Zealandia

Zealandia is a mostly submerged landmass in the southwest Pacific Ocean that consists of the continental crust of New Zealand, New Caledonia, and numerous underwater plateaus and ridges. It covers about 4.9 million square kilometers, and only about 5 percent of its area is above sea level, including the North and South Islands of New Zealand and a few other small islands. Zealandia is bounded by the surrounding ocean basins and is separated from Australia by deepwater trenches and ridges.

Geology: Zealandia is geologically distinct from the adjacent oceanic crust; it is built of continental crust that is thicker and less dense than oceanic crust. Its crustal thickness is estimated at 20–40 km, compared with 5–10 km for typical oceanic crust. It contains granitic and other continental-type rocks, and its crustal margins show evidence of past tectonic rifting and subsidence. The region forms a coherent crustal block that is separate from the surrounding tectonic plates, though it lies largely below sea level.

History and status: Zealandia began to break away from the supercontinent Gondwana during the Late Jurassic to early Cretaceous, about 83–85 million years ago. Subsequent subsidence and sea-level rise left most of the landmass submerged within tens of millions of years. The term Zealandia was coined in the 1990s by geologists studying this region. In 2017 a study argued that Zealandia meets several criteria for continental status; some scientists endorse labeling it a continent, while others describe it as a submerged continental crust or microcontinent. It is sometimes referred to as Earth's eighth continent, though that designation is not universally accepted.

Significance: Zealandia provides insight into continental crust formation, the breakup of Gondwana, and submarine geology. Its recognition as a continent highlights debates about how to classify landmasses that are mostly underwater, and it has implications for geography, geology, and plate tectonics.