Geoscience Reference
In-Depth Information
to it. When it has cooled, you push a finger into it. If it is still liquid, there's nothing for it
but to lick your finger and let the marmalade continue to simmer. But after a while, as the
brew approaches its setting point, a sample on the plate will crinkle up as you push your
finger into it like a miniature continental collision. And it's not a bad model for the way
continents behave on large scales. Compressed to fantastic pressures by overlying rocks,
and possibly heated from beneath, rocks subjected to the lateral force of a colliding contin-
ent will tend to fold rather than to fracture. And the incredible masses of rock involved will
be strongly affected by gravity so that the steepest folds will sag under their own weight
into over-folds, rather as the skin on custard, or indeed marmalade, would do.
The Earth is not flat
Another reason why flat continents cut from an atlas do not fit together very well is that
they are supposed to represent plates on the surface of a sphere. They get distorted in the
map projection. But it is not easy to slide rigid plates about on the surface of a sphere
either. You cannot simply move them in straight lines because there are no straight lines
on a sphere. Each motion is in effect a rotation about an axis cutting through the sphere.
But there are still difficulties. One is finding a frame of reference among all the jostling
plates. Another is accommodating different rates of sea floor spreading. A simple model
might invoke an axis similar to the Earth's rotation axis for the opening of the Atlantic and
the relative motions of the Americas away from Africa and Europe. But that would require
creating Atlantic Ocean crust like a segment of orange skin, wide at the equator and nar-
rowing smoothly towards the poles. The rate of sea floor spreading does vary, but not in
a convenient way like that. The result is transform faults; breaks in the crust thousands of
kilometres long, offsetting segments of the mid-ocean ridge.
Frames of reference
With the evidence of sea floor spreading and the mechanism of mantle convection, plate
tectonics rapidly became established at the centre of modern Earth sciences. But even today
there are geologists who object to the term 'continental drift' because of its associations
with the time when the mechanism had not been properly explained and few believed it.
But, once people were prepared to accept it, evidence for past plate motions became ob-
vious. There was the geological evidence of rocks of the same type split apart and now
lying on opposite sides of an ocean. There was evidence from living and fossil remains of
the times different populations became isolated from one another or when they were able
to cross between continents. For example, it is just over 200 million years since Australia
parted company from parts of Asia such as Malaysia and Indochina. Since then, mammals
