Discovering how tectonic plates are recycled off the coast of New Zealand

Underwater volcanoes provide insights into tectonic plate shift in the Pacific

Processes inside the Earth are often difficult for researchers to follow. An international team of researchers including members from Geozentrum Nordbayern at FAU and the GEOMAR Helmholtz Centre for Ocean Research in Kiel were able to draw conclusions about the movement of submerged tectonic plates at a depth of up to 100 kilometres using samples from the floor of the Pacific Ocean, allowing them to answer questions about the history of the tectonic plates to the north of New Zealand. Their study has recently been published in the international journal Nature Communications.*

The Kermadec Islands are located in the Pacific Ocean around 1000 kilometres north of New Zealand. A long chain of underwater volcanoes in close proximity to one another also belongs to the islands. They sit on the eastern edge of the Australian Plate which ends at the Kermadec trench. Further east is the Pacific Plate which is moving west at a rate of 5 to 24 centimetres per year and becoming submerged under the Australian plate at the Kermadec Trench. Experts call this a subduction zone.

A group of researchers from New Zealand, Australia and England, as well as Geozentrum Nordbayern and GEOMAR Helmholtz Centre for Ocean Research Kiel, recently proved that chains of volcanoes in subduction zones can provide information about the extent to which plates have been submerged. This allows certain processes in the subduction zone near New Zealand to be explained in more detail.

The results also contribute to a better understanding of plate tectonics and can help improve assessments of earthquake zones like the one off the coast of New Zealand. ‘Our investigations show that even a 20 kilometre thick oceanic crust can be swallowed and that this part of the crust which has been swallowed influences the volcanic activity on the surface,’ explains co-author Prof. Dr. Karsten Haase from Endogenous Geodynamics, Geozentrum Nordbayern, at FAU.

The researchers made use of a special characteristic of the Pacific Plate for their study. To the north-east of New Zealand, the plate is thicker due to a 15 to 23 kilometre high volcanic crust known as the Hikurangi Plateau. As the Pacific Plate submerges under the Australian Plate, there is a particularly high amount of friction in this thicker area. This causes parts of the Hikurangi Plateau which are submerged to break off and subsequently rise into the mantel wedge above due to the pressure and temperature there. This mixture of rock is then becomes part of the magma in the Kermadec volcanoes.

‘However, the Hikurangi rock’s unique chemical signature is preserved during this process,’ explains Prof. Dr. Kaj Hoernle from GEOMAR, co-author of the study. ‘We use geophysical and geochemical methods to analyse samples from the magma layer on the ocean floor which allowed us to determine where and to what extend the Hikurangi Plateau has been pushed under the continental plate.’

The researchers found traces of the isotopes which are typical of the Hikurangi Plateau in several Kermadec volcanoes on the Australian continental plate spread over large distances. ‘This shows that, in the past, the Hikurangi Plateau underneath the Australian continental plate moved much further north and was originally much larger than previously thought,’ says Kiel-based marine geologist Professor Hoernle. ‘In this way, the magma from the volcanoes allows us to form a picture of the tectonic plates’ past. The composition of the magma is like a fingerprint of the source,’ he explains.

*doi.org/10.1038/ncomms5923

Further information:

Prof. Dr. Karsten Haase (FAU)
Phone: +49 9131 8522616
karsten.haase@fau.de

Prof. Dr. Kaj Hoernle (GEOMAR)
khoernle@geomar.de