Australia Has Its Ups And Downs

While plate tectonics - the theory of continental drift - has been known for decades, a team of Australian and American scientists has added a revolutionary new insight into the flexing of the planet's outer crust.

A 3D computer model developed by Dr Louis Moresi of CSIRO Exploration & Mining's Solid Mechanics Research Group, Dr Michael Gurnis of Caltech and Dr Dietmar Müller of the University of Sydney, has revealed the ponderous upheaval of eastern Australia starting 100 million years ago which turned what was once a great inland sea into dry land.

Their breakthrough has resolved a long-standing puzzle for geologists - why large parts of Australia were flooded when global sea levels were low and why, when sea levels rose again, Australia was high and dry.

"The answer is that we are literally living on the scum of the earth," Dr Moresi explains."The continents are composed of a thin crust of the lightest rocky material, and they both drift and bob up-and-down in response to the stirrings of enormous convective currents driven by radioactive decay in the Earth's mantle below them."

Despite its huge surface area, Australia's crust is only 20-50 kilometres deep. This is supported on a layer of relatively cool mantle a couple of hundred kilometres thick.

"Amazingly, parts of Western Australia have remained continually afloat for some 2-3 billion years, because they are lighter still - being formed largely from the original cold mantle material from that early time," he says.

Geologists have long understood that where two plates collide, one dives beneath the other, causing outbursts of volcanic activity and the rise of mountain ranges like the Himalayas or Andes. But for a long time it was not clear exactly what drove the process.

"When faced with a puzzle like this, the first thing an earth scientist might do is seek another example and try and work out what causes any differences between the two cases, Dr Moresi explains.

"Since there is only one Earth, we had to compare our surface with what is happening on the other rocky planets - Mars, Venus and Mercury. But as they are not perfect analogies for our planet, we also decided to build a computer model to show what we think is going on inside the Earth, and how it affects the surface."

The outcome is a spectacular full-colour movie, spanning more than 100 million years and showing - in just a few minutes - how Australia breaks free of Antarctica to race north, and how its inundated eastern half bobs vertically upward, draining the Jurassic seas from its surface.

At the same time the model accurately predicts how a deep pucker, caused by enigmatic forces inside the planet, developed in the Southern Ocean seabed between Australia and Antarctica.

The team's dynamic model reveals that the sinking of eastern Australia was caused by a second tectonic plate being drawn beneath the Australian plate, where it stagnated, sucking the landmass above it vertically downward some 350 metres and allowing the seas to flood lowlying parts of Queensland, NSW, Victoria and central Australia.

As the continent drifted north it gradually escaped from the influence of the underlying plate, and surged back up again, shrugging off the shallow seas that had covered it. The layer of new sediment they left behind made the land surface higher still, and remained dry when global sea levels rose once more.

The team's research was published in a recent edition of the international journal Science and has been acclaimed a world breakthrough in understanding and predicting the timing of vast geological events.

"This work opens up a huge new frontier in which the motions of plates can be predicted from computer models of the Earth's internal heat engine, in much the same way as scientists use similar models to study climate change," Caltech's Dr Gurnis says.

The new science of modelling plate tectonics provides the missing link between planetary science - the understanding of how planets form and evolve - and geology, which looks at processes on a more local scale.

It is expected to have particular application in the hunt for oil and minerals and in reconstructing vast events in Earth's history, Dr Moresi says.

More information:
Dr Louis Moresi, CSIRO Exploration & Mining
louis.moresi@dem.csiro.au Vision opportunity:

Animated computer graphic showing drift and rise of Australian continent between 125 million years ago and the present day. View on the world wide web at: http://www.ned.dem.csiro.au/research/solidMech/Citcom/DEMO/(scroll to the bottom of the pag