Macquarie Island

The seafloor, standing up

Macquarie Island is the only island made entirely of oceanic crust: every rock on it formed on or beneath the ocean floor.^[2] It is the one place where the Macquarie Ridge, an undersea ridge running from about 61°S up towards New Zealand, breaks the surface, pushed up where the Indo-Australian plate meets the Pacific plate.^[2][3]

Under the ocean floor lies about 6 km of crust, and under that the mantle, the thick layer of hot rock that makes up most of the Earth. On Macquarie, rock from the upper mantle is being raised above sea level as it happens, which UNESCO says happens nowhere else on Earth.^[3][1] The tilting and uneven uplift of the island have brought up every level of the crust, down to 6 km below the seafloor, so a geologist can walk across a section of the ocean’s floor and the mantle under it.^[3]

The mantle rock is in the north of the island. Most of the southern three-quarters is basalt, including wide fields of pillow lava that erupted on the ocean floor.^[2][3]

Slices of old ocean floor called ophiolites turn up on land in other places, but every other one was thrust onto a continent while mountains were being built, and was crushed and changed on the way. Macquarie’s rocks are still attached to the seafloor they formed in, and barely deformed. Geologists treat the island as the link between those continental ophiolites and the ocean crust they came from.^[4][3]

Still rising

The island’s geological story began about 10 million years ago and is still going on.^[3] Macquarie itself came above the waves only about 600,000 to 700,000 years ago. Old beaches, now stranded high above the shore, show it has kept rising, by an estimated 0.8 mm a year.^[2] Earthquakes strike about once a year.^[1]

Off the east coast the seabed drops steeply into the Macquarie Trench, more than 5,500 m deep; to the west it slopes down to about 3,500 m.^[2] On land, a plateau 150 to 300 m high runs down the island’s spine, dotted with lakes and tarns, above a coastal terrace of deep, soggy peat.^[2][3]

Sealers, rabbits and a recovery

The Australian sealer Frederick Hasselborough sighted the island in 1810 and named it after Lachlan Macquarie, then governor of New South Wales. It was a seal-hunting centre until 1919, and fur seals were nearly wiped out by the 1830s.^[1] In 1911 Douglas Mawson’s Antarctic expedition used it as a halfway point for the first radio link between Australia and Antarctica.^[5]

It became a nature reserve in 1933, and a research station has run there since 1948.^[1][5] Nobody lives there permanently. It is the only known breeding ground of the royal penguin.^[1]

Rabbits were brought in as food in the 1870s and, a century later, were stripping the vegetation. A virus released in the late 1970s cut their numbers, so the island’s feral cats turned to native seabirds; the cats were wiped out between 1985 and 2000, and the rabbits boomed again.^[1] A seven-year programme finally rid the island of rabbits, rats and mice in 2014, and the plants are growing back.^[5]

Bird flu

On 28 August 2026 the Australian government said it would take the station’s 24 staff off the island on the icebreaker Nuyina. Thousands of elephant seals breed there, the H5 strain of bird flu can kill more than 90% of their pups, and officials expected many to die in the breeding season. They said the risk to people was low, but living among dying animals was not.^[6][7] The Nuyina was to collect them in early October, and the government said it was looking at ways to watch the island from afar, hoping to send staff back in coming seasons.^[8] The Maritime Executive reported it would be the first time the station had been left empty since 1948, weeks after an A$370 million plan to rebuild it was announced.^[9]

In mid-October Australia’s Chief Veterinary Officer confirmed the island’s first case of H5N1, found in samples from an elephant seal and a kelp gull.^[7] On Heard Island, another Australian sub-Antarctic territory, more than 13,000 seal pups had died of the virus.^[7][9]