C a s e

H i s t o r i e s

totemperature gradientmetamorphism,and possibly
dueto snow melting orsettling beneath them;

*fine grainedash particlesmigratedownwards from their
original layer into previously clean snow. Snow pit and
spring observations suggest this migration occurs faster
on more northerly aspects and with rain-soakedsnow,
so meltwaterand/orcapillary action may be involved;
Someoftheseprocessesmaysoftenorweaken snowby
snow grain metamorphism and disruption orweakening
ofbondsbetweensnowgrainsorbyaltering grain size,
shape ortexture (Conway and Raymond, 1993).
Depression offreezing pointsdownto-0.5±0.1ºChas
beenmeasuredforsomeashes(Table2)anddownto-
1.3±0.5ºC in one saturated very weak snow layer(Figure
3).The largest depression measured was for a 1:1 (weight
for weight) made-up slurry of the 17 June 1996 ash but the
dilution factoris important.The -1.3±0.5ºC temperature
could be explained by meltwaterin contact with the ash
dissolving more salt as it spread down the sloping upper
surface oftheimperviousashlayer.Thefreezing point
depression varied between layers because the amount of
salt presentin the ash depositsvaries according to whether
the Crater Lakewas presentwhen the ash waseruptedand
ashgrainsize(S.CroninandW.Giggenbach,personal
communications).

STABILISATION OF ASH-INDUCED WEAKNESSES

IN SNOWPACK

Changes in the snowpack after the June 1996 eruption, in-
cluding formation of ice layers and snow melt,helped to

stabiliseash-induced weaknesses.Aclear(congelation)
ice layer formed in place of the slush over the 17 June ash
and bonded it strongly into the snowpack (Figure 3).Clear
layers were observed forming around ash layers as a result
of ash-induced meltwaterfreezing(e.g. 8-9 July ash layer,
Figure 3).Rimeformedthicklayersonsomeotherash
deposits before they were buried by snow.Early snow melt
during warmnortherlyrain allowedrockanchorstore-
emerge below about 2000 m.

Fig. 3.A sequence of three snow pits on northern slopes of Ruapehu

during the 1996 eruption period, showing some influences of ash layers

and time on the snow pack.

• differentparticles of tephra, andtephraandsnow grains
  may not bond well to each other because of differences
  in particle size and shape;
• thin dark ash layers orash particles on the snow sur-
  face absorb solar radiationand enhancemelting (Major
  and Newall,1989);

• ash layers on thesnow surfacemay also cool radiatively
  and freezesolid while exposedthere, perhaps influenc-^MANAGEMENT
  ing subsequent energy and moisture transfer;^{Immediate and potential volcanic and secondary hazards}
• penetrating solarradiation might be absorbed bybur-^{were managed by a sequence of volcano and slope moni-}
  ied ash layers andcause melting but temperature meas-^{toring,information dissemination including public warn-}
  urementsdid notprovideconsistentevidence ofthis^{ingsandsignage,andclosureofvulnerablepartsof}
  (Figure 3);^{skifields.Skifield evacuation and road closures ensured}

• soluble chloride and othersalt material in the ash de-^{few peoplewere ableto accessthe higher risk zones around}

press thefreezingpointcausingmelting at temperatures^{the crater(Figure 1).Existing scientificinformationon}

below 0ºC (Table 2) and probably alter moisture transfer^{volcanic hazards (Figure 1),and management by consen-}

by affecting vapour pressure and/or thin films between^{sus between thedepartment, ski operators, police andcivil}

snow grains;^{defence were effective tools.}
Managementof secondaryhazards wassubsumed by the
*lateralspreadingofmelt(orbrackish)wateroccurseruption managementat first because existing information
within low density snow or at stratigraphicboundariesonsecondaryhazardswasalmostnonexistantin1995.
(Conway and Raymond,1993)such as between snowSufficientinformationwasabletobedevelopedduring
and underlying fine-grained and/orfrozen ash layers.1996forwarnings to be issued.Explosiveswere used to
This weakensbonds betweengrains.Figure 3 illustratesremoveweaknessesabovesomeashlayersandtest
the presence of a weak, saturated layer of round grainssnowpack strength associated with them (Table 3).
on top ofthe 17 June ash layer;

*weaklayerswithfacetted snowcrystalsorthinvoid
spaces develop beneath some ash layers (Figure3) due

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