14 Feb 2024 : Daily Answer Writing
Q1) Explain the primary lines of evidence supporting the theory of continental drift. How did it contribute to expanding our understanding of Earth’s history. Also discuss the limitations of continental drift theory. (250 Words/15 Marks)
ANSWER
Continental Drift Theory is a theory of geomorphology given by Alfred Wegner in 1920s to explain the formation of continents and their existing shape and form. Wegner opined that initially (up until 200 million years before) all continents formed a single continental mass (PANGAEA) and a mega-ocean surrounded the same (PANTHALASSA).
Primary lines of evidence supporting the theory of continental drift are:
- Geographical similarity seen by the jig-saw fit of the continental boundaries.
E.g., North America-Europe, South America-Africa.
- Geological evidences denote that the Caledonian and Hercynian Mountain systems of the western and eastern coastal areas of the Atlantic are similar and identical.
- Same types of rocks of similar age found on the coasts of Brazil and western Africa, indicating common existence in the geological past.
- 4. Placer deposits of gold are found on coast of Ghana but their source veins lie in Brazilian plateau which indicates continuous landmass at some point of time.
- Common plant and animal species found in different continents.
E.g., skeletons of Mesosaurus are found in southern cape province of South Africa and Iraver formations of Brazil.
- Glacial deposits of tillite have been found in Africa, India, South America, etc. which indicates similar geological history of these landmasses.
Even though partially, continental drift theory expanded our understanding of Earth’s history, as:
- Climatic changes: The theory strives to explain major climatic changes happened in the geological past of Earth.
E.g., Carboniferous glaciation of major parts of the Gondwanaland.
- Mountain building: As per this theory, mountains were formed at the frontal edges of floating and drifting continental blocks due to friction and resistance offered by Sima layer.
- Origin of the Island Arcs: Wegener’s theory attributed formation of Island arcs to the differential rates of continental drift.
E.g., portions of North and South Americas, while they were moving westward, were left behind and the island arcs of West Indies and southern Antilles were formed.
- Basis for future theories: Continental Drift theory cemented the notion that continents are amenable to drifting, which set the scientific foundation for future theories.
E.g., plate tectonic theory.
Figure: Drifting of the continental plates
Despite its pioneering zeal, the continental drift theory has limitations on more than one fronts, as can be seen from:
- The forces applied by Wegener (the force of buoyancy and tidal force of the sun and the moon) are not sufficient to drift the continents apart.
- Contradictory views:
- Wegener has described several contrasting view-points. Initially, sialic masses (continents) were considered by Wegener as freely floating over ‘sima’ without any friction offered by ‘sima’.
- Later, he has described forceful resistance of offered by ‘sima’ in the free movement of sialic continents to explain the origin of mountains along the frontal edges of floating continents.
- Both coasts of the Atlantic Ocean cannot be completely refitted. Thus, the concept of juxtaposition’ or ‘jig-saw fit’ cannot be validated.
- Wegener has not elaborated the direction and chronological sequence of the displacement of the continents. Further, he did not describe the situations of pre- carboniferous times.
- It left many questions unanswered.
E.g., what kept Pangaea together till its disruption in Mesozoic era? Why did the process of continental drift not start before Mesozoic era?
Though most point of Wegener’s theory was rejected but its central theme of horizontal displacement was retained. Moreover, the postulation of plate tectonic theory after 1960 is the result of this continental drift theory of Wegener.
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