known as tillites, and glacial striations indicate the direction of ice flow.
• Implication: If the continents were in their current positions, a massive glaciation event covering nearly the entire globe, even extending across the equator, would be required. However, there is no corresponding evidence of such extensive glaciation in the Northern Hemisphere during this period; instead, coal seams in cold regions like Antarctica suggest a previously tropical climate. Wegener proposed that these continents were once clustered around the South Pole in Gondwana, allowing for a single, extensive ice sheet whose remnants are now scattered across different continents due to drift.

5. Paleomagnetic Evidence (Post-Wegener Development)

While not part of Wegener's initial evidence, subsequent paleomagnetic studies provided strong support for continental drift and formed a crucial link to plate tectonics.

• Observation: Studies of remanent magnetism in ancient rocks on different continents revealed "polar wandering curves" that differed for each continent. This indicated that the magnetic poles appeared to have moved over time.
• Implication: A more plausible explanation than actual polar wandering was that the continents themselves had moved relative to a relatively stable magnetic pole. The alignment of magnetic minerals within ancient rocks indicated their position relative to the magnetic pole at the time of their formation, and discrepancies between continents could only be reconciled by their relative movement.

The cumulative weight of these geological evidences, particularly the congruence of coastlines, fossil distribution, and matching rock formations, formed a powerful argument for continental drift. Although Wegener could not fully explain the mechanism, his meticulous compilation of data laid the undeniable foundation for the later development and acceptance of the theory of plate tectonics, which provided the missing driving force of mantle convection.