Fresh light cast on Zealandia
SCIENTISTS have shed fresh light upon the sunken continent our country sits upon, finding it was once transformed by a process they’ve likened to a ‘‘massive, superslow earthquake’’.
Zealandia has been known to geologists for decades but it has only been in recent years that it gained public recognition, after scientists announced that it should count as the planet’s seventh — albeit smallest — continent.
New Zealand to the south and New Caledonia to the north are the only major land masses of the otherwise mostly underwater Zealandia, which, at 4.9 million sq km, is about twothirds the size of Australia.
Its continental crust is mostly from 10km to 30km thick, which is thinner than the 30km to 45km of the six other continents, yet thicker than oceanic crust, which is about 7km thick.
The first scientific drilling expedition to Zealandia in 1972 hypothesised it was underwater after its crust was stretched, thinned and ripped away from the ancient supercontinent of Gondwana about 85 million years ago.
Although evidence remained compelling that this was at least part of the case, new samples collected and tested by
Victoria University’s Prof Rupert Sutherland and his colleagues have pointed to a fresh factor.
That was a dramatic change Zealandia underwent during the formation of the Pacific Ring of Fire about 50 million years ago.
The Pacific Ring of Fire is a zone of volcanoes and earthquakes that result from the geological process of subduction, where a tectonic plate sinks back deep into the Earth.
The process by which the zone formed has always been a mystery.
‘‘We propose that a ‘‘subduction rupture event’ ’’ propagated around the whole of the western Pacific at that time,’’ Prof Sutherland said.
‘‘We suggest the process was similar to a massive superslow earthquake that resurrected ancient subduction faults that had lain dormant for many millions of years.
‘‘This concept of asubduction resurrection’ is a new idea and may help explain a range of different geological observations.’’
As a result of the Pacific Ring of Fire, ‘‘things that were in 1000m of water came up to sea level and then subsided down to be more than 1000m deep again’’, he said.
‘‘The permanent effects included the New Caledonia Trough that comes all the way to Taranaki.’’
Prof Sutherland’s team, which took part in a nineweek voyage as part of the International Ocean Discovery Programme, included more than 30 scientists from around the world.
They used a 300tonne drill to make six boreholes up to 900m below the sea floor, from which rock and sediment cores were collected and analysed for clues about the timing and length of Zealandia’s uplift.
‘‘We used fossils from three of the sites to show that northern Zealandia became much shallower and likely even had land areas between 50 and 35 million years ago,’’Prof Sutherland said.
‘‘At about the same time, two other sites subsided into deeper water, and then the whole region subsided by at least a kilometre to its present depth.’’ . — The New Zealand Herald