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Geography and nomenclature The Australian continent is the smallest and lowest-lying of the Earth's continents, having a total land area of some 8,560,000 square kilometres. Though the Commonwealth of Australia occupies much of the continent and is often mistaken for being the entire continent, Australia and adjacent islands are connected by a shallow continental shelf covering some 2,500,000 square kilometres including the Sahul Shelf•• and Bass Strait and half of which is less than 50 metres deep. As Australia the country is largely comprised of a single island, and comprises most of Australia the continent, it is sometimes informally referred to as "the island continent", especially for marketing purposes. • Pedagogic simplification also results in many school children being taught that the continent and the country are synonymous, resulting in the misconception being perpetuated. Geology The continent primarily sits on the Indo-Australian Plate. The lands were joined with Antarctica as part of the southern supercontinent Gondwana until the plate began to drift north about 96 million years ago (mya). For most of the time since then, Australia-New Guinea has remained a single landmass. Biology As the continent drifted north, unique flora and fauna developed. Marsupials and monotremes also existed on other continents, but only in Australia-New Guinea did they out-compete the placental mammals and come to dominate. Bird life also flourished, in particular the ancestors of the great passerine order that would eventually spread to all parts of the globe and account for more than half of all living avian species. Animal groups such as macropods, monotremes, and cassowaries are endemic to Australia. There were three main reasons for the enormous diversity that developed in both plant and animal life. For about 40 million years Australia-New Guinea was almost completely isolated. During this time, the continent experienced numerous changes in climate, but the overall trend was towards greater aridity. When South America eventually separated from Antarctica, the development of the cold Antarctic Circumpolar Current changed weather patterns across the world. For Australia-New Guinea, it brought a marked intensification of the drying trend. The great inland seas and lakes dried out. Much of the long-established broad-leaf deciduous forest began to give way to the distinctive hard-leaved sclerophyllous plants that characterise the modern Australian landscape. For many species, the primary refuge was the relatively cool and well-watered Great Dividing Range. Even today, pockets of remnant vegetation remain in the cool uplands, some species not much changed from the Gondwanan forms of 60 or 90 mya. Eventually, the Australia-New Guinea tectonic plate collided with the Eurasian plate to the north. The collision caused the northern part of the continent to buckle upwards, forming the high and rugged mountains of New Guinea and, by reverse (downwards) buckling, the Torres Strait that now separates the two main landmasses. The collision also pushed up the islands of Wallacea, which served as island 'stepping-stones' that allowed plants from Southeast Asia's rainforests to colonise New Guinea, and some plants from Australia-New Guinea to move into Southeast Asia. The ocean straits between the islands were narrow enough to allow plant dispersal, but served as an effective barrier to exchange of land mammals between Australia-New Guinea and Asia. Although New Guinea is the most northerly part of the continent, and could be expected to be the most tropical in climate, the altitude of the New Guinea highlands is such that a great many animals and plants that were once common across Australia-New Guinea now survive only in the tropical highlands (where they are severely threatened by overpopulation pressures). See also | ||||||||||
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