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A geophysicist in the US has created a computer model that appears to
answer one of the fundamental questions of plate tectonics: what forces
caused Pangaea, the Earth’s original single supercontinent, to break up
into today’s continents.

Pangaea began to split apart roughly 180 million years ago. ‘Neither
the existence of Pangaea nor the fact that our continental plates are moving
has been in dispute for more than 20 years,’ says John Baumgardner of Los
Alamos National Laboratory in New Mexico. ‘But this is the first three-dimensional
model that begins from a small set of basic principles and accounts for
the main features of this geophysical drama.’

The basic energy source driving large-scale geological changes is heat:
the immense reservoir of thermal energy in the Earth’s interior, augmented
by the decay of radioactive elements, induces convection currents in the
mantle-the 3000-kilometre-thick layer of silicate rock between the core
and the crust.

As well as modelling movements across the surface, Baumgardner’s model
also includes vertical movements within the mantle. ‘Pangaea was surrounded
by ocean floor that was colder, and thus denser, than the rock below it,
so it had a natural tendency to sink,’ he says.

The model starts with this ocean floor subducting, or sinking along
the margins of Pangaea. The drag created by this pulls the adjacent, more
buoyant continental plates outwards from the body of Pangaea so they override
the adjacent ocean floor. This mechanism is still at work, which is why
North and South America continue to drift westward, toward subduction zones
and away from Africa and Europe.

Baumgardner has been refining his model since 1981 and its complexity
requires the power of a Cray Y-MP supercomputer. The present version of
the model divides the earth’s mantle into some 1.3 million volume elements,
each about 100 kilometres across. The conditions in each cell, such as temperature,
density and pressure, determine the forces that deform the material in it
and these forces combine to move large volumes of material.

But the oceanic plates that are sinking are less than 80 kilometres
thick, so the present model can not resolve them. ‘We are now in the process
of implementing the model on the new generation of massively parallel supercomputers,’
says Baumgardner. These speed up calculations by dividing work between
many processors. ‘These new machines will allow us to investigate features
in a global model that have length scales on the order of 25 kilometres,’
he says.