Geoscience Reference
In-Depth Information
ations in thickness of layers, and, we now know, continuous exchange of material between
layers. Where our planet departs from the perfect onion model is where most off the interest
and excitement in modern geophysics lies, and where we can find the clues to the processes
that drive the system.
4. The main 'onion' layers in a radial section of the Earth.
Lava lamps
Do you remember those lava lamps of the 1960s and numerous later revivals? They make
a good model for the processes at work within the Earth. Whilst they are switched off, a
layer of red gloop sits at the bottom of a layer of transparent oil. But turn on the lamp and
the filament in its base warms the red gloop so it expands, becomes less dense, and begins
to rise in elongated lumps to the top of the oil. When it has cooled sufficiently, it sinks back
down. So it is in the Earth's mantle. Heat from radioactive decay and from the Earth's core
drives a sort of heat engine in which the not-quite-solid rock of the mantle slowly circulates
over billions of years. It is this circulation that drives plate tectonics, causes the continents
to drift, and triggers volcanoes and earthquakes.
