Geoscience Reference
In-Depth Information
ude. Other clues come from tiny grains of a mineral called zircon. These carry within them
products of radioactive decay that date the periods of tectonic activity when they formed.
For the Alexander Terrane, they reveal two major mountain-building episodes, 520 and 430
million years ago. Eastern Australia was the site of mountain-building at both these times,
whilst North America was quiet. Conversely, western North America was active 350 milli-
on years ago, a time when the Alexander Terrane seems to have been dormant. The Alex-
ander Terrane began to split from Australia 375 million years ago and formed a submarine
ocean plateau, at which time various marine animals were fossilized there. About 225 mil-
lion years ago, the Terrane began moving northward at 10 centimetres a year. This contin-
ued for 135 million years, whereupon North American fossils start to appear as the Terrane
reached its present latitude and collided with Alaska. It is even possible that it brushed past
the coast of California on its way, scraping off material from the California Mother Lode
gold belt. If that is correct, the Alaskan gold rush may have been to the same rocks as the
Californian gold rush, but displaced 2,400 kilometres to the north.
Continental pile-up
We have heard about several different types of plate boundary. There are the spreading
centres of mid-ocean ridges, the transform faults perpendicular to the ridges, and the sub-
duction zones where ocean lithosphere takes a dive under a continent. All are comparat-
ively narrow, well-defined zones that are relatively easy to understand and explain with
simple diagrams. But there is one type of plate boundary that is more complex and where
the idea of the tectonics of rigid plates breaks down: intercontinental collisions. Where
ocean crust is involved it is comparatively easy. As long as it is cold, it will be dense
enough to sink down into the mantle at a comparatively steep angle of about 45 degrees.
Continental crust won't sink, just as a cork floating in the sea stays afloat in spite of all
waves breaking over it. Continental crust also deforms more easily than ocean lithosphere.
So, when continents collide, it is more like a serious traffic accident.
A good example is how India joined Asia. For hundreds of millions of years, India had
been one of the partners in a complex country dance across the southern hemisphere fea-
turing Africa, Australia, and Antarctica. Then, about 180 million years ago, it broke away
and began to drift northwards. There are some spectacular mountain ranges down the west-
ern side of India, the Western Ghats and Deccan Traps. A strange feature of them is that,
although sea is quite close to the west, the major rivers that drain these ranges flow to the
east. Another puzzle came when Professor Vincent Courtilliot of the University of Paris
started to look at palaeomagnetism in the basalt rocks of which the hills are made. He had
been working in the Himalayas and wanted some comparisons from further south. He ex-
pected to find millions of years' worth of palaeomagnetic data in the thick basalt layers,
