JOIN USCan you even imagine that all continents on Earth were joined together, at one point in time? C Sivaram explains the formation of present day land masses and the physical phenomena behind them
Just 100 years ago, in 1915, German geologist and meteorologist Alfred Lothar Wegener postulated and developed the Theory of Continental Drift, also publishing a book on the origin of continents and oceans in the same year.
That the Earth’s huge land masses constituting the continents which appear rigidly fixed on the globe could actually be shifting around constantly (sometimes bumping into each other) appeared unacceptable to the majority of geologists then. The theory began gaining acceptance 50 years later, especially after the development of plate tectonics and related ideas like the sea floor spreading hypothesis of Harry Hess.
Now, if one studies the positions and shapes of the large continents on the globe, it appears quite evident that if North and South America could be pushed towards western and southern Europe and western Africa, they could all fit together like pieces in a jigsaw puzzle.
This observation was made many years before Wegener used this kind of evidence (such as the fit of eastern coast of South America into western coast of Africa as well as similarities in the distribution of rock types, geological structures, flora and fauna etc.) to suggest that the present distribution of the continents resulted from the breaking up of one or two greater land masses.
This implied that the earth’s continents once formed a single mass dubbed Pangaea more than 200 million years ago. This supercontinent later broke up into Laurasia (the northern land mass) and Gondwanaland (in the south). Gradually, the continents arranged themselves into the present pattern that exists.
When Wegener published his theory of continental drift 100 years ago, it caused major controversies. However, the evidence is now in his favour and there is little doubt that the continents have drifted widely and continue to do so. Indeed, 100 million years ago, India along with Australia and Antarctica, was part of Gondwanaland and close to the South Pole.
Even 60 million years ago, India was fully below the equator and has been drifting northwards. In 10 to 20 million years from now, the world as we know (at least in the geographical sense) may be beyond our recognition. Even high precision measurements from LAGEOS (laser geodynamic satellites) have established beyond doubt that continents are drifting constantly at rates of 10-20 cm a year.
But what causes the continental drift? The cause is just under our feet. The earth’s crust floats on the liquid rocky mass below it. Like logs of wood floating on water, the continents of the earth’s crust are made mostly of relatively lighter material like granite float on the denser molten rock constituting the mantle below it. The ocean floors which are also part of the Earth’s crust float on the molten mantle along with the continents. Thus, continents of solid rock float on a sea of molten magma, sinking a little into the liquid.
What lies beneath
The earth is covered by a relatively thin crust (like the skin of an apple surrounding the fruit). The top crust is not a complete single shell, but a mosaic of several rigid segments called plates. These tectonic plates form the continents and oceans. The tectonic plates do not just float, they also move around. The earth’s mantle extends to about 2000 km. The temperatures at its base may be several thousand degrees, causing molten rock to rise to the surface.
While rising, they slowly cool and sink back into the depths, giving rise to a giant conveyor belt of molten circulating matter supplying titanic forces to move the continents. Speeds at which the plates move vary with time and plate. Typically, plates can move at speeds of 2-10 cm a year.
Tectonic plates carry the continents and oceans like gigantic rafts floating on the denser mantle. The asthenosphere plates with an average thickness of a 100 km float on the asthenosphere, moving relative to one another at several centimetres a year. The study of plate tectonics involves the motion of lithospheric plates which have moved throughout geological time resulting in the present positions of the continents.
Plate activity
There are presumed to be six major plates associated with the continents, i.e. Eurasian, American, African, Pacific, Indian and Antarctic. Plate margins coincide with regions of seismic and volcanic activities. Most of the earthquakes are caused by movements between tectonic plates. As the plates move, the compacted rocks grind against each other, causing pressures to build up, which, if exceeds the strength of the rock, can cause their rupture.
The Earth’s crust then shifts suddenly, making the ground shake, while seismic waves spreading from the rupture can cause damage even far away. Areas where the edges of tectonic plates meet are at maximum risk from earth quakes because of friction from adjoining plates, (like the San Andreas Fault). Builders along such fault lines are expected to follow strict codes (in US, Japan etc.). Detailed maps of such edges have been prepared thus far.
Again, in Wegener’s theory, constant changes on earth’s surface, like the formation of mountain ranges arise mainly due to the constant shifting of the continents. It was earlier thought that cooling and contraction of the earth over millions of years caused the topography changes. In the language of plate tectonics, for instance, the Himalayas (including Mt. Everest) formed when the Indian plate collided with the Eurasian plate,
30 million years ago.
Living proof
The fact that sea shells and molluscs are present in these mountain tops shows that they were once part of the ocean. The Himalayas, a comparatively younger mountain range, is indeed getting taller as the Indian plate continues to push northwards (this has been measured from satellites). The spreading of the sea floor is again due to plates drifting apart.
Thus, large scale topographical changes on the earth’s surface (like formation of mountain ranges, sinking of large land masses) are all explained mainly by plate tectonics.
Why is the earth’s interior so hot, making the mantle a viscous liquid and the outer core a dense metallic liquid? The answer lies in the fact that radioactive isotopes in the earth’s interior release around forty terawatts of energy annually. Again, the earth is perhaps unique in having plate tectonics.
None of the other terrestrial planets (Mars, Venus and Mercury) have it. This again is attributed to our vast reservoirs of water, enabling the oceanic plates to be sub-ducted below the continental plates. It has also been debated whether the Earth always had plate tectonics and how crucial it was for the evolution of life.
In short, it has been 100 years since the revolutionary idea that all large scale changes on the face of the earth, like rise of mountains, islands, earth quakes, titanic volcanic eruptions etc. are caused by vast continental masses floating like ice floats on a denser molten rock below, colliding with each other literally causing titanic changes.
Wegener’s picture is now the standard model in geology, joining the other three standard models, i.e. the DNA biosynthesis, big bang cosmology and unification of fundamental forces in particle physics.