THE LIE OF THE LAND

Earth's sixth largest country by area at 7,682,300 square kilometres, its low mountain ranges and generally flat topography reflects the age of its landscape and extensive erosion which has occurred over millions of years.

The continent lies at the centre of the large Indo-Australian plate. As mountain ranges are formed by tectonic plate collisions, the absence of plate boundaries within our landmass are the reasons why we have low mountains and infinitesimal earthquake activity.

During it's long geological jouney, Oz has drifted north. 600 million years ago it was part of the Gondawana supercontinent, flanked by India in the west and Antarctica in the south. In Jurassic times (150 million years ago) a rift appeared between Oz and Antarctica.  50 million years later the Tasman sea was formed and 45 million years later, Oz broke away to begin its leisurely journey north. Soon it collided with the Eurasian plate and the Pacific plate resulting in cataclysmic volcanic eruptions that gave birth to the young, high landforms of New Guinea and Indonesia complete with growing mountains and nascent volcanos.

The Oz landmass consists of three main topographical regions. The Western Plateau covering about two-thirds of the land, the Central Lowlands and Eastern Highlands where three-quarters of the population live today. Some of the world's most ancient rocks are found in the Western Plateau, dated at more that 4000 million years old. Our oldest geological formations, the crustal, around which the rest of the continent grew are the Pilbara Block in the northwest and the larger Yilgarn Block in the southwest which was formed about 3500 million years ago. Sediments accumulating over them raised the block above sea level while erosion of these landforms deposited large quantities of iron on the seabed forming thick layers of iron-rich rocks found today in what is the Hamersley Range.

Around 200 million years ago, due to vocanic activity and plate movement, new areas of crust rose above the sea in northern and central Oz. These youngsters, along with the Yilgarn and the Pilbara grew into three large masses in the far west, the north and the south. 900 million years later they bonded to form the West Australian plateau. Soon after, the plateau was covered by two ice ages about 730 million year ago. And, 600 million years later, shock waves created by the formation of Gondwana created a vast mountain range known as the Petermann Mountains. Snow-fed rivers fed sediments down to the plains below to build the inselbergs we know as monoliths today ie: Ayers Rock, Mt Connor, The Olgas and others.

Oz was much wetter during the late Mesozoic and early Cenozoic ages, when (15 million years ago), numerous rivers flowed across the Western plateau. Extensive erosion resulted, leaving chains of salt pans scattered across the land. Some of these ancient rivers formed when Oz was part of Gondwana. Today, their headwaters may lie in Antarctica ...

At the plateau's southern edge lies the featureless and flat Nullabor Plain. Consisting mainly of limestone, it was formed under shallow seas during the early to mid-tertiary period about 60 million years ago. It was then uplifted during the Miocene Epoch, 25 million years ago. Today the Plateau is covered by large deserts that are sand, clays or stones deposited by water or wind erosion. Occasional rainfall has formed floodplains along desert streams but the region has few rivers. Most of them rise at the edge of the plateau and flow to the western and northern coastlines.

The Central Lowlands lie from the Gulf of Carpentaria to the Murray-Darling delta, from the Western Plateau to the Eastern Highlands. The land comprises layers of sediments left over million of years ago by rivers and seas. During the Cretaceous Period (145-165 million years ago), an enormous sea covered much of the northern Central Lowlands. Numerous rivers emptied into this sea which subsequently flowed into the Southern Ocean. The Central Lowlands consist of a narrow belt that abuts the Western Australian plateau along the Darling fault escarpment, an enormous fault line created by gigantic rifts around 200 million years ago.

As the seas retreated and the climate became more arid, rivers dried up to form an inland drainage network that empties into lakes including Lake Eyre. The sedimentary rocks of the Central Lowlands contain the Great Artesian Basin, an enormous aquifer that covers about 22 per cent of the continent. It is the major source of underground water in the continent and is replenished by rivers that flow down from the Eastern Highlands.

The Eastern Highlands, incorrectly known as The Great Dividing Range, extends from northern Queensland to Tasmania. It is not a single mountain range but a series of plateaus, mountains and volcanos. They formed about 540 million years ago when an ocean plate on the eastern side of Oz plunged under the continental plate pushing up volcanic islands as sediments collected along the plate boundaries. These continued to accumulate and build over the next 200 million years (the Permian and Triassic Periods) gradually forming the eastern third of Oz. During this period, the folding of the crust created sedimentary basins where plant-life occurred which then  turned into rich coal deposits slowly ......

Volcanic activity continued. Parts of the highlands consist of volcanic deposits formed as the Indo-Australian plate slowly moved northwards over hot-spots where plumes of magma burst through the crust like a blowtorch. This northward movement is confirmed by the fact that the oldest volcanic rocks lie to the north and the youngest to the south. Those in Mt Nebo in Queensland, for example, are about 32 million years old while those at Mt Gambier are between 1400 and 4700 years old. 

During the late Mezozoic and early Cenozoic eras (about 15 million years ago), parts of eastern Oz were uplifted between 600-3000 metres. Bounded by faults and folds, this area includes The Blue Mountains. Extensive erosion of the Eastern Highlands has exposed huge granite intrusions including Mt Kosciuszko, Mt Buffalo and the granite chain from Armidale to Tenterfield.

Many areas of the Eastern Highlands are separated from the Lowlands by escarpments. Numerous short rivers rise and flow from the highlands ending in large bays. These bays were formed when sea levels rose at the end of the last ice age flooding valleys in the southeast.

The land was still evolving ..... as man arrived just a mere 60,000 years ago.