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

3W_3G-P19

Date/Time: July 23 Poster

Proximal pyroclastic density current deposits at a glaciated composite volcano: MountRuapehu, New ZealandJames D Cowlyn1 , Ben M Kennedy1 , Darren M Gravley1 , Shane J Cronin2 , Natalia Pardo2 , Graham S
Leonard3 , Dougal Townsend3
1

Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand,2
Volcanic Risk Solutions, Massey University, Private Bag 11 222, Palmerston North, New Zealand,
3GNS Science, PO Box 30368, Lower Hutt, New ZealandE-mail: james.cowlyn@pg.canterbury.ac.nz

Young (<13.6 ka BP cal.) proximal pyroclastic density current (PDC) deposits at Ruapehu Volcano, New Zealandare described for the first time. The deposits result from some of the largest known eruptions at Ruapehu and its
subsequent transition to vulcanian and phreatomagmatic activity. They provide insight into the emplacement andpreservation of small to medium volume PDCs at glaciated composite volcanoes, reflecting complex interactions
with palaeomorphology and possibly also changing ice cover at the volcano. Five distinct PDC episodes havebeen observed. 1.) The oldest units are massive pumiceous deposits including a full spectrum of dense to highly
expanded pumices that represent collapsing plinian columns sourced from Ruapehu’s Northern Crater. These
are correlated with a sequence of plinian fall deposits preserved on the ring plain. Directly overlying these isunit 2.), an unusual <30m thick variably welded grey-black PDC deposit that comprises rounded monolithologic
scoriaceous clasts supported by a vesicular ash matrix. Unit 2 is interpreted to represent a collapsing near-ventspatter pile deposited on the steep upper flanks. Transport over ice may have been significant in producing theunusual rounding of the clasts and vesicular matrix, but the incipient welding and presence of well developed
cooling fractures suggest that extensive cooling did not occur during transport. The role of ice in the emplacementof this flow may be important for understanding the pyroclastic flow hazard at similar glaciated composite volcanoes
worldwide. 3.) A younger locally exposed pumiceous PDC deposit containing fibrous and colour banded pumicesis thought to correlate to the last known plinian episode at Ruapehu, associated with the opening of the presently
active South Crater. 4.) Overlying this is a sequence of small PDC deposits with dense juvenile clasts that may
represent flows generated from vulcanian eruptions sourced from smaller degassed magma pockets remaining
after the plinian events. This sequence is capped by 5.) a thin (<1m) black PDC deposit containing distinctivelarge (<1m) cauliform and breadcrust bombs. The complete PDC sequence reveals systematic temporal changes
in magma composition, density, and the role of magma mingling during a period of significant change at Ruapehu.
Complex interactions with palaeomorphology and possibly surface ice appear to have been important factors in thegeneration, emplacement and preservation of the PDCs, and these effects are important for understanding the until
now overlooked PDC hazard at Ruapehu and other glaciated composite volcanoes worldwide.

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