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Recycling the Crust

When a New Zealand volcano like Ruapehu or White Island erupts, it could be spitting out material that originated from other New Zealand mountains three million years earlier. It is almost certain that some of the molten rock flowing out of the volcano has been part of a remarkable recycling process involving New Zealand mountains, erosion, rivers, channels under the sea, subduction zones and volcanoes.

Over this three-million-year period, rock material can complete a unique cycle, involving a journey of several thousand kilometres, and end up back where it started. Victoria University geologist John Gamble, working with colleagues Dr Ian Wright (a Victoria graduate) and Dr Lionel Carter from NIWA, helped put together the evidence for this scenario.

"This recycling seems to be a highly unusual phenomenon," Gamble says.

New Zealand mountains -- notably the Southern Alps -- are young by geological standards, and subject to rapid uplift and erosion. Pieces of rock of varying sizes are worn away from mountainsides by the forces of erosion and weathering, and many will be carried along by streams and rivers till they reach the ocean.

This sediment can be caught up in a massive system of underwater channels -- up to five kilometres deep -- and currents that Dr Gamble and his colleagues have christened the Eastern New Zealand Oceanic Sedimentary System (ENZOSS).

There are three main routes the sediment can take on its way to the subduction zone in the Kermadec Trench, north-east of the East Cape. The trench marks the boundary between the Pacific and Australian tectonic plates which form the surface of the Earth in this region. The three possible routes involve channels up to 1400km long.

Along the seven-kilometre-deep Kermadec Trench, the sediment is taken into the Earth's interior on the surface of the Pacific Plate, which is "subducting" beneath the Australian Plate. This material can then travel to New Zealand's volcanic zone, be erupted and begin its cycle all over again.

How do the scientists know all this is happening? Oceanographers can map the channels and trace the movement of the sediment along the sea floor. The identity of the sediment has been confirmed by analysis of the minerals; for example, the sediment from the sea floor and rocks from the Southern Alps contain amounts of a certain type of mica.

However the mica analysis cannot be used to trace the sediment after it gets subducted, because the mica will undergo reactions in the deep interior of the Earth. Instead, John Gamble came up with a method that involved measuring the ratio of two isotopes of lead and the concentrations of certain trace elements such as rubidium, thorium and caesium. His analysis showed that the lead isotope ratios and key trace element ratios in sediment from the bottom of the Kermadec Trench were similar to those in rocks on volcanoes along the southern Kermadec Island Arc such as Tangaroa, and Clark and Ruapehu volcanoes in New Zealand.

"These analyses show clearly that at least some of these materials can be traced all the way from the mountains and volcanoes, down the rivers, through the submarine channels to the subduction zone in the Kermadec Trench, and -- most remarkably -- back up through the volcanoes," Gamble says.

"It's a near-incredible journey, which we estimate takes about three million years. The amount of sediment flowing through the system would have varied greatly through geological time depending on how much sediment was entering the ocean through the rivers, and on the balance between uplift and erosion"

The process has been described in articles in the prestigious earth science publications Geology and Journal of Petrology.