Magma and Lava of Mt Ruapehu


Mt Ruapehu produces eruptions of high explosivity, but they are not the biggest like the eruption that produced Lake Taupo 26,500 years ago.  However, they can be larger than those that created Rangitoto Island 600 years ago in Auckland’s Hauraki Gulf.

This is due to the composition of the magma and the amount of water contained in it.  Magmas are generated from partially melted source rocks which can include subducted oceanic crust and overlying sediments plus the lower continental crust.  Andesites, which are the most common rocks produced by Ruapehu eruptions, are differentiated products of basaltic magmas that may have become contaminated and mixed with magmas generated by partial melting of the surrounding crust.  In short, the higher the water content, the higher the explosivity of the eruption.

The fallout from eruptions is as follows:

Ash – less than 2mm diameter particles

Lapilli – 2-64mm diameter particles

Bombs – greater than 64mm diameter plastically deformed blobs of magma

Blocks – greater than 64mm diameter solid rock fragments

The temperatures of these rocks at eruption are 1100 – 1250 degrees Celcius

During the 1995/96 eruptions enough ash and lapilli built up on the rim of Crater Lake to allow an increase in lake water volume.  This debris, termed tephra, is unconsolidated and allowed as much as 1000 cubic metres per day to seep out of the lake prior to the 18 March 2007 lahar.


About lilyennis

Author, artist, martial artist, flautist, and cat fanatic. I live in Thames, NZ with four beautiful orange tabbies, a seal Siamese, a cockatoo and a husband. When I'm not agonising over my writing I'm Scottish Country Dancing or visiting art galleries and art trails with friends. I have been vegan since December 2013 and am loving it!

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