Geoscience Reference
In-Depth Information
Scum of the Earth
Continental crust is very different from the crust that floors the oceans. Ocean crust is pre-
dominantly magnesium silicate, whereas the continents contain higher proportions of alu-
minium silicates. They also contain less iron than the denser material of the mantle or of
the ocean floor. As a result, they float, albeit on the semi-solid mantle rather than in liquid.
And they can be thick. The ocean crust is a fairly uniform 7 kilometres thick, but the con-
tinents can range from 30 to 60 kilometres or more. And, like the ocean lithosphere, they
are under-plated by a thick layer of cold, hard mantle. Just how deep the roots of continents
do go is still a subject of controversy that, in the end, probably comes down to definitions.
But continents are also a bit like icebergs: there's a lot more below ground than we can see
above. And the higher they rise in mountain ranges, by and large the deeper they go be-
neath.
Drifting continents
With the benefit of hindsight, the knowledge of mantle convection, and the evidence of sea
floor spreading, it is very easy to see that the continents have moved over geological time
relative to one another. But it was not always so convincing. In spite of James Hutton's
ideas about mountain-building and the rocks cycle, it was a long time before any mechan-
ism could be suggested. Between 1910 and 1915, the American glaciologist Frank Taylor
and the German meteorologist Alfred Wegener proposed the hypothesis of continental drift.
Yet no one could imagine a way in which the continents could drift like ships at sea through
the seemingly solid, rocky mantle. For nearly half a century, supporters of continental drift
were in the minority. But the theory's few supporters were working hard. Alex du Toit in
South Africa was building up evidence of similar rock structures between southern Africa
and South America, while Arthur Holmes, a British geophysicist, proposed mantle con-
vection as a mechanism for the drift. It was not until the 1960s, when the oceanographers
got to work, that the debate was settled. Harry Hess proposed that convection beneath the
ocean crust might cause the sea floor to spread out from mid-ocean ridges, and Fred Vine
and Drum Matthews provided the magnetic evidence of sea floor spreading. It was papers
by Tuzo Wilson in Canada, Jason Morgan at Princeton, and Dan McKenzie at Cambridge
that brought the evidence together into the theory of plate tectonics.
Plate tectonics explains the surface of the Earth in terms of the motions of a small number
of rigid plates which move relative to one another, interacting and deforming along their
boundaries. It is not that the continents are drifting free but that they are carried on plates
which extend far deeper to include the mantle lithosphere, typically 100 kilometres thick.
The plates are not restricted to the continents, but include the slabs of ocean floor as well.
