Geology and geography

Situated in the geographical region of Oceania, Australia is the smallest continent in land area.

The continent includes a continental shelf overlain by shallow seas which divide it into several landmasses—the Arafura Sea and Torres Strait between mainland Australia and New Guinea, and Bass Strait between mainland Australia and Tasmania. When sea levels were lower during the Pleistocene ice age, including the Last Glacial Maximum about 18,000 BC, they were connected by dry land. During the past 18,000 to 10,000 years, rising sea levels overflowed the lowlands and separated the continent into today's low-lying arid to semi-arid mainland and the two mountainous islands of New Guinea and Tasmania.

With a total land area of 8.56 million square kilometres (3,310,000 sq mi), the Australian continent is the smallest, and second-lowest human inhabited (after Antarctica) continent on Earth. The continental shelf connecting the islands, half of which is less than 50 metres (160 ft) deep, covers some 2.5 million square kilometres (970,000 sq mi), including the Sahul Shelf and Bass Strait.

As the country of Australia is mostly on a single landmass, and comprises most of the continent, it is sometimes informally referred to as an island continent, surrounded by oceans.

Geological forces such as tectonic uplift of mountain ranges or clashes between tectonic plates occurred mainly in Australia's early history, when it was still a part of Gondwana. Australia is situated in the middle of the tectonic plate, and therefore currently has no active volcanism.

The continent primarily sits on the Indo-Australian Plate. Because of its central location on its tectonic plate Australia doesn't have any active volcanic regions, the only continent with this distinction. 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. For most of the time since then, Australia–New Guinea remained a continuous landmass. When the last glacial period ended in about 10,000 BC, rising sea levels formed Bass Strait, separating Tasmania from the mainland. Then between about 8,000 and 6,500 BC, the lowlands in the north were flooded by the sea, separating New Guinea, the Aru Islands, and the Australian mainland.

A northern arc consisting of the New Guinea Highlands, the Raja Ampat Islands, and Halmahera was uplifted by the northward migration of Australia and subduction of the Pacific Plate. The Outer Banda Arc was accreted along the northwestern edge the continent; it includes the islands of Timor, Tanimbar, and Seram. Papua New Guinea has several volcanoes, as it is situated along the Pacific Ring of Fire. Volcanic eruptions are not rare, and the area is prone to earthquakes and tsunamis because of this. Mount Wilhelm in Papua New Guinea is the second highest mountain in the continent, and at 4,884 metres (16,024 ft) above sea level, Puncak Jaya is the highest mountain.

The Australian continent, being part of the Indo-Australian Plate (more specifically, the Australian Plate), is the lowest, flattest, and oldest landmass on Earth and it has had a relatively stable geological history. New Zealand is not part of the continent of Australia, but of the separate, submerged continent of Zealandia.

New Zealand and Australia are both part of the Oceanian sub-region known as Australasia, with New Guinea being in Melanesia.

Papua New Guinea, a country within the continent, is one of the most culturally and linguistically diverse countries in the world. It is also one of the most rural, as only 18 percent of its people live in urban centres. West Papua, a province of Indonesia, is home to an estimated 44 uncontacted tribal groups. Australia, the largest landmass in the continent, is highly urbanised, and has the world's 14th-largest economy with the second-highest human development index globally. Australia also has the world's 9th largest immigrant population.