Academics solve cold case

The riddle of why the Taupo super volcano was once a choker during its biggest eruption may be solved thanks to geological detective work by a team of Victoria University scientists.

Known as the Oruanui eruption, the massive eruption took place about 27,000 years ago, throwing out 530 cubic kilometres of magma and creating a huge hole, known as a caldera, now filled by Lake Taupo.

Super volcano. Photo: Supplied.

The Oruanui eruption is the world's youngest super-eruption, with a Volcanic Explosivity Index of 8, and an explosive force greater than the combined power of all the nuclear weapons ever created.

The eruption is divided into 10 episodes with a number of times when the eruption stopped, particularly between the first and second episodes when there was a quiet period, probably for several months.

A major puzzle for geologists has always been why this massive eruption started, stopped, and then started again on several occasions, when it is clear that there was an enormous reservoir of magma (molten rock) only 4-8 km deep beneath the Taupo area that was primed for eruption.

All the other super volcanoes just blew. Why Taupo took a breather of several months after first clearing its throat is a cold case that's required considerable detective work.

A solution to this riddle has been found, and is presented in a paper titled “The invisible hand: Tectonic triggering and modulation of a rhyolitic super eruption” in the prestigious international journal ‘Geology', published by the Geological Society of America.

It's jointly authored by PhD student Aidan Allan, Professor Colin Wilson, and Dr Marc-Alban Millet, from Victoria University's school of Geography, Environment and Earth Sciences, and Dr Richard J. Wysoczanski from NIWA.

The basis for the break through in the case is the discovery that the pumice layers from the early stages of the Oruanui eruption contain a mixture of two distinct different compositions – the magma from Taupo and small amounts of magma from a separate volcanic system centred about 15km north east of today's township of Taupo, and known imaginatively as the North East Dome System.

This second system doesn't erupt very often and produces only minor amounts of pumice, together with small volcanic domes where the magma has oozed out on to the Earth's surface.

The subsurface magma pools of the dome system are not only different in composition to the massive body beneath Taupo, but they also contain an additional mineral, biotite, a form of mica that is missing from the Taupo magma.

The evidence from the pumice deposits, coupled with detailed analytical work in the Victoria University geochemical laboratories suggests the mixing was created by tectonic processes that transported the north east dome magma along a dike towards Taupo. Limited physical mingling of the two magma types indicates they came in contact in the conduit simultaneously with eruption.

The passage of the dike is thought to have been controlled by a large-scale rifting event, similar to but larger than that which accompanied the 1987 Edgecumbe Earthquake in the Bay of Plenty. The evidence points to the stress changes linked with the rifting acting to drive the north east dome magma sideways through the Earth's crust along a fault line (rather than erupting it above its source) and triggering the Oruanui eruption.

During the first episode of the eruption, enough magma was released so that pressure in the vast reservoir of molten rock in the magma chamber dipped back below the eruption threshold and the vent sealed itself shut. Because Taupo is part of an active tectonic region, the tearing apart of the Earth's crust occurred repeatedly, starting the second episode, and then causing a run-away effect into the third episode.

This work shows that tectonic forces can trigger and control large eruptions by affecting the ways in which magma chambers behave, without necessarily leaving any discernible sign in the geological record. Only the presence of a "foreign" magma in the Oruanui deposits allowed the tectonic control to be identified.

The issue for modern volcanologists is that if “under recognised” tectonic processes have the potential not only to initiate large explosive eruptions, but also to prematurely suppress or stop the eruption altogether, being able to accurately declare when a large explosive eruption has truly ended may be a significant challenge.

This study is one of four on super volcanoes and their eruptions in New Zealand and the USA being funded through support from the Marsden Fund of the Royal Society of New Zealand, while Aidan Allan has been supported by a Bright Futures Top Achiever Doctoral Scholarship.

The authors also thank John Townend, Euan Smith, and James Muirhead for discussions, plus Cindy Ebinger, Calvin Miller and Bill Collins.


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