Scientists Find Seafloor Hotspots South Of Tonga

Scientists Find Seafloor Hotspots South Of Tonga

A joint New Zealand and Australian oceanographic expedition has found eight hydrothermally active submarine volcanoes and five seafloor vent fields south of Tonga.

It is the first time the 430km stretch of seabed south of Tonga has been systematically investigated for hydrothermal activity related to submarine volcanism.

The 20-day expedition on the Australian research ship, Southern Surveyor, was funded by the Australian government. It involved scientists from New Zealand, Australia, Japan, Germany, and Tonga.

" The expedition confirmed that the Tonga-Kermadec Arc is one of the most volcanically active stretches of seabed in the world," said expedition co-leader and chemical oceanographer Gary Massoth of the Institute of Geological and Nuclear Sciences Ltd (GNS).

" The evidence we collected also strengthens our belief that volcanic arcs are very important sources of heat, dissolved minerals, and other chemicals entering the ocean."

The area studied is unusual in that it is one of only four places on Earth where arc volcanism and spreading ridge volcanism occur adjacent to each other.

" This is significant because the relationship between the two magma systems is poorly understood. We don't know how the two magma systems differ, if at all, and what sort of interaction there may be beneath the seafloor. The data and samples we collected may help resolve this," Mr Massoth said.

The volcanoes they investigated, some larger than Mount Ruapehu, were mapped for the first time late last year by a German expedition.

" Most of the volcanoes are stunningly beautiful structures and several have multiple venting sites."

Investigation of the adjacent spreading ridge area, which lies within 25km of the active volcanic arc front in places, found five discrete fields of hydrothermal venting.

" This is a large number of vent sites over a relatively short distance. It's consistent with our understanding of the area as highly active."

The mineral-rich plumes from the vents rose up to 200m from the seafloor indicating the venting temperature could be as high as 350degC.

" At that temperature, a number of metallic minerals dissolve like sugar in tea. It's our expectation that these metal-rich fluids may precipitate on the seafloor to form iron, copper, zinc, lead, and possibly gold and silver ore deposits."

The instrument used to detect and collect samples of the volcanic plumes was built by GNS scientists. Because of their expertise, and their custom-built "plume sniffer", the GNS scientists have been invited to participate in numerous international expeditions.

They returned to GNS in Lower Hutt today with 1400 plume samples from the voyage which will take several months to analyse and interpret.

GNS scientists have now completed systematic seabed surveys of one-half of the Tonga-Kermadec Arc which stretches 2500km between New Zealand and Tonga.

They anticipate that the entire stretch may contain as many as 90 hydrothermally active submarine volcanoes making it one of the world's great arc volcano systems.

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