Geoscience Reference
In-Depth Information
spanning many palaeomagnetic reversals. Instead, he discovered that they were all magnet-
ized in the same direction, suggesting that they must have erupted within one short period
of at most a million years. Keith Cox at Oxford University and Dan McKenzie at Cam-
bridge worked out what must have happened. India was once a larger continent, and as it
drifted north, it passed over a mantle plume at just the time it was producing a huge pulse
of magma. This caused the continent to dome up. But the Deccan Traps are just on the
east side of this dome. Rivers could not drain to the west since that lay uphill. Unimagin-
able volcanic eruptions produced several million cubic kilometres of basalts in the space of
thousands of years. Eventually, the activity split the continent in two. The Indian subcontin-
ent we know today is just the northeastern portion of that. The rest lies under the sea in the
huge basalt bank between the Seychelles and the Comoros. It turns out that this volcanic
outpouring occurred around 65 million years ago, around the time of the Cretaceous/Ter-
tiary boundary, and the extinction of many animal groups including the dinosaurs. Maybe
it was not an asteroid after all that killed them but the pollution and climate change that
resulted from these incredible volcanic eruptions.
Meanwhile, what remained of the subcontinent continued northwards, closing the great
oceanic gulf of the Tethys and eventually ramming into Asia. Whilst the ocean lithosphere
of the Tethys was dense and could subduct back into the mantle beneath Asia, the contin-
ental crust could not. The two continents first came into contact about 55 million years ago,
but a continent has such momentum that nothing is going to stop it dead in its tracks. The
closing speed was about 10 centimetres per year. It slowed to around 5 centimetres per year
but has continued colliding ever since, like a vehicle crash test played in very slow motion.
During this time, the Indian subcontinent has moved a further 2,000 kilometres north. The
first thing to happen was a pile-up of sediments and a thickening of the crust in a series of
under-thrusts as slabs of Indian continent wedged beneath Asia, like the debris in front of a
bulldozer. This thick continental material gave rise to the high Himalayas.
