Where it is and what it carries
The Australian Plate covers about 46 million square kilometers, or 9% of Earth’s surface. It carries all of Australia and New Caledonia, almost all of Fiji, about 38% of New Zealand, about a third of Papua New Guinea and 8% of Indonesia. Most of its area is ocean floor: the eastern Indian Ocean, the Tasman Sea and the Coral Sea.
Many maps still show a single Indo-Australian Plate. The PB2002 model splits it in two, because the Indian Plate and the Australian Plate rotate slightly differently. They meet in a broad zone of folded and faulted seafloor beneath the central Indian Ocean, where they converge at only about 0.5 cm/yr.
What happens at its edges
The longest boundary, 11,154 km, follows the Southeast Indian Ridge between Australia and the Antarctic Plate. New seafloor forms there as the plates spread apart at about 6.8 cm/yr. To the west, the southern Central Indian Ridge separates it from the Somali Plate at about 4 cm/yr.
The northern edge is where the plate is destroyed. Along the Sunda Trench south of Java, Australian seafloor sinks beneath the Sunda Plate at about 5.8 cm/yr. Farther east, the continental shelf of northern Australia is too thick and buoyant to sink, so it collides with arcs and microplates: the Timor Plate, the Banda Sea Plate, the Bird’s Head Plate, the Maoke Plate and the Woodlark Plate. This collision raised the mountains of New Guinea. At its far northwest corner, a short subduction segment also touches the Burma Plate.
In the southwest Pacific, the plate sinks east beneath the New Hebrides Plate at the New Hebrides Trench. At about 11.4 cm/yr, this is its fastest boundary. Its border with the Pacific Plate changes style along its length, with relative speeds of about 2 to 10 cm/yr. South of New Zealand, Australian seafloor subducts at the Puysegur Trench. Across the South Island, the Alpine Fault carries the plates past each other as a continental transform. Farther south, the Macquarie Ridge links the boundary toward Antarctica.
Around Fiji and the Lau Basin, the plate borders a cluster of microplates: the Kermadec, Tonga, Niuafo’ou, Futuna, Conway Reef and Balmoral Reef plates. Most of these boundaries are young spreading ridges and transform faults in back-arc basins.
From Gondwana to the tropics
Australia was once part of the supercontinent Gondwana, joined to Antarctica along its southern coast. The two began to rift apart in the Cretaceous, and once a deep ocean opened between them, Australia sped north. Today it is one of the fastest-moving continents, traveling roughly 7 cm/yr.
Chains of extinct volcanoes along eastern Australia get younger toward the south, a record of the plate sliding north over a hotspot. Younger volcanism continues in the Newer Volcanics Province of Victoria and South Australia, which last erupted around 2900 BCE.
Earthquakes and volcanoes
The dataset lists 246 earthquakes of magnitude 6.5 or larger on the plate since 1900, three of them magnitude 8 or more. The largest is the 2018 Fiji earthquake (M8.2), which struck about 600 km down in the Pacific seafloor that has sunk beneath the Fiji and Tonga region. A second deep quake, M7.9 at 671 km, followed near Fiji weeks later. South of New Zealand, the 2004 Tasman Sea earthquake (M8.1) and the 1989 Macquarie Ridge earthquake (M8.0) ruptured the seafloor near the Pacific boundary. In 2016 an M7.8 quake broke the Indian Ocean floor southwest of Sumatra, far from any trench.
Most of the plate’s 13 volcanoes in the dataset sit in New Zealand’s North Island. Ruapehu (2,797 m) last erupted in 2007 and Tongariro in 2012, both in the Taupo Volcanic Zone, where the crust is stretching. Fiji has Taveuni and Nabukelevu, and the Auckland Volcanic Field lies beneath New Zealand’s largest city.