Volcanism in New Zealand

New Zealand experiences significant volcanic activity due to its location along the subduction zone of the Pacific and Australian plates. As the subducting plate is heated, it releases its surface water into the mantle above, which lowers the mantle’s melting point and causes magma to form. The magma rises to the surface and erupts, forming volcanoes like Mount Ruapehu (Smith).

This diagram demonstrates how the subduction of the Pacific Plate beneath the Australian Plate produces magma, which erupts on the surface in volcanoes like Mount Ruapeu and Taupo.
http://www.cdemhawkesbay.govt.nz/hawkes-bay-civil-defence-emergency-management-group/earthquakes_idl=2_idt=496_id=1814_.html

Ruapehu is classed as a stratovolcano (or composite volcano) because it was formed by a mixture of andesite lava and ash layers. When Ruapehu erupts, the magma cools and breaks up quickly within the crater lake, spewing ash and larger tephra such as bombs and blocks into the air (Ruapheu). In the video below, watch as tephra rains down during the 1996 eruption.

In 1953 a mix of lake water and volcanic material broke through the crater and created a lahar, which flowed down the volcano and took out a railroad bridge just as a train was crossing, killing 150 people (Mount). Below is a photo of a more recent lahar on Ruapehu.

 

In 2007, a lahar broke through the tephra which had previously contained it within the crater and ran down the side of the volcano. Since the 1953 disaster, early warnings systems have successfully prevented any additional lahar fatalities.
 http://www.radionz.co.nz/collections/u/when-the-siren-goes/volcanoes/lahars

Further north along the subduction zone is Lake Taupo’s giant rhyolite caldera. The caldera was formed following the Oruanui eruption 26,500 years ago when the volcano collapsed upon its empty magma chambers. 

 

The Orunanui eruption created the Taupo caldera, which is now filled with water. At over 600 square kilometers, Lake Taupo is the largest lake in New Zealand.
http://www.stuff.co.nz/dominion-post/news/6784815/Taupo-super-eruption-secrets-revealed

The most recent major Taupo eruption was in 200 AD and was much smaller. Still, ash spewed over 50 kilometers into the air and a 100 meter-deep pyroclastic flow enveloped the surrounding area (Taupo). Luckily, New Zealand was not yet inhabited by humans. The diagram below compares the volume of Oruanui and Taupo to other major eruptions.

The Oruanui eruption produced 1000 times the volume of the 1980 Mt St Helen's eruption.
http://www.teara.govt.nz/en/diagram/8716/taupo-eruptions

New Zealand’s largest city contains over fifty cinder cone volcanoes. They are monogenetic, meaning that they only erupt once. Unlike Taupo and Ruapehu, they were created by a hot spot, an area of weak crust where magma sometimes rises to the surface. As the earth's crust moves over the hot spot, a new volcano could form in Auckland, which would be devastating for the city (Auckland).

Imagine if Mt Eden erupted today. Luckily that will probably never happen because it is a monogenetic volcano, but the hot spot beneath Auckland is still active, meaning that a new volcano could form within or near the city.

Works Cited

“Auckland Volcanic Field.” GNS Science. N.p., n.d. Web. 13 Oct. 2014.

“Mount Ruapehu Erupts.” Waikato Regional Council. N.p., n.d. Web. 13 Oct. 2014.

“Ruapehu.” GNS Science. N.p., n.d. Web. 13 Oct. 2014.

Smith, Richard, David J. Lowe, and Ian Wright. “Volcanoes - Pacific Ring of Fire.” Te Ara - The

    Encyclopedia of New Zealand. N.p. 13 Jul. 2012. Web. 13 Oct. 2014.

“Taupo.” GNS Science. N.p., n.d. Web. 13 Oct. 2014.