Geological Heritage
Nature & People
Events over hundreds of millions of years have created the landscapes of the Great Eastern Ranges. The driver of this change is plate tectonics. The Earth's surface floats above a molten sea of dense magma. Convection currents in the magma and a sinking ocean floor push continents together and rip them apart. The Great Eastern Ranges have undergone periods of mountain building. Seas have risen and inundated parts of the land to create the great layers of sedimentary rocks that form the Sydney Basin and other sedimetary landscapes.
145 Million Years Ago
Australia was once part of Gondwana, the supercontinent. Around 145Â million years ago Australia and Antarctica were separated by the movement of great geological plates driven by immense forces deep within the Earth's molten core. This created the Southern Ocean about 100Â million years ago. Australia is still moving north at a rate of 5Â cm/year.
80 Million Years Ago
The Tasman Sea was opened only 80 million years ago, when eastern Australia was separated from the now-submerged Lord Howe Rise. The buoyant land mass of eastern Australia rose at a rate of 0.01 mm per year. This does not sound like much, but over 80 million years it is a rise of 800 m. This process is thought to be responsible for the most recent uplift of the Great Eastern Ranges. There was some volcanic activity when the Tasman Sea opened. Sites in the Great Eastern Ranges where the legacy of this activity can be see are at Mount Dromedary near the historic town of Tilba on the far south coast, and on the Monaro Range, near Cooma.
20 Million Years Ago
Volcanic activity was occurring from 20 million to 4,000 years ago at a number of sites within the Great Eastern Ranges. Examples are the caldera at Mount Warning and the younger volcanic rocks of the Monaro Volcanic Province. This activity is thought to be the result of a 'hot spot', an area where lava is forced to the surface from deep in the Earth. Australia was moving north at a rate of 6.5 cm/year, but the magma source remained stationary beneath the moving crust, so a plume kept feeding new volcanoes, each further south than the last.
What Goes Up Must Come Down!
In the Border Ranges erosion has dissected and removed up to 90% of the volcanic caldera around Mount Warning. This has created creeks and rivers that transport volcanic material to the sea. Erosion has created intricate drainage patterns and carved deep gorges that have in some places isolated plateaux – such as those in the Greater Blue Mountains and Morton National Park – and is responsible for the dramatic cliffs and waterfalls of the Great Escarpment. Plate tectonics and erosion continue to shape the landscapes of the Great Eastern Ranges.
