- During WW II, it was discovered that the ocean floor had some unique relief features like ridges, trenches, seamounts, shoals etc.
- Ridges and trenches gave insights into natural boundaries between various lithospheric plates (tectonic plates). These important discoveries led to the field of tectonics in geology.
- Tectonics is the scientific study of forces (convection currents in the mantle) and processes (collisions of the lithospheric plates, folding, faulting, volcanism) that control the structure of the Earth’s crust and its evolution through time.
- It is basically about understanding the large-scale deformation of the lithosphere (crust and upper mantle above asthenosphere) and the forces that produce such deformation.
- It deals with the folding and faulting associated with mountain building; the large-scale, gradual upward and downward movements of the crust (epeirogenic movements); the growth and behaviour of old cores of continents known as cratons; and sudden horizontal displacements along faults.
Continental Drift Theory (CDT)
- Continental drift refers to the movement of the continents relative to each other.
Polar wandering (similar to Continental Drift Theory)
- Polar wandering is the relative movement of the earth’s crust and upper mantle with respect to the rotational poles of the earth.
Seafloor Spreading Theory (SFST)
- Seafloor spreading describes the movement of oceanic plates relative to one another.
Plate Tectonics (PT)
- Plate tectonics is the movement of lithospheric plates relative to each other.
Convection Current Theory (CCT)
- Convection current theory forms the basis of SFST and PT. It explains the force behind plate movements.
Plate Tectonics – Breakup of Pangea
- Alfred Wegener suggested continental Drift Theory in the 1920’s.
- According to Continental Drift Theory there existed one big landmass which he called Pangaea which was covered by one big ocean called Panthalassa.
- A sea called Tethys divided the Pangaea into two huge landmasses: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys.
- Drift started around 200 million years ago (Mesozoic Era, Triassic Period, Late Triassic Epoch), and the continents began to break up and drift away from one another.
Watch the video for quick and better understanding
- According to Wegener, the drift was in two directions:
- equator wards due to the interaction of forces of gravity, pole-fleeing force (due to centrifugal force caused by earth’s rotation) and buoyancy (ship floats in water due to buoyant force offered by water), and
- westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener).
- Wegener suggested that tidal force (gravitational pull of the moon and to a lesser extent, the sun) also played a major role.
- The polar-fleeing force relates to the rotation of the earth. Earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth (greater centrifugal force at the equator).
- Centrifugal force increases as we move from poles towards the equator. This increase in centrifugal force has led to pole fleeing, according to Wegener.
- Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters (tides explained in detail in oceanography).
- According to Wegener, these forces would become effective when applied over many million years, and the drift is continuing.
Evidence for CDT
Apparent Affinity of Physical Features
Apparent Affinity of Physical Features
- The bulge of Brazil (South America) seems to fit into the Gulf of Guinea (Africa).
- Greenland seems to fit in well with Ellesmere and Baffin islands of Canada.
- The west coast of India, Madagascar and Africa seem to have been joined.
- North and South America on one side and Africa and Europe on the other fit along the mid-Atlantic ridge.
- The Caledonian and Hercynian mountains of Europe and the Appalachians of USA seem to be one continuous series.
Continuous Very Old Fold Mountain Chain
- Coastlines are a temporary feature and are liable to change.
- Several other combinations of fitting in of unrelated landforms could be attempted.
- Continental Drift Theory shifts India’s position too much to the south, distorting its relationship with the Mediterranean Sea and the Alps.
- The mountains do not always exhibit geological affinity.
Causes of Drift
- The gravity of the earth, the buoyancy of the seas and the tidal currents were given as the main factors causing the drift, by Wegener.
- This is illogical because for these factors to be able to cause a drift of such a magnitude, they will have to be millions of times stronger.
Polar wandering (Shifting of Poles)
- The position of the poles constantly drifted (due to plate tectonics).
- Poles may have shifted, not necessarily the continents.
Distribution of Fossils across the Gondwanaland
- Presence of glossopteris vegetation in Carboniferous rocks of India, Australia, South Africa, Falkland Islands (Overseas territory of UK), Antarctica, etc. (all split from the same landmass called Gondwana) can be explained from the fact that parts were linked in the past.
- Similar vegetation is found in unrelated parts of the world like Afghanistan, Iran and Siberia.
Distribution of Fossils
- The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
- Mesosaurus was a small reptile adapted to shallow brackish water. The skeletons of these are found only in South Africa and Brazil. The two localities presently are 4,800 km apart with an ocean in between them.
Rocks of Same Age Across the Oceans
- The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa.
- Rocks of the same age and similar characteristics are found in other parts of the world too.
- Tillite deposits are sedimentary rocks formed out of deposits of glaciers.
- The Gondwana system of sediments are found in India, Africa, Falkland Island, Madagascar, Antarctica and Australia (all were previously part of Gondwana).
- Overall resemblance demonstrates that these landmasses had remarkably similar histories.
- Rich placer deposits of gold are found on the Ghana coast (West Africa) but the source (gold-bearing veins) are in Brazil, and it is obvious that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
- Wegener failed to explain why the drift began only in Mesozoic era and not before.
- The theory doesn’t consider oceans.
- Proofs heavily depend on assumptions that are generalistic.
- Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents.
- Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift.
Though scientifically unsound on various grounds, Wegener’s theory is a significant milestone in the study of tectonics, and it laid a strong foundation for future the theories like seafloor spreading and plate tectonics.