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Topography of Zealandia. The linear ridges running NNE and SW away from New Zealand are not considered part of the continent, nor are Australia (upper left), Fiji or Vanuatu (top centre).^[1]

Zealandia (IPA: /ziːˈlæːndiə/), also known as Tasmantis or the New Zealand continent, is a nearly submerged continent that sank after breaking away from Australia 60-85 million years ago^[2] and from Antarctica between 130 and 85 million years ago. It may have been completely submerged about 23 million years ago,^[3] and most of it (93%) remains submerged beneath the Pacific Ocean.

Zealandia is 3,500,000 km² in area; this is larger than Greenland or India, and almost half the size of Australia. It is unusually long and narrow, stretching from New Caledonia in the north to beyond New Zealand's sub-antarctic islands in the south (from latitude 19° south to 56° south,^[2] analogous to ranging from Haiti to Hudson Bay or from Sudan to Sweden in the northern hemisphere). New Zealand is the largest part of Zealandia that is above sea level, followed by New Caledonia.

The major submerged parts of Zealandia are the Lord Howe Rise, Challenger Plateau, Campbell Plateau, Norfolk Ridge, Hikurangi Plateau and the Chatham Rise. Smaller provinces include the Louisiade Plateau, Mellish Rise, Kenn Plateau, Chesterfield Plateau, and Dampier Ridge.^[4] The seemingly separate Gilbert Seamount (northwest of Fiordland) is also part of the New Zealand continent,^[5] while how strongly Bollons Seamount (south of the Chatham Islands) remains connected to Zealandia is unknown.

Zealandia supports substantial inshore fisheries and contains New Zealand's largest gas field. Some parts of Zealandia may be mined in the future.

Unlike most continents, Zealandia is largely made up of two nearly parallel ridges. The ridges rise above the sea floor to depths of 1,000-1,500 meters, with infrequent rocky islands rising above sea level. The ridges are continental rock, but are lower in elevation than normal continents because their crust is thinner than usual (only about 20 kilometers thick), and consequently they don't float as high above the earth's mantle.

About 25 million years ago, the southern part of Zealandia (on the Pacific Plate) began to shift relative to the northern part (on the Indo-Australian Plate). The resulting displacement by approximately 500 kilometres (300 mi) along the Alpine Fault is evident in geological maps.^[6] Compression across the plate boundary has uplifted the Southern Alps. Further north, subduction of the Pacific Plate has led to extensive volcanism, including the Coromandel and Taupo Volcanic Zones.

Volcanism on Zealandia has also taken place repeatedly in various parts of the continent before, during and after it rifted away from the supercontinent Gondwana. Although Zealandia has shifted ~6000 km to the northwest with respect to the underlying mantle from the time when it rifted from Antarctica, recurring intracontinental volcanism exhibits magma composition similar to that of volcanoes in previously adjacent parts of Antarctica and Australia. This volcanism is widespread across Zealandia but generally of low volume apart from the huge mid to late Miocene shield volcanoes that developed the Banks and Otago Peninsulas. In addition, it took place continually in numerous limited regions all through the Late Cretaceous and the Cenozoic. However, its causes are still in dispute. During the Miocene, the northern section of Zealandia (Lord Howe Rise) might have slid over a stationary hotspot, forming the Lord Howe seamount chain.

A petrified log at Curio Bay.

New Caledonia and New Zealand lie at the northern and eastern ends, respectively, of the ancient continent. Lord Howe Island is of fairly recent volcanic origin. These land masses are two outposts of the Antarctic Flora, including Araucarias and Podocarps. The islands have no native land mammal fauna. ^[7]

During glacial periods more of Zealandia becomes a terrestrial rather than a marine environment. At Curio Bay, logs of a fossilized forest closely related to modern Kauri and Norfolk Pine, can be seen that once grew on terrestrial Zealandia about 180 million years ago during the Jurassic period.^[8] These were buried by volcanic mud flows and gradually replaced by silica to produce the fossils now exposed by the sea.

• The coast and beyond, including map of the New Zealand continent, from GNS Science

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