What is a tectonic plate?

A tectonic plate is a large segment of Earth's lithosphere (the rigid outer shell of the planet) that moves slowly over the underlying mantle. Earth's surface is divided into about 15 major plates and several smaller microplates, and the slow movement of these plates over millions of years builds mountains, opens oceans, and triggers earthquakes and volcanic eruptions.

Australia sits on the Indo-Australian Plate (sometimes treated as the Australian Plate where it is moving independently of the Indian Plate). The plate covers the Australian continent, the Indian Ocean to the west, the Tasman Sea to the east, and parts of the Indian subcontinent and southern Indian Ocean. It is moving roughly north-north-east at about 7 centimetres a year, faster than any other continental plate, which is why GPS coordinates for Australian features are updated every few decades.

Australia's place in the middle of its plate, far from active boundaries, is the reason it has very few volcanoes, very few major earthquakes, and no fold-mountain ranges of recent age. The Great Dividing Range, the Australian Alps, and even the Flinders Ranges are eroded remnants of much older mountain-building events. The most active boundary affecting Australia lies far to the north along the Banda and Sunda Arcs in Indonesia and Papua New Guinea, where the Indo-Australian Plate dives beneath the Eurasian and Pacific plates, generating volcanoes and large earthquakes that occasionally send tsunamis toward the Australian coast.

The eastern edge of the plate runs through New Zealand, where the Australian Plate meets the Pacific Plate along the Alpine Fault, giving New Zealand its earthquakes and Southern Alps. Some 50 to 100 million years into the future, plate-tectonic models suggest Australia will collide with south-east Asia, fundamentally reshaping the continent's geography.