Holy Careening Continents, Batman!

Reconstruction of Gondwana in the Permian, showing the modern thicknesses of continental lithosphere. From Kumar et al., 2007.

A very cool paper in today's Nature (link below) deals with the question of India's movement after it broke away from Gondwana about 140 million years ago. Plate tectonics has a bit of a tendency to leave me gasping for breath, because the forces involved are just so incredible. Great chunks of the planet's surface get ripped up by colliding masses of rock bigger than all imagining, at scales at which living organisms just become negligible.

Most of you are probably aware that the Earth's crust is divided into large chunks called plates. The plates are moving in all different directions - magma wells up from the more fluid mantle under the crust on one side of the plate, cools down to form crustal rock which is carried with the movement of the plate to the other side, where things can suddenly get interesting.

Crustal rock falls into two categories. Oceanic crust is heavier, while continental crust is more buoyant (obviously, 'buoyant' is a relative term when you're dealing with rock). When a travelling piece of oceanic crust runs out of plate, it sinks down beneath the opposing plate, and is remelted back into the mantle. Continental crust, however, doesn't do this - it's too light to sink and so it simply bobs in place, like a cork in water. When two pieces of continental crust on opposing plates run into each other, neither is able to go beneath the other, and an almighty crash occurs. The force of these rockmasses pushing into each other pushes the rock at their junction upwards, and many of the world's mountain ranges represent present or past collisions of continental crust. The Himalayas and their backing ranges are where India is currently ploughing into Asia, while the Alps were formed as Italy (with the weight of Africa behind it) pushed up against the rest of Europe.

Eventually, the continental collision runs out of steam, and the plate boundary becomes fused to form a supercontinent. This isn't necessarily the end of the story, however. Supercontinents have formed many times over the Earth's history, only to fracture at a later date and scatter pieces of continental crust back in different directions again. The mechanisms for this are unclear, but the leading idea is that heat building up underneath the continental mass rises up as a mantle plume, ripping apart the overlying continent. The Great Rift valley in eastern Africa represents a current plate fracture in action. The supercontinent of Gondwana contained the present-day landmasses of Africa, South America, Antarctica, India and Australia. before its division over the course of the Mesozoic.

For the most part, this was a fairly sedentary process. Africa and Australia drifted off at about 2 to 4 cm per year. Antarctica seems to have been fairly happy where it was, and hardly moved at all. India, however, was a different matter - it couldn't wait to escape the family home, and tore off at incredible speeds of up to 20 cm a year. Its eventually collision with Asia about 50 million years ago slowed it down somewhat, but it still travels at 5 cm per year, faster than all the other Gondwanan fragments.



Topographical view of India and the Tibetan Plateau (from 100gogo), showing the line of India caning into Asia.

How was India able to achieve this massive speed? Today's paper by Kumar et al. explains that. Using seismic readings, they measured the thickness of the continental crust in the Gondwanan fragments. The deepest part of Africa has a continental thickness of more than 250 km. Antarctica and Australia have thicknesses of about 150 km. India, in contrast, has a thickness in places of less than 100 km. It seems that it is the absence of that deep keel that has allowed India its free movement.

The presence of deposits such as diamond-bearing kimberlites that form at very deep temperatures in India indicate that its crust wasn't always so thin. Kumar et al. suggest that the mantle plume that caused the breakup of Gondwana may have melted away the deeper parts of India before flinging it northwards like a dart into Asia's underbelly.

REFERENCES

Kumar, P., X. Yuan, M. R. Kumar, R. Kind, X. Li & R. K. Chadha. 2007. The rapid drift of the Indian tectonic plate. Nature 449: 894-897.