New Zealand is located on the boundary between two tectonic plates. Magma can come to the surface and erupt when the subducting plate melts.
Lying along the plate boundary means that there are more earthquake activity that may happen. The Pacific plate under New Zealand is pushing into the Indo-Australian plate and is being forced down into the Earth's mantle, which creates subduction where the plates melt.
The magma that rises from this "subduction" zone produces a line of active volcanoes to form as the volcanic arc. New Zealand's volcanoes are part of the larger zone of active volcanoes at boundaries that rim the Pacific Ocean, known as the "Pacific Ring of Fire."
But what shocks many today, is the discovery that the North Island of New Zealand actually sits upon a part of the largest volcanic outpouring on Earth.
Hot Rocks from the Mantle Rise to the Surface
Around 120 million years ago, a giant plume of hot rock detached itself from the core-mantle boundary, about 3,000 kilometres below Earth's surface, which rose rapidly to the surface as a "superplume," the new study revealed.
Geophysicist Professor Tim Stern of Victoria University explained that this ancient plume connected the deep interior of the Earth with the surface of the planet.
According to Stern, "in the 1970s, geophysicists proposed that the Earth's mantle was undergoing a churning motion, rather like a lava lamp, and hot blobs of buoyant rock rose up as plumes from as far as the Earth's core."
Moreover, the melting of this rock near the surface of the Earth could be the cause of so much volcanism, similar to what was observed in Iceland or Hawaii.
Larger volcanic outpourings have happened before in the history of geology, of which the largest known is the southwestern Pacific in the Cretaceous Period during the time of the dinosaurs. The volcanic outpouring has caused the formation of the continent-sized underwater plateau.
But the movements of the tectonic plates cause the plateau to split, with one fragment of it drifted to the south forming the Hikurangi Plateau. This now underlies the North Island and also the shallow ocean offshore.
Together with colleague Associate Professor Simon Lamb, Stern studied the speed of seismic waves through these rock layers to know their origins and features.
The researchers said that the seismic pressure "P" waves triggered by either earthquakes or man-made explosions travel through the mantle rocks beneath the Hikurangi Plateaufaster than observed in the seafloor, about 9 kilometers per second.
"A peculiar feature of these high speeds is that they are equally high for seismic vibrations travelling in all horizontal directions, but much lower for those vibrations travelling vertically upwards," they added.
All Part of the Same Superplume
They used the differences in horizontal and vertical speed to match the Hikurangi Plateau rocks with those of the Manihiki Plateau north of Samoa and the Ontong-Java Plateau north of the Solomon Islands and found that they have the same speed characteristics. This means that they were all part of the same superplume.
This creates the largest volcanic outpouring on Earth in a region over 2,000 kilometers across, Stern said.
Meanwhile, Lamb said that the associated volcanic activity might have played an important role in Earth history. It could have influenced the planet's climate and also the evolution of life by triggering mass extinctions.
This study gave evidence that such plume occurred and also a fingerprint method to detect fragments of the largest plumes of all superplumes rising from near the Earth's core.
