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Kosciuszko explained: mystery of the Snowy Mountains solved

They may not be very tall, but they're ours. And it seems we finally know how Kosciuszko and the Snowy Mountains came to be.

For years geologists have had competing, unsatisfactory theories explaining the formation of the Great Dividing Range.

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Mystery of the Snowy Mountains explained

University of Sydney researchers have an explanation for how the Great Dividing Range was formed when Australia's eastern continental edge 'rebounded' 100 million years ago.

A team at the University of Sydney now believes it has solved the mystery of how Australia's long, wide, low range of mountains formed.

Most mountain ranges, such as the Himalayas or European Alps, are formed by tectonic plates colliding, but not the Dividing Range. Until now, no one was really sure what formed them. Unusually high gravity readings in the eastern highlands of Australia supplied a clue.

"The gravity field led us to suspect the region might be pushed up from below so we started looking at the underlying mantle: the layer of rock between the Earth's core and its crust," said Professor Dietmar Muller.

Professor Muller and his team used a geodynamic model never applied to Australia's east coast before. Backed by high-powered software developed at the California Institute of Technology and the University of Sydney, Professor Muller merged models of plate tectonics with convection models of the warm mantle moving underneath the Earth's crust.


They found that two dynamic uplift events caused by convection in the Earth's mantle – one about 100 million years ago and the second 50 million years later – led to the present structure of the Snowy Mountains.

Professor Muller said: "The reason why the gravity field over the eastern highlands is higher than expected is the upward displacement of the lower crustal boundary (the crustal root of the mountains). This moves the dense mantle of the Earth higher up, resulting in an increase in gravity field strength."

Until now the dominant theory explaining the uplift along the Dividing Range is that it was caused by "rebound" to erosion on a previously much larger, very ancient mountain range.

The idea being that as mass is removed from the top of mountains, equilibrium is upset causing upward movement.

However, Professor Muller said: "We can tell that this doesn't work because the gravity fields of the eastern highlands are larger than they ought to be."

So rather than a mountain system in equilibrium, Professor Muller said: "This tells us that the eastern highlands are pushed up from underneath."

Other theories have suggested the Dividing Range was formed by drift over volcanic "hot spots" or the range is the remnant of a rift valley caused by separation with Zealandia to the east.

"The hot spot theory doesn't explain the uplift. It explains the occasional volcanic eruptions [such as at the Warrumbungles, Glasshouse Mountains] but the eastern highlands are far too extensive to have been caused tiny hotspots.

"And the Dividing Range is too wide to be the remnant of a rift valley."

According to Professor Muller's model, as Australia and Antarctica separated 150 million years ago, the eastern edge of our continent was dragged down as it collided with the Pacific plate to the east.

About 100 million years ago, this "subduction" process stopped, causing the eastern edge of Australia to rebound upwards between 400 and 600 metres.

Then 50 million years ago, Australia's drift north accelerated dramatically to about seven centimetres a year. This saw the east of the continent straddle the edge of a Pacific mantle upwelling known as the Pacific Superswell.

This "superswell", which more recently caused volcanic islands such as Tahiti and Fiji to form, led to a second uplift event in the Snowy Mountains of about 700 metres.

"The northern highlands [in present day Queensland] experienced a continuous history of uplift [from 100 million years ago] ... The southern highlands [including the Snowy Mountains] started interacting with the edge of the large pacific upwelling about 40 to 50 million years later, resulting in a two-phase uplift history," the report said.

The findings of Professor Muller's team were published in Earth and Planetary Sciences.

Other geologists unconnected to the research find it convincing, including Professor Richard Arculus, a geologist at the Australian National University.

"We have taken undergraduates on geological field trips to the Snowy Mountains for many years, but never had a really satisfactory explanation for them of how the range formed," Professor Arculus said.

"This new model is a far more definitive explanation for the Great Dividing Range than we have had until now; the model is comprehensive in terms of accounting for the sequence of events that have occurred in the past, and the origins of the forces that have been involved."

Craig O'Neill, a geologist at Macquarie University, said: "The Muller paper makes a pretty convincing case that the difference in timing of the uplift in the north and south of the Eastern Highlands is due mostly to convection currents underneath.

"[This explanation] is more than likely entirely correct. It's a beautiful bit of work."