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.