Voyage investigates explosive behaviour of Kermadec volcanoes
Marine geologists investigating the past behaviour and hazard risk of volcanoes in the Kermadec Arc, northeast of the Bay of Plenty, have discovered two new submarine volcanoes near Raoul Island.
The scientists will return tomorrow from a successful collaborative expedition mounted by the National Institute of Water & Atmospheric Research (NIWA) and the University of Auckland. The 23–day voyage onboard NIWA’s deepwater research vessel Tangaroa investigated volcanoes on the two largest Kermadec Islands – Raoul and Macauley – and their submerged flanks.
At Raoul, two new submerged caldera volcanoes were found. Both volcanoes have relatively small craters, some 4 km in diameter (just under a fifth of Ruapehu’s crater diameter and half that of Wellington Harbour). One crater is very deep, measuring about 1000 m from the rim to the crater floor. Both volcanoes appear geologically young – in the order of thousands of years old – but laboratory analysis of sediments will be needed to properly assess their age.
‘We wanted to get a better understanding of the processes of construction and destruction of submerged volcanoes to underpin assessments of the hazard risk they pose’, says NIWA marine geologist Dr Ian Wright, who led the voyage. ‘We focussed on Macauley and Raoul because they represent the more explosive and potentially destructive volcanoes in the Kermadec Arc.’
The Kermadec Arc, on the Pacific Ring of Fire, has been the source of vigorous and explosive volcanic activity over the past several thousand years. Its most northerly island, Raoul, hit the headlines in March 2006, when it erupted suddenly.
‘Macauley and Raoul are interesting because at times their magma has been rich in silica and volatiles, making them prone to violently erupt and form calderas, posing significant hazards’, says volcanologist Professor Colin Wilson, who led the Auckland University team. ‘Scientists are finding that these explosive types of submarine volcanic eruptions are more common than previously expected.’
To get a picture of the volcanoes’ past eruption histories – including the age, size, and direction of the eruptions – the 20–member expedition took sediment samples and mapped the contours of the volcanoes both above and below sea level (the latter using sophisticated multibeam sonar technology).
A series of sediment cores taken from east and west of both islands revealed at least six eruptions from the two islands, recorded as centimetre thick layers up to 100 km from the islands.
At Macauley, new seafloor mapping has identified very large volcanic debris flows with waves of sediment up to 50 m in height near the island. These travelled 50 km or more along the seafloor from a very large eruption 6 300 years ago.
The University of Auckland team working on Raoul Island for a week have mapped inside the Raoul crater following the 2006 eruption. ‘This will give us clues as to how the eruption occurred’, says Professor Wilson.
The expedition also collected material (mostly pumice) thrown out by the volcanoes making up Raoul and Macauley Islands, and Healy submarine volcano to test theories about the effects of water pressure on the explosiveness of the eruptions. ‘The theory is that eruptions occurring in shallow water (such as around Macauley) are more violent because the dissolved gases in their magma expand violently in the low water pressure,’ explains Dr Wright. ‘Analysis of the material thrown out by these three volcanoes will indicate how explosive their eruptions have been in the past. This work will help us understand how and when large hazardous eruptions might occur in the future.’
The volcanic studies are funded by the Foundation for Science, Research & Technology and the Marsden Fund.
The voyage was also used to assist the Department of Conservation (DoC) in provisioning their base on Raoul Island, and to carry researchers from DoC and the University of Auckland to study seabirds following the successful eradication of rats from both Raoul and Macauley Islands.
Tangaroa is due in at Burnham Wharf (Evans Bay, Wellington) at 09:00 on Wednesday 23 May.
The Kermadec volcanic arc
- The Kermadec volcanic arc forms a greater than 1000 km sector of the ‘Pacific Ring of Fire’ northeast of the Bay of Plenty, with extensions southward to the New Zealand volcanoes of the Taupo Volcanic Zone, and northward to the submarine (submerged) and subaerial (emergent) volcanoes of the Tofua Arc (Tonga).
- Most of the Kermadec sector is submarine, with only the partially emergent volcanoes of Raoul, Curtis, and Macauley islands being above sea level. These island volcanoes are active, and include eruptions from Raoul Island in modern times.
- Much of the eruption record of these islands is basaltic (involving basalt lavas, which are low in silica). In addition, the eruptive histories of the islands have been punctuated with intensive periods of more silicic (involving lava flows high in silica), explosive, and caldera-forming eruptions. The best example of the latter is the Sandy Bay Tephra that includes sequences of pyroclastic flows exposed above sea level on Macauley Island. The source vent of the Sandy Bay Tephra is the Macauley caldera (with a caldera floor of approximately 900 m water depth) on the lower western submarine flanks of the Macauley edifice.
Eruption types and implications for hazards
- New multibeam mapping reveals a complex seafloor geology of the Macauley caldera and outer flanks, but the structure and composition of the caldera, nature of pumice fill, and mechanisms of the submarine pyroclastic ‘mass-flows’ remains equivocal.
- It’s also unclear whether the original eruption happened above sea level and then collapsed to form a submarine caldera, or began as a submarine eruption that exploded above sea level to form volcanic deposits now found on the island. Both eruption mechanisms have major implications for the evolution of the Kermadec volcanic arc and hazard risk.
- Piston coring downwind of both Raoul and Macauley volcanoes provides further opportunity to determine the age, stratigraphy, dispersal direction, and eruptive size of the larger silicic events.
- Discoveries of submarine silicic volcanism along the Kermadec Arc, and elsewhere globally (e.g., south of Japan) show that submarine pumice-forming explosive eruptions are surprisingly common. Such eruptions involve silica-rich magmas (despite being under water) and many are large enough to form calderas and represent a significant hazard.
Influence of water depth/pressure
- Submarine silicic calderas, recognised also from the southernmost sector of the Kermadec Arc (Healy and Brothers), can be pyroclastic and explosive within water-depths of approximately 900–1000 m. These eruptions are unusual because pressure in water depths of up to 1.5 km should suppress the foaming of magma to form pumice and hence inhibit explosive eruptions. Although such eruptions occur, they cannot be observed directly and there are few measurements from the eruption products to help understand the physical processes whereby magma foams and breaks up underwater.
- Healy, Macauley and Raoul volcanoes present a unique circumstance, which allow these processes in submarine explosive eruptions to be modelled. All three volcanoes have erupted similar silicic magmas within the last 10 000 years in deeper marine, shallower marine, and above sea level settings respectively.
- Eruption products from the three different volcanoes (and hence different hydrostatic pressures) will be analysed to pin down the degassing processes driving these explosive eruptions.
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