C a s e

H i s t o r i e s

Table 1.Summary of secondary (2º) events detected and estimated number of days involved, to 1 September 1996.Events are grouped and/or

counted according to type, significance, temporal separation (eg each slab avalanche is recorded separately but point releases and sluffs are

groups) and eruption phase.

SECONDARY HAZARDTYPES,NUMBERS

ANDLOCATIONS

Many secondary events orsets of events have been docu-
mented to date (n=120, Table1).They have included: slab
avalanchesdueto ballistic block impacts, lahars, airwaves,
ashfall and weaknesses due to ash layers;pointreleases
due to impacts and ash layers;wet snow avalanches due
to ash-induced melting; ice block fall due to undercutting
by lahars and warm waves;a range of types of secondary
lahars (mud and debris flows);and ash sluffs.
Almostall (116)ofthese events were on MtRuapehu
and all except one slab avalancheon WhakapapaSkifield
started above 2000m and within 3 km of the centre of the
crater.Two events were noted on each of the neighbour-
ing volcanoes Ngauruhoe and Tongariro 11-14 kmnorth-
northeast of the crater.On Ruapehu the start zonesof only
9 events were below 2500 m and all these were on north to
east facing slopes.The two events recorded on Ngauruhoe
(2287 m) both started above 2000 m.The two very small
eventsonTongariro(1967m)were at1700mandmany
small events probably went undetectedat these altitudes.
Events have been classified into six types and two cat-
egories - immediate or delayed(Table1).Immediate events
wereproduceddirectlyand immediately by eruptionprod-
ucts such as a ballistic block impacting the snow, whereas
delayed events occurred days to months afterindividual
eruptions.About 18 percent of the events were triggered
directly, but most (82%) were delayed.
The events discussed in this paper are only a sample of all

258

thatwouldhave occurred.Butall seasons,aspects and
altitudeswere abletobeexaminedastheeruptionand
secondary processesextended forover15monthsand a
wide range of topography.

TYPESANDTRIGGERSOF IMMEDIATE

SECONDARY HAZARDS

The largest andpotentially most hazardousof the immedi-
ate events were hardslab avalanchestriggered by ballistic
blocks and lava bombs (up to 5 m in diameter).Impacts of
suchmaterialerupted during 17-18June1996setoffat
leastsixsuchavalanches including aclass 3+inCrater
Basin below Tahurangi.Immediate slope failure occurred
onmostaspectsexceptsouthalthoughprojectiles were
ejected in all directions.Similarballistic impacts during
othereruptions inspring1995andJuly1996produced
numerous point releases and small sluffs rather than hard
slab avalanches.A weak facetted layerwas found in the
snowpack at Turoa on 19 June 1996(M.Brown,personal
communication)sothesnowpack musthavebeenmore
unstable in the colder early winterconditions.Two hard
slabavalanchesweretriggeredbylahars(3July,23
September1995) again in colder situations and aspects.
Onlyononeoccasion(17-18June)didhotballistics
appeartocreate sufficientlyintensemeltingtoproduce
what were interpreted as very small,non-hazardous sec-
ondary lahars.The more intenseandturbulent pyroclastic
hazardsofthekind whichcreate hazardouslaharselse-
where (Major and Newall,1989) did not occur.