A chance discussion at a recent Perth conference between geologists from around the world has led to a new theory being posited - and published in an article in the prestigious journal Science on January 2, 2009.
Professor Peter Cawood, acting head of The University of Western Australia's School of Earth and Environment, and one of the article's main authors, said he and his fellow conference attendees were talking at the Australian Earth Science Convention about when and at what rate did the Earth's continental crust form.
Many have argued that the continents have developed during rapid periods of growth related to continental amalgamation separated by relative quiescence. However, Professor Cawood and co-workers argue that these collision events represent periods of preservation, not generation, and that throughout much of Earth's 4.5 billion year history the formation of the Earth's crust was relatively continuous.
Professor Cawood, President of the Geological Society of Australia, said the Earth was unique in the solar system in having a bimodal distribution of crust - oceanic crust and continental crust. This, along with the Earth's atmosphere and hydrosphere, developed through continental drift. He said changes were still occurring - in 75 million years Australia was likely to be surrounded not by ocean, but by desert. It would be part of China and much closer to North America.
Professor Cawood said while people were innately curious in the Earth and its formation, the issue of climate change had brought their interest to the fore.
"The rate of climate change is now so much faster than we can observe in the rock record," he said. "There have been extremes in the past - once the Earth was a ‘snowball' with solid ice at the equator - but at this rate we can't support our current lifestyle, already causing mass extinctions of flora and fauna. The Earth will survive climate change - but humanity may not."
Professor Cawood said WA was a good place to be a geologist. The Murchison preserved the oldest fragments of the Earth's history - 4.4 billion year-old zircon crystals whose isotopic composition proved that even so long ago there was water, perhaps in shallow oceans. And in the Pilbara were 3.6 billion year-old stromatolites, or algal forms, that were the oldest confirmed record of life on Earth.
"The Earth is unique in the solar system in that its surface has a bimodal height distribution, with an average 125 m above sea-level and 4 km below," Professor Cawood said.
"The repository of the history of the Earth is in the 4.5 billion year-old buoyant continental crust, most of which is produced by plate tectonics. The oceanic crust is a relatively young 200 million years old. When people plot the history of our planet they get pulses of activity because they are seeing the rate of collisions but not, as we suggest in our article, the times of crust generation."
Professor Cawood's co-authors are Professor Chris Hawkesworth, Dr Craig Storey and Dr Bruno Dhuime from the University of Bristol, and Dr Tony Kemp from James Cook University, Townsville. Their article is entitled "A Matter of Preservation".