Geoscience Reference
In-Depth Information
Chapter 5
Drifting continents
As a child I used to enjoy helping my mother to make marmalade. I confess that I still like
making it occasionally myself. But now, when I stare into the preserving pan of simmer-
ing fruit and sugar, I can't help imagining that I am seeing our planet's evolution, greatly
speeded up with one second perhaps representing ten or even a hundred million years. When
the jam is gently simmering on a slow heat, convection cells establish, with columns of hot
marmalade rising to the surface and spreading across it. With them comes some scum, a fine
sugary foam, which is not dense enough to sink back down but collects in rafts on the calmer
areas of the surface. This foam is a bit like the Earth's continents. It starts to form quite early
on in the process and slowly builds up and thickens. Occasionally, the convection pattern
beneath changes and the scum splits apart. Sometimes the rafts of scum run together and pile
up even thicker. Of course, we shouldn't take this analogy too far. The timescales and the
chemistry are altogether different; by and large geologists don't find sugar crystals in granite
or orange peel xenoliths in basalt. But it's an image worth holding in mind as we consider
the scum of the Earth: the continents.
