Eruption in the Kermadec Islands

Yesterday’s eruption at Raoul Island in the Kermadec Island chain is a sharp reminder that New Zealand sits astride a very active part of the Earth’s surface.

The eruption was small by New Zealand standards, but no doubt was harrowing for the Department of Conservation (DOC) scientists who experienced it. Five of the scientists were evacuated to Auckland by helicopter last night but one, who was carrying out a routine testing of water temperatures inside the volcano when Green Lake erupted, is still missing.

Scientists have been living on Raoul Island since early last century, studying weather and geology and reporting their findings back to New Zealand. Observations made at the island provide an early warning of weather and geological trends which will affect the main islands of the country.

Prior to automation of weather observations, Raoul was home to staff from the Meteorological Service who spent six month assignments recording weather and geological data and radioing it back to their counterparts in New Zealand. With automation of weather recording and improved communication via satellite, the weather station was automated and geological observations devolved to other scientific groups working on the island. The DOC scientist missing since the eruption seems to have been engaged in measuring activity inside the crater of the volcano.

These measurements of water temperature and gas emissions provide valuable information on a volcano’s state of activity when they are analysed by vulcanologists. GNS scientists regularly take such measurements at New Zealand’s active volcanoes such as Ruapehu and White Island and at other thermal “hot spots” such as Okataina and Lake Taupo which are actually vast volcanoes in their own right.

GNS Science (rebranded from the Institute of Geological and Nuclear Sciences last year) operates at least one seismometer in the Kermadecs, at Raoul Island. Oddly, data from the device does not appear for public scrutiny on their website, Geonet. In fact, the New Zealand public was better informed about earthquake activity in the Kermadecs when Victoria University’s Research School of Earth Sciences (RSES) operated a website on the nation’s earthquake activity during the 1990s. The RSES site faded when IGNS and EQC started their Geonet website.

New Zealanders must now go to the United States Geological Survey website to gain information on earthquakes in New Zealand territory to the north and south of our three main islands. The data is posted spasmodically, and is sometimes 3 or more days out of date.

Raoul Island, part of the Kermadec island chain is about 1200 km north-east of Auckland. The islands sit on the Tonga Microplate, a fractured part of the much larger Australian tectonic plate which carries the North Island and the western part of the South Island.

A long oceanic trench stretches from south of Samoa to an area east of Wairarapa and marks the collision point of the massive Australian and Pacific tectonic plates. In the north it is called the Tonga Trench, but is known as the Kermadec Trench further south, and is named the Hikurangi Trench where it passes along the North Island’s eastern coast. The area near the trench is perhaps better-known to the public as the section of the Pacific’s “Ring of Fire” that stretches from the Pacific islands to New Zealand.

The trench is caused by the Pacific Plate subducting (sliding under) the Australian Plate. As it does so, at the rate of several millimetres a year, it pulls the edge of the Australian Plate down, creating a deep canyon 8,000 metres deep thoughout most of its length until it starts rising to about 5,000 metres depth to the north-east of East Cape.

The subduction process is not smooth, occurring in spasmodic lurches which we feel and record as earthquakes. As the Pacific Plate slides down deeper into the molten mantle of the Earth, it cracks and is melted as part of the planet’s own recycling scheme. The heating liberates water and magma which rise to create volcanic areas in the Australian Plate above. Hence the long line of volcanoes, most of which are undersea vents, which stretch from the Kermadecs down through White Island to Lake Taupo, the planet’s most active and productive rhyolite volcano.

Scientists studying earthquakes and eruptions tend to regard them as separate processes, owing to their deep understanding of the activity that they observe. However, in the broader sense, both earthquakes and eruptions are simply expressions of the same process – tectonics – and are thereby linked. It is therefore possible to accept that a large earthquake or series of earthquakes can lead to an eruption, as faults move and alter paths for magma, hot gases and super-heated water to rise to the surface. Conversely, it is also possible to accept that an eruption alters pressures in the planet’s crust nearby and can change stress in nearby earthquake fault zones.

Raoul’s volcano is located in a zone of high earthquake activity, where deep and shallow earthquakes have been occurring for some months. Whether this activity will continue or propagate south along the Kermadec Trench remains to be seen.

New Zealand currently has three active volcanoes – Raoul, White Island and Ruapehu. Mt Tongariro shows signs of low-level earthquakes which began late last year. Mt Ngauruhoe, which was very active up until 1975 has been quiet since. Mt Taranaki (Egmont), which last erupted in 1755 is also quiet. The caldera volcanoes at Taupo and Okataina (which includes Mt Tarawera) are currently lakes which exhibit thermal and seismic activity.

Wanganui, where vulcanologists confidently expect a volcano to be located at some time in the distant future has just experienced its 4th swarm of earthquakes in less than 12 months.

It is a complex and fascinating picture of activity which waxes and wanes as different components become active.

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