Oruanui outbreak

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Ash plume from the Oruanui eruption, as one might have seen it from space 26,500 years ago.

The Oruanui eruption of the Taupo in the Taupo Volcanic Zone on the North Island of New Zealand was the world's largest eruption in the last 70,000 years and one of the world's largest eruptions in the last 250,000 years , with a volcanic explosion index of eight . This means that the Taupo is classified as a super volcano .

The eruption occurred about 26,500 years ago in the late Pleistocene and produced around 430 cubic kilometers of ash-fall deposits, 320 cubic kilometers of pyroclastic flow deposits (predominantly ignimbrites ) and 420 cubic kilometers of primary (directly related to the extraction products of this eruption) intracaldera deposits of 1170 total Cubic kilometers of tephra , which corresponds to 530 cubic kilometers of bubble-free magma . Much of what is now Lake Taupo was created in direct connection with the Oruanui eruption. The magma extraction took place alternately through several chimneys, which are now in the subsurface of Lake Taupo.

The Oruanui eruption shows two unusual characteristics: an episodic course and a broad spectrum of deposits, both in terms of ash falls and pyroclastic flow deposits.

Deposits

stratigraphy

The eruption deposits have been subdivided into ten mappable units (“phases”), of which the tenth and youngest is the most poorly preserved in the area, but the most thick phase, as has been determined by locally well preserved profiles. The breaks in activity between the phases extend for a maximum of weeks to a few months, but mostly only for hours.

Ash falls

Distal deposits from the outbreak, i.e. those at a greater distance from the center of the outbreak, consist exclusively of ash. They can be found almost all over the North Island and in the northeast of the South Island of New Zealand . These deposits with a total thickness of more than two meters have been preserved on the North Island. On the Chatham Islands , which are around 1,000 kilometers southwest of Lake Taupo, there are still 18 centimeters thick deposits of ash dust that came from the most recent phases of the eruption.

Pyroclastic flow deposits

Proximally, i.e. with increasing proximity to the center of the eruption, pyroclastic flow deposits are temporarily stored in the ash falls or form the predominant part of the Oruanui sequence. They mostly consist of partly stratified, partly massive ignimbrites, cover a total area of ​​300 square kilometers, locally reach thicknesses of more than 200 meters and can be found e.g. Sometimes at a distance of up to 80 kilometers from the shores of Lake Taupo.

The Oruanui Caldera

The Oruanui Caldera was formed during the most recent stages of the eruption. Today, Lake Taupo, which is over 600 square kilometers in size, was previously believed to be more or less identical to the caldera. Geophysical explorations, however, produced a somewhat more detailed picture of the lake bedrock. The actual caldera is an area of ​​only 140 square kilometers in the central area of ​​the lake. This area is surrounded by an edge zone of 228 square kilometers, the so-called "collapse collar". The southern part of the lake is formed by an asymmetrical trench structure measuring 155 square kilometers . While the subsidence of these three areas of the lake is directly related to the Oruanui eruption, the 47 square kilometer extreme northeast of the lake is more recent and was formed during an eruption 1,800 years ago.

consequences

The enormous amounts of volcanic material that were extracted during the eruption and deposited in the area surrounding the volcano had a major impact on the subsequent landscape development in the region. The periglacial climate that prevailed in New Zealand at the time of the Oruanui eruption (corresponds to the Vistula glacial period in northern Europe) had an unfavorable effect on the development of a vegetation cover around the Taupo volcano after the eruption ended. The result was intense erosion of the pyroclastic deposits, as a result of which large quantities of this material were carried away by rivers and deposited again at their lower reaches. This led u. a. to the fact that the Waikato River shifted its original course through the Hauraki Plains into the Hauraki Gulf to the west into the Hamilton Basin and now flows into the Tasman Sea.

literature

  • Colin JN Wilson (2001): The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview. Journal of Volcanology and Geothermal Research. Vol. 112, pp. 133-174, doi : 10.1016 / S0377-0273 (01) 00239-6 .
  • Vern Manville, Colin JN Wilson (2004): The 26.5 ka Oruanui eruption, New Zealand: a review of the roles of volcanism and climate in the post-eruptive sedimentary response. New Zealand Journal of Geology & Geophysics. Vol. 47, No. 3, pp. 525-547, doi : 10.1080 / 00288306.2004.9515074 .
  • Colin JN Wilson, S. Blake, BLA Charlier, AN Sutton (2006): The 26.5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body. Journal of Petrology. Vol. 47, No. 1, pp. 35-69, doi : 10.1093 / petrology / egi066 .

Web links

  • The Taupo volcano. Story: Volcanoes. The Encyclopedia of New Zealand, (accessed July 14, 2013)