Continental Drift Theory

Q.Critically evaluate the continental drift hypothesis of A.Wegner Q.Bring out distinctions between the continental drift theory and the plate tectonic theory. Q.Discuss the limitations of the theory of continental drift and show how the theory of plate tectonics is an improvement over it.

This theory marks departure in previously hold notion that continents are static and mountains are wrinkles in the crust.

                Though concept of drift of continents was given earlier too, but the full fledged theory which stirred the imagination of scholars worldwide is attributed to Alfred Wegner of germany who gave his theory in 1912.

ASSUMPTIONS

1. Earth interior is made of 3 layers : sial,sima and nife (Edward Suess Scheme)

2. Continental crust is made of sial and is less denser than sima which lies just below it. Continental crust is floating over sima.oceanic crust is upper layer of sima.
3. During carboniferous period, all the landmass was united into single landmass called Pangea and was surrounded by a single water body called panthalassa.
4. Pangea was moving north-westward.
5. Northward movement was due to buoyancy and gravitational force acting on the ellipsoidal pangea was not along the same line and didn’t cancel out.Their resultant was towards north.
6. Westward movement of continent was due to tidal pull of sun and moon.

THEORY

1. Flight from the Poles: carboniferous period.
2. Opening of tethys: Angarland and Gondwanaland
3. Formation of continents and oceans
4. Formation of mountains: Rockies,Andes,Alpine ranges of europe
5. Formation of island arcs:  Japan,Phillippines,west Indies,etc.

EVIDENCES

1. Jig-Saw fit: Various Coasts surrounding oceans,especially Atlantic ocean, appears to fit together as if they were once united and later separated. For instance,african west coast and south american east coast.

2. Geological evidence: Various mountain ranges and geological structures witness continuity on different continents separated by ocean, For example, appalachian mountains in Northeastern part of america and mountain systems of ireland,wales and north western europe are similar.

Du Toit after analysis of eastern coast of south america and western coast of africa confirmed that they are geologically similar.

3. Carboniferous Period Glaciation: Evidence of carboniferous glaciation of south africa, falkland,brazil,peninsular india,australia,antarctica,etc indicates they were part of same landmass during carboniferous period.
4. Fossils: Fossils and Vegetation remains on eastern coast of south america and western coast of africa found out to be similar.
5. Flora: Presence of glossopteris in india, australia,south africa,falkland,antarctica,etc indicates they were once united as a single landmass
6. Lemmings behaviour: Lemmings(found in scandinavia) appear to commit suicide by jumping of the norwegian coast is explained as continuity of migration behaviour towards landmass west of norway which was once connected and now has drifted apart.

CRITICISM

• Plate drifting: Later it was found it is not continent which are drifting but it is plate consisting of both continental and oceanic crust are moving.
• Forces:Tidal force is too weak force for the westward drift of continent.If tidal force is as large as assumed by Wegner,it has potential to gradually stop rotation of earth.

Similarly differential buoyancy and gravitational force is not enough for northward movement. If force was so large it would have ultimately  resulted in accumulation of all the land mass near equator.

• Contradictory views: It was initially assumed sial is moving over sima without friction but later mountains like andes and rockies were said to formed by the friction offered by sima layer on sial layer.
• Complete sequence of events: Alfred Wegener didn’t provide what occurred before pangea and events before carboniferous period.
• Too much simplification:Painting whole canvas with single stroke of brush

APPRAISAL

• Departure from thermal contraction theory
• Blow to static continent notion.
• Triggered many research and theories and stirred the imagination of scholars.