The theory of continental drift is most associated with the scientist Alfred Wegener. In the early 20th century, Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes plowing through oceans and into each other.
Tectonics Plates
The existence of distinct relief features on the ocean floor, such as ridges, trenches, seamounts, shoals, etc., was discovered during World War II.
Trenches and ridges provided information on the natural boundaries between different lithospheric plates (tectonic plates). These significant discoveries gave rise to the geological field of tectonics.
The science of tectonics examines the forces (mantle convection currents) and mechanisms (collisions of lithospheric plates, folding, faulting, and volcanism) that shape the Earth’s crust and affect how it has changed through time.
The main focus is on comprehending the forces behind the lithosphere’s large-scale deformation (the crust and upper mantle above the asthenosphere).
It covers mountain-building-related folding and faulting, large-scale, progressive upward and downward crustal movements (epeirogenic movements), the development and behaviour of ancient continent cores known as cratons, and abrupt horizontal displacements along faults.
Important concepts of tectonic processes
The system of ideas behind plate tectonics theory suggests that Earth’s outer shell (lithosphere) is divided into several plates that glide over the Earth’s rocky inner layer above the softcore (mantle). The plates act like a hard and rigid shells compared to Earth’s mantle.
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.
Continental Drift Theory (Alfred Wegener, 1922)
In the 1920s, Alfred Wegener proposed the continental Drift Theory. The continent of Pangaea, which was encircled by the vast ocean Panthalassa, was one large landmass, according to the Continental Drift Theory. Pangaea was split into two enormous land masses by a sea called Tethys: Laurentia (Laurasia) to the north and Gondwanaland to the south. The continents started to separate and drift apart some 200 million years ago (Mesozoic Era, Triassic Period, Late Triassic Epoch).
Forces behind the drifting of continents, according to Wegener
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 (the earth rotates from west to east, so tidal currents act from east to west, according to Wegener).
- Wegener proposed that tidal force—the moon’s and, to a lesser extent, the sun’s gravitational pull—also played a significant effect.
- The earth’s rotation is related to the polar-fleeing force. Earth has a bulge at the equator and is not a perfect sphere. The earth’s rotation is to blame for this bulge (greater centrifugal force at the equator).
- As we travel from the poles to the equator, centrifugal force increases. According to Wegener, this rise in centrifugal force has caused poles to escape.
- Oceanic tides are caused by the moon and sun’s attraction, which is known as tidal force (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 in support of Continental Drift Theory
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 the 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 the USA seem to be one continuous series.
Criticism
- 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.
Criticism
- 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).
Criticism
- Poles may have shifted, but not necessarily the continents.
Botanical Evidence
- The presence of glossopteris vegetation in Carboniferous rocks of India, Australia, South Africa, Falkland Islands (Overseas territory of the UK), Antarctica, etc. (all split from the same landmass called Gondwana) can be explained by the fact that parts were linked in the past.
Criticism
- 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 the Same Age Across the Oceans
- The belt of ancient rocks of 2,000 million years from Brazil’s coast matches with those from western Africa.
Criticism
- Rocks of the same age and similar characteristics are found in other parts of the world too.
Tillite deposits
- Tillite deposits are sedimentary rocks formed out of deposits of glaciers.
- The Gondwana system of sediments is 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.
Placer Deposits
- 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.
Drawbacks of Continental Drift Theory
- Wegener failed to explain why the drift began only in the 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 for 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.
.
Read More Articles on Geography
You Can Follow on Youtube – Score Better
Join Us on Telegram For More Update
.