C a s e

H i s t o r i e s

Secondaryalpinehazardsinducedbythe1995-1996eruption ofRuapehuVolcano,NewZealand Harry(J.R.)Keys Department of Conservation, Private Bag, Turangi, New Zealand

Key Words:secondary hazards,lahar,avalanche,volcanic

ash,skifield management

ABSTRACT

Hazards duetothe 1995-1996eruptionincluded lahars,
ballistic blocks and ashfall, and a wide variety of second-
ary hazards inducedby them, includingslab and wet snow
avalanches (up toclass3),pointreleasesand secondary
lahars.Secondaryevents (n=120)werenoted overa 15
monthperiod.Moststartedwithin3kmofthecrater,
although two avalanchesoccurred 11 km away.
Secondarylaharsweremosthazardousbuttravelled
down predictable paths.Ash thicknesses as thin as 5 cm
created hazardous secondary lahars that were associated
with heavyrainduringthespring-autumnof 1995/96.More
such events, andwet snow avalanches, seem likely during
the 1996 spring or 1996/97summer.

INTRODUCTION

Mt Ruapehu (2797m,Figure 1) is the highest mountain in
theNorthIsland and New Zealand'smostvisitedalpine
area.More than 10,000 people will be skiing on its three
skifields within 5 km of the activecrater on the busier days
of the late June-November ski season.Climbers and other
visitors frequently visit the summit area with itsglaciers
and a 10⁷m³lake that normally occupies the vent.
Ruapehuis a mostly andesitic composite volcano which
is very active by world standards, even for andesite volca-
noes (Houghton et al., 1987).There have been 11 signifi-
cant eruptionsthis century including the 1995-1996 erup-
tion.Eruptions permanently ejecting more than 10⁶m³of
lake watercreate significant hazards in the skifield areas
and occur on average every 20-50 years while smallerbut
still potentially hazardous eruptions occur more frequently
(Hancox etal.,1995;Otway etal.,1995).Howeveronly
two ski seasons (1995 and 1996) have been disrupted sig-
nificantly byvolcanic activityin 42years ofcommercial
skiing on the mountain although ashfalls during the 1945
eruption interferred with non-commercial skiing.
The combination of a popular mountain and a very ac-
cessible,frequently active crater lake surrounded by per-
manent snow and ice isunique in the world.Managing
this part of Tongariro National Park World Heritage Area
requiresa goodunderstanding oftheeffects ofvolcanic
activity on the mountain.These include secondary haz-
ards which are produced, or induced, by volcanic hazards
and are the subject of this paper.

RUAPEHU CLIMATE

Ruapehu(39º17'S,175º34'E)isanexposedmassifwith
prevailing west to southwest winds.It is 85 kmfrom the
coast andhasatemperate, maritime climate.At Whakapapa
Village(1119 m) the mean annualair temperature is +7.2ºC
withmeandailytemperaturesof2.4-8.1ºCintheJuly-
Novemberskiseason(NZMeteorologicalService1983).

At the village mean annualrainfall is 2838 mm.Snowline
isdowntoabout1600mfromApriluntilSeptember-
Novemberwith measured total snow depths up to 4 m at
2000 m.The snowpack is equitemperaturefor most of the
season (Irwin, 1991)with numerous ice layers.

THEPHASES AND VOLCANIC HAZARDSOF THE1995-96
ERUPTION

Forthe purposes ofthis paperthe eruption so farcan be
dividedintosixphases.Thefirstandfifthwere
noneruptive.Table 1 summarises the five which caused
secondary events and Figure 1 showsthe extent ofmain
life-threatening volcanic hazards.
The fourth phase (Period D on Table 1) deposited a se-
quenceof ash layers on the mountain upto >1500 cm thick
(Figure 2).The sixth phase ejectedballistic rocks up to 1.4
km,depositedafurthersequence ofashand produced
audible airwaves.

Fig. 1.Map of Mt Ruapehu showing maximum ballistic ranges in "large" and

"medium" sized eruption scenarios (Houghton et al., 1987), maximum range

downwind of golfball sized ejecta in 1995-96, and 1995 lahar paths.

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