Young at heart
DON’T BELIEVE EVERYTHING you hear about Australia. She is talked about as a timeless ancient land, but many of her surfaces are new.
Kakadu National Park has a sandstone plateau more than a thousand million years old, but the famous wetlands below it, crowded with water lilies and magpie geese, go back only 4000 years. In a geological sense, they appeared yesterday.
At that time, South Australia’s Mt Gambier was a live volcano. In western Victoria, several volcanoes have erupted between 30,000 and 40,000 years ago*, producing new cones and flows of lava across the land. Some must have been witnessed by Aboriginal people, whose descendants, not surprisingly, have legends about them. More eruptions are expected in the near future.
The Great Barrier Reef is less than 9000 years old. It began forming after the last ice age ended when seas began rising to the levels they are today.
Australia has a plant older than the reef. King’s holly (Lomatia tasmanica), a suckering shrub in Tasmania, comprises a single clone believed to be more than 40,000 years old. A plunket mallee (Eucalyptus curtisii) growing in southern Queensland is estimated to be 4000 to 9000 years old, and Western Australia has a eucalypt thought to date back more than 6000 years.
The icea ages account for most of the youthful landscapes. During the last glacial peak 25,000 years ago, seas dropped 120m, joining mainland Australia to Tasmania and New Guinea. Only a handful of Australia’s islands remained islands at this time. Had Melbourne existed back then it would have been an inland town, far removed from the sea.
Tasmania became an island again about 14,000 years ago, and New Guinea well after that. Almost all low-lying coastal landscapes, including beaches, mangrove swamps and shallow bays, have formed recently as sea levels stabilised and sediments accumulated.
The world has been through many glacial cycles, so although Kakadu’s wetlands and the Great Barrier Reef are very young, there were older wetlands and reefs in the same places at warm times in the past. Indeed, the Great Barrier Reef began growing on an older reef platform.
Despite her many youthful landscapes – which include the Simpson Desert (one million years old) – Australia deserves to be called an ancient land. Western Australia boasts the Yilgarn Craton, one of the oldest areas of exposed land on Earth. The Kimberley is also very ancient. Zircon found east of Shark Bay was dated as 4400 million years old which, at the time it was announced, was the oldest rock found anywhere in the world.
Tim Low speaks to ABC Radio Darwin 1057 about Kakadu’s youthful side.
*This number has been edited from the original, which said “over the past 30,000 years” after new information came to light.