IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P1_3G-O16

Room A4

Date/Time: July 21 15:45-16:00

Transformation of blast-like hydrothermal jets into highly mobile pyroclastic surges. New
insights from the August 2012 Te Maari eruption
Eric Christophe Breard, Gert Lube, Shane J Cronin
Massey University, New ZealandE-mail: E.C.P.Breard@massey.ac.nz

Cold (Bandai-type) pyroclastic surges are a particularly hazardous type of pyroclastic density current. They cancomprise spectacular mobility in terms of their runout, their ability to surmount topographic barriers and theirextremely destructive and lethal potential. In comparison to magmatic and phreatomagmatic/phreatic surges, ourknowledge on the generation mechanisms and on the flow behaviour of cold surges remains largely fragmentary.
The August 6 2012 eruption of Te Maari volcano, Tongariro (New Zealand), caused by the sudden decompressionof the hydrothermal system below the crater, produced highly mobile cold surges that travelled and spread for 2.1
and 2.3 km along wide sectorial arcs to the E and W, respectively. In the W, surges reached and caused damage
beyond the popular Tongariro Crossing Track, largely affecting Tourist businesses in the Central North Island of NZ
until today.
Direct observations of the eruption, geophysical and geochemical monitoring data, and detailed mapping of the
pyroclastic successions over the entire 6.3 square km surge inundation area allows: (a) a quantitative reconstructionof the eruption sequence comprising an initial debris avalanche, two surge units and associated ballistics strew
fields related to two discrete and directed blast-like explosions that also formed the new fissure-type crater, andfinal fall deposits from a third, plume-forming explosion through the main Upper Te Maari vent; (b) a reconstructionof the surge generation mechanism through a longitudinal transformation of two high-velocity, discrete, blast-likehydrothermal jets expelled at 30 to 60 degrees to the horizontal; and (c) describing and interpreting associatedtransitional sedimentary facies changes along the main blast or surge sectors from massive breccias, a three-partite
surge bed succession over a narrow, medial region with anti-dune formations and a wider distal region of normal
dunes and planar surge forms.
Here we will give a brief description of the eruption sequence. We will show and discuss detailed quantitative
datasets of the laterally and longitudinally evolving surge bed forms including dune and bed geometry data, deposit
distribution, grain-size, particle density and componentry data and relate these to the explosion mechanisms at
source, to the space- and time-variant formation of a highly density stratified surge, and to local interaction with 5
successive ridges and valleys in the surge runout path.
Through integrating surge thickness and grain-size data we will also present computations of the bulk/initial
grain-size distribution of the two hydrothermal blasts/surges. This rarely obtainable piece of information will be
used to model the absolute, vertically and longitudinally variable mass partition of the stratified surge flow. The
model allows us to finally discuss general aspects of surge transport and deposition as well as surge destruction
behaviour that is characteristic for mobile cold surges.

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