Two processes, continental collision and volcanism, were responsible for most of the world's mountain ranges. Australia's Great Dividing Range, however, happened without either, puzzling geologists as to its source. Finally, they have an answer, one that may also explain highlands elsewhere.
Mountains are not really Australia's thing. The continent's highest peak, Mount Kosciuszko, is just 2,228 meters (7,310 feet) high, and can be walked up without climbing equipment. Nevertheless, the Great Dividing Range is impressive in extent, running 3,500 kilometers (2,175 miles) down the eastern coast. However, neither common mountain-building processes could have built it.
“Continental collision leaves tell-tale signs,” the University of Sydney's Professor Dietmar Müller told IFLScience. Normally you would see deformed crust, but there is no sign of this in eastern Australia. “Volcanism is far too scant. There are scattered extinct volcanoes, but not nearly enough to explain the uplift of the entire highlands.”
One clue lies in the area's strong gravitational field. Anyone walking up Mount Kosciuszko wouldn't notice it, but they need to work a fraction harder than someone climbing a similarly sized peak elsewhere.
“If you lift up the crust, the dense mantle comes closer to the surface, and the excess density results in a strengthened gravitational field,” Müller said. The effect increases the strength of gravity by a few thousandths of a percent, but that was enough to alert Müller's team.
On any other continent, a mountain not much more than two kilometers (1.2 miles) high wouldn't deserve a monument, but this is as high as Australia gets. Peter Zaharov
In Earth and Planetary Science Letters, Müller explained that the mountains are the product of two successive rounds of uplift. From 120 to 80 million years ago, East Gondwana began to break up and its continental slab sank. "Eastern Australia was drifting over a subducted plate graveyard, giving it a sinking feeling," co-author Dr. Kara Matthews of Oxford University said in a statement. "But around 100 million years ago subduction came to a halt, resulting in the entire region being uplifted, forming the Eastern Highlands."
Müller explained that sinking as a result of subduction is followed by a rebound. However, this effect only raised the area by 400-600 meters (1,300-1,900 feet). The northern highlands continued to rise, albeit slowly, while the southern end didn't change much, aside from some erosion.
"Then, about 50 million years ago Australia's separation from Antarctica accelerated and it started moving north-northeast, gradually taking it closer to a vast mantle upwelling called the South Pacific Superswell," said co-author Dr. Nicolas Flament. "This provided a second upward push to the Eastern Highlands as they gradually rode over the edge of the superswell."
The team's modeling has been confirmed by a study of river systems in the Snowy Mountains at the southern end of the Great Dividing Range, which show signs of having cut through the mountains in a two-stage process. Erosion records from offshore basins also support the theory.
The paper refers to highlands in Brazil and southern and eastern Africa as possibly having been lifted up by similar processes.