Some three years ago, Zealandia was identified as a continent. Now, results from a scientific drilling expedition, reported in February 2020, show that the largely submerged Zealandia continent, which stretches across five million square kilometres beneath the southwest Pacific Ocean, was shaped by two tectonic events: first, it was ripped away from Australia and Antarctica, and then, it was carved by forces that started the Pacific Ring of Fire.
Zealandia, believed to be Earth’s seventh continent, experienced dramatic elevation changes between about 50 million and 35 million years ago, according to a new analysis of samples, collected during the International Ocean Discovery Program (IODP) Expedition 371 in 2017. The study says that this topographic upheaval may have been due to a widespread reactivation of ancient faults linked to formation of the western Pacific’s Ring of Fire.
The continent is said to stretch some 1.9 million sq. miles, or 4.9 million sq. km, once making up some 5 per cent of the area of the supercontinent Gondwana. Gondwana is the name given to an ancient supercontinent that included Antarctica and Australia. Roughly, 94 per cent of the area of Zealandia is said to be submerged.
The first scientific drilling expedition for collection of samples in the place where Zealandia is believed to have been, took place in 1972 by Australia, New Zealand, and New Caledonia. The results suggested tectonic forces stretched and thinned Zealandia’s crust until it was ripped from the ancient supercontinent Gondwana. The deep ocean, the Tasman Sea, intervened between them.
Detailed surveys, during the 1990s and 2000s, carried out to establish sovereignty over the Zealandia continental mass by New Zealand, Australia, and France, have however, resulted in other observations. In 2017, a nine-week expedition of 32 scientists began into the southwest Pacific as part of the International Ocean Discovery Program (IODP) aboard the research vessel JOIDES Resolution. It had the aim of focusing on the difference between Zealandia and the other continents. The expedition 371 of the IODP was undertaken to take new samples to test the hypothesis that formation of the Pacific Ring of Fire played a key role in shaping Zealandia.
After the theory on tectonic forces in the 1970s, Zealandia was seen as simply cooling and subsiding. But fossils in the drill cores collected by the team indicated that during the early Cenozoic, portions of northern Zealandia rose some 1 – 2 km while other sections subsided about the same amount before the entire continent sank another km deep underwater.
The timing of these topographic transformations perhaps coincided with a global reorganisation of tectonic plates evidenced by the bend in the Emperor-Hawaii seamount chain, the reorientation of numerous mid-ocean ridges, and the onset of subduction—and the related volcanism and seismicity—in a belt that still encircles much of the western Pacific.
The study has stated that the early signs of the Ring of Fire were almost simultaneous throughout the western Pacific. This timing also predates the global tectonic plate reorganisation. To find an explanation for how subduction began across such a broad area in such a short time, it has proposed a ‘subduction rupture event,’ which has been likened to a massive, super-slow earthquake. Though nothing has been proved about this new theory of ‘subduction rupture’, the expedition scientists believe the event resurrected ancient subduction faults that had lain dormant for many millions of years.
The team concludes that though they don’t know where or why, but something did happen to start off that locally induced movement, and when the fault started to slip, like in an earthquake, the motion rapidly spread sideways onto adjacent parts of the fault system and then around the western Pacific. However, unlike an earthquake, the researchers say that the ‘subduction rupture event’ may have taken more than a million years to unfold. Ultimately, perhaps Zealandia’s sedimentary record could help determine how and why this event happened and what the consequences were for animals, plants, and global climate.
The findings were published in the journal Geology.
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Sidelight
There is a difference between Zealandia and the other continents. More than half the surface area of Earth’s other six continents is composed of low-lying land and shallow seas, and they have relatively narrow mountain ranges and steep continental slopes in the deep ocean. In contrast, Zealandia is mostly hidden beneath more than one km of water and could be classified as more than 90 per cent continental slope.
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Sidelight
The Pacific Ring of Fire is a zone of volcanoes and earthquakes running along the west coasts of north and south America, past Alaska and Japan, and then through the western Pacific to New Zealand. The violent geological activity in this zone reflects deeper unrest at the boundaries of tectonic plates, caused by ‘subduction processes’ —where one tectonic plate converges on another and sinks back deep into the earth.
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