Bull Volcanol (2000) 62 : 278±293 DOI 10.1007/s004450000096

RE SEA RCH AR T ICLE

V. Manville ´ K. A. Hodgson ´ B. F. Houghton J. R. (H.) Keys ´ J. D. L. White

Tephra, snow and water: complex sedimentary responses at an active snow-capped stratovolcano, Ruapehu, New Zealand

Received: 8 June 1999 / Accepted: 18 April 2000 / Published online: 16 August 2000  Springer-Verlag 2000

Abstract A feature of small-scale explosive volcanism at stratovolcanoes is the rapid destruction of primary near-vent pyroclastic deposits by sedimentary processes. A protracted series of explosive eruptions of moderate volume from September 1995 until July 1996 at Mount Ruapehu in New Zealand, its largest eruptive episode this century, afforded an opportunity to study these remobilisation processes in detail. All significant sub-plinian eruptions occurred in mid-winter, forming metre-thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow. Subsequent events demonstrated the variety and complexity of the erosion processes that remobilise primary pyroclasts in such a setting. These processes arose from the complex interactions of tephra with snow and ice, and liquid water in varying proportions, and were very diverse in nature and scale. Their effectiveness can be gauged from the fact that there is almost no stratigraphic record of any of the >40 eruption episodes recorded in the past 100 years at Ruapehu. Syn-eruptive remobilisation processes included the generation

Editorial responsibility: M. Rosi V. Manville ()) ´ K. A. Hodgson ´ B. F. Houghton Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand E-mail: [email protected] Fax: +64-7-3748199 V. Manville ´ J. D. L. White Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand J. R. (H.) Keys Department of Conservation, Turangi, New Zealand Present address: K. A. Hodgson Department of Natural Resources Engineering, Lincoln University, P.O. Box 84, Lincoln, New Zealand

of eruption-triggered lahars by the ejection of hot water from the Crater Lake. Post-eruptive interactions mainly remobilised fall deposits from proximal areas, and included rain-triggered lahars, which were among the largest and most hazardous events with the greatest distal impacts. Keywords Stratovolcano ´ Explosive volcanism ´ Tephra remobilisation ´ Lahars ´ Ruapehu ´ Volcanic hazards

Introduction Eruptions at snow-clad volcanoes frequently cause dangerous syn-eruptive floods and lahars (Major and Newhall 1989) when pyroclastic flows or tephra falls catastrophically melt summit glaciers and snow caps (Waitt et al. 1983; Waitt 1989; Pierson 1985; Pierson et al. 1990; Trabant et al. 1994), when lava is extruded beneath thick ice (Gudmundsson et al. 1997) or when eruptions eject water from a crater lake (Healy et al. 1978; Cronin et al. 1997a). Glaciers and snowfields are often strongly modified during the course of the eruption (Thouret 1990; Brugman and Meier 1981), generating unusual mass flows which consist of mixtures of snow, ice, water and clastic material (Pierson and Janda 1994; Waitt et al. 1994; Cronin et al. 1996). These flows have variously been termed ªmixed avalanchesº (Pierson and Janda 1994) or ªsnow slurry laharsº (Cronin et al. 1996, 1997a), and their deposits recorded as ªunusual ice-diamictsº (Waitt et al. 1994). Such potentially hazardous flows are important in the remobilisation and redistribution of proximal pyroclastic ejecta (Pierson and Scott 1985; Pierson 1985). Post-eruptive sedimentary processes mediate the redistribution of pyroclastic material from the volcano and influence its preservation potential, as well as long-term watershed hydrology and hence glacier stability. They involve complex interactions between

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Fig. 2 Mount Ruapehu viewed from the northwest on 24 September 1995. Deposits from the 23 September Surtseyan jets discolour the summit area. The thin but more widespread fall from the convective plume accompanying the explosions is visible on the left flank. The black valley-filling deposits to the right are two eruption-triggered lahar deposits in the tributaries of the Whakapapaiti. (Photo courtesy of L. Homer)

Fig. 1 Location map for Ruapehu, showing localities referred to in the text

tephra, snow ice, and water in a diverse series of environments under seasonally changing climatic conditions. Despite their importance, they have received relatively little attention relative to syn-eruptive processes (e.g. Driedger 1981). In contrast, the post-eruptive erosion and remobilisation of tephra from nonfrozen substrates is relatively well known from both temperate (Collins et al. 1983; Collins and Dunne 1986; Leavesley et al. 1989) and tropical zone cone volcanoes (Segerstrom 1960; Waldron 1967; Okkerman et al. 1985; Rodolfo et al. 1989; Rodolfo and Arguden 1991; Major et al. 1996; Scott et al. 1996; Thouret et al. 1998). This paper considers the extended sedimentary response to a protracted series of moderate-sized explosive eruptions in 1995±1996 from the snow-clad stratovolcano Ruapehu in the Taupo Volcanic Zone, New Zealand (Fig. 1). Ruapehu is an ideal setting in which to study such syn- and post-eruptive remobilisation processes: it is frequently active, seasonally snow covered and its slopes are easily accessible. During 1995±1996, approximately 0.1 km3 of juvenile pyroclastic material and other debris was ejected. The thickest and coarsest deposits accumulated within 2 km of the

vent on the steep upper slopes of the volcano (Fig. 2), whereas fine ash was deposited as far as 250 km downwind from a high plume. Regular fieldwork enabled evolving remobilisation processes to be tracked through time. Physical setting Ruapehu is a large, highly active, dominantly andesitic stratovolcano (Fig. 1; Hackett and Houghton 1989). All historic activity has occurred in South Crater (Cole and Nairn 1975), occupied until 1995 by a hot acidic lake with a volume of approximately 9”106 m3 (Christenson and Wood 1993), which overflowed into the Whangaehu River. The 2797-m-high summit supports six major glaciers that descend to 2000-m altitude. Below 1800 m the slopes are covered with sparse alpine vegetation and are cut by deep, steep-walled valleys. Slopes below approximately 1500 m support thin forest and tussock, and grade to a 6- to 15-kmwide ring-plain (Palmer 1991; Palmer et al. 1993; Donoghue et al. 1995). The mountain lies 85 km from the coast and has a temperate, maritime climate with prevailing west to southwest winds that strongly influenced the primary distribution of 1995±1996 tephras. Average rainfall, at nine stations between 629- and 1100-m elevation ranges from 1059 to 2851 mm/year (Thompson 1984). Most rain falls during short-lived, high-intensity storms, and flooding of the ring-plain valleys occurs during all storms exceeding 100 mm of rain. Snow accumulates above approx-

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imately 1600 m between April and November, reaching >4 m thickness at 2000-m altitude. The snow pack is equitemperature for most of the season, with numerous ice layers (Irwin 1991). 1995±1996 eruption chronology Historic activity at Ruapehu is characterised by small, but frequent, explosive phreatic, phreatomagmatic, and rarer dome-building eruptions (Healy et al. 1978; Nairn et al. 1979; Hackett and Houghton 1989), with at least 40 episodes recorded since 1945. Deposits from eruptions as recent as 1975 are not recognisable on the volcano (Hackett and Houghton 1989) due to the efficiency of the processes discussed here. The 1995±1996 events form the largest historical eruption episode; approximately 18 eruptions of at least this intensity have occurred over the past 1800 years (Donoghue et al. 1997). The 1995 eruption had two main phases (Table 1). The early phase was influenced by the presence of Crater Lake and was characterised by water-rich Surtseyan jets, eruptiontriggered lahars, and convective steam and ash plumes. Lahars were generated by the displacement of Crater Lake water over the outlet into the Whangaehu Valley, or by the explosive ejection of lake water, lake floor debris and some pyroclastic material onto the upper slopes of the volcano where large amounts of snow were incorporated (Bryan et al. 1996; Cronin et al. 1996, 1997a). Emptying of Crater Lake marked the beginning of the second phase, characterised by sustained ash production under drier vent conditions that

generated subplinian tephra falls. Activity waned early in November 1995, and water began to pond on the crater floor, refilling Crater Lake. Eruptive activity resumed on 17 June 1996, removing the incipient lake. Small ash-producing eruptions alternated with Strombolian fire-fountaining until early July when activity declined. Tephra volumes erupted during 1996 were much less than during the previous year. Of 12 individual eruptions which deposited measurable ash layers, only the events of 17 June deposited >0.1 m of tephra at any site.

Tephra distribution and volume Tephra from the more sustained phases of the 1995±1996 eruptions was deposited on the northern and eastern flanks of Ruapehu under the influence of prevailing winds, producing a series of overlapping lobes with variations in thickness and grain size (Fig. 3). Only the three largest eruptions deposited significant volumes of material distally. These tephras, produced after emptying of Crater Lake, were coarser and more poorly sorted than September 1995 tephra and up to ten times more voluminous (Fig. 4). This contrast in physical properties strongly influenced subsequent remobilisation processes. The volume of tephra in each catchment on Ruapehu was calculated from the isopach maps (Fig. 3) using the trapezoidal method (e.g. Froggatt 1982). In 1995, the 43-km2 Whangaehu catchment received the greatest quantity of pyroclastic material (ca. 6.4”106 m3), largely concentrated near the vent, fol-

Table 1 Chronology of the 1995±1996 volcanic activity at Ruapehu volcano (Ruapehu surveillance group, Institute of Geological and Nuclear Sciences). Sedimentary events are indicated in italics Date 1995 January 29 June 18 and 20 September 23 September 25 September 27 and 29 September 7 October 11±12 October 14 October 16±29 October 28 October to 1 November 2±15 November 1996 15 June 17±18 June 27 June 6±8, 10 and 11 July 20 July

Event Weak steam explosions, heating cycle in Crater Lake Small phreatic explosion destroys monitoring equipment Single moderate-sized phreatic explosions and lahars Single explosion, blocks thrown 1.5 km, lahars generated in Whangaehu, Whakapapaiti, and Mangaturuturu catchments Multiple explosions, 10-km-high plume, near-continuous lahar in Whangaehu catchment Explosive eruptions generate lahars in Whangaehu catchment Sustained 15-min explosion, eruption column to 8 km height Sustained 8-h explosive eruption removes the residual Crater Lake and distributes ca. 2”107 m3 of tephra along a northeasterly directed dispersal axis to 250 km downwind Sustained explosive eruption disperses ca. 1”107 m3 of tephra along a southeasterly directed dispersal axis Strong gas emissions, declining tephra output Rainfall remobilises tephra deposits on the volcano forming large lahars Volcanic activity ceases, Crater Lake begins to reform Resumption of seismic tremor activity Continuous explosive eruptions remove incipient Crater Lake and sustain a 20-km-high eruption column. Ash fall along a northerly directed dispersal axis for >200 km. Lahar 8 km long in Whangaehu Valley eruptions generate plume to 6 km Moderate eruptions, plumes to 6 km, local ash fall plume to 10 km, ballistics to 1.4 km, minor ash fall in Taupo; last notable eruptive activity

281 Fig. 3 Isopach maps for the proximal tephra of the 1995 and 1996 eruption episodes show strong control on plume direction by prevailing winds

lowed by the 81-km2 Wahianoa (2.7”106 m3) and 53-km2 Mangatoetoenui (2.0”106 m3) catchments. All three areas were subject to frequent post-eruptive remobilisation events. The sectoral distribution of tephra and access difficulties restricted studies of tephra/snow/water interactions and tephra remobilisation to the downwind Whangaehu and Mangatoetoenui catchments during October 1995 to May 1996. Resumption of activity in 1996 deposited a further approximately 1”106 m3 of tephra in the 135-km2 Whakapapa catchment to the north (Fig. 3), prompting additional work in this area.

Tephra/snow/water interactions During 1995±1997, a variety of tephra/snow ice/water interactions were observed at Ruapehu (Table 2), operating over a range of scales spanning nine orders of magnitude and time frames of minutes to years.

These interactions and remobilisation processes were controlled by numerous factors, including tephra thickness, grain size distribution, and stratigraphy; slope angle, aspect, and nature of the substrate; and climatic parameters such as mean temperature, diurnal temperature range, insolation and precipitation. Many interactions involved the remobilisation and reworking of pyroclastic fall material immediately after its emplacement. Other processes only became active with time as tephra thicknesses increased with subsequent eruptions, or as winter passed into spring. The full range of remobilisation events at Ruapehu (and other snow-clad volcanoes), including avalanches and lahars, have components that may be plotted on a ternary diagram (Fig. 5), enabling transformations marked by changes in the relative proportions of the components to be indicated (i.e. flow bulking by erosion, debulking by deposition, melting of snow-ice content). Similar ternary diagrams have been previously applied in the classification of periglacial slope

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Fig. 4 Selected tephra grain size data from the 1995±1996 eruptions of Ruapehu. Locations of samples are indicated by letters in Fig. 3. Numbers in parentheses refer to distances from the vent. A,B the contrast between early phreatomagmatic tephra (A), and later subplinian tephra (B) erupted under dry vent conditions. C Bulk sample of 1995 tephra from the Mangatoetoenui Glacier. This material is representative of pyroclastic deposits remobilised by rain-triggered lahars. D Proximal 1995 tephra. E 17 June 1996 tephra deposited on the summit area of Ruapehu, coarse-grained and hence resistant to remobilisation

deposits (Blikra and Nemec 1998), but not to dynamic transformations between multiphase gravity flows of the type discussed in this paper. The term ªsyn-eruptiveº refers to processes synchronous with eruptive

activity; ªpost-eruptiveº refers to processes active following deposition of the tephra unit that is being reworked. Fig. 5 Ternary diagram shows the relative proportions of the three major components recognised in the reworked 1995±1996 Ruapehu deposits: tephra particles (T); snow and ice (S); and liquid water (W). Diagram to the right shows the positions occupied by various single-, two- and three-component processes. Inset triangles on subsequent figures indicate components involved in each reworking process. (An example is shown on the left.) Starting compositions are shown as open circles, transformations by arrows, and final compositions following interactions and transformations as filled circles. Circle diameter is proportional to the volume of the deposits

283 Table 2 Catalogue of tephra±snow±water interactions at Ruapehu and their dominant features Type of remobilisation

Transport distance

Volume of individual event

Primary lahars

4±>100 km

50 000±1”106 m3

Avalanches and ice falls

10±100 m

1±1000 m3

Eolian

10 m to many kilometres

Slope angle ()

Ash thickness (cm)

Distribution

Nature of hazard

Causes

Syn-eruptive

>34

Stream catchments Immediate heading near the summit

Explosive volcanism through Crater Lake

Crater basin, Whangaehu channel

Explosion shockwaves and undercutting by primary lahars

Immediate

Concentrated on ridges and steep slopes

Strong winds coincident with ash-producing eruptions; tephra admixed with simultaneous snowfall, or blown into depressions and sheltered areas

<2

Ubiquitous for tephra-on-snow

Diurnal freeze±thaw processes

>30

<4

Ubiquitous for tephra-on-snow

Diurnal freeze±thaw processes

>15±22

<2

Tephra-on-snow

Diurnal freeze±thaw processes

Post-eruptive Surface tension accretion

millimetres to centimetres

Downslope creep

millimetres to centimetres

Micro-debris flows

10 cm to 10 m

1±1000 cm3

>10 Eolian

10 m to many kilometres

Sheetwash/ rilling

10±100 m

Slumping, translational sliding

1±10 m

Rain-triggered lahars

Ash-induced avalanches

Heavy rain on thicker tephra Summit and ring-plain of Ruapehu

Delayed/ disjoint

Strong winds acting on dry tephra deposits

>24

>10

Tephra deposits on regolith substrates

Incision of tephra deposits by surface run-off

10±1000 m3

>15±22

>20

Thick tephra Delayed deposits above a detachment surface

Rainfall loading of tephra deposits

100 m to many kilometres

10±1”106 m3

15±39

>20

Thick tephra Delayed deposits above a detachment surface, large stream valleys

Triggered by heavy rainfall on steep, unstable tephra deposits; correlation between size and frequency as well as volume of tephra in catchment and intensity of rainfall; occurred in spring±summer

10±200 m

1±1”104 m3

53±67

>0.5

Steep slopes above Delayed/ 2000 m and within disjoint 3 km of crater

Buried tephra layers form weaknesses in snowpack. Failure triggered by freeze±thaw, explosion shockwaves, ground shaking

30±34

Syn-eruptive interactions Syn-eruptive mass flows of tephra, snow ice and water in various proportions were generated repeatedly by explosive volcanic activity. They included a variety of

11 km away on Ngauruhoe

events and processes such as lahars, avalanches and ice falls, triggered by a range of mechanisms (Table 2). The destabilising effect of buried layers of tephra in seasonal snowpack increased the frequency of subsequent eruption-triggered mass flows.

284 Fig. 6 Deposits of the 23 September 1995 eruption-triggered lahars on the eastern flank of Ruapehu. The lahar path to the left was generated by the displacement of water via the Whangaehu Valley outlet channel, the path to the right by the explosive ejection of crater lake water onto the Whangaehu glacier. See Fig. 5 for explanation of triangle. (Photo Courtesy of L. Homer)

Primary (eruption-generated) lahars

Avalanches and ice falls

Numerous lahars were generated in the catchment draining the Crater Lake (Fig. 6), and others, during the 1995 eruption sequence (Cronin et al. 1996, 1997a). These lahars, which consisted of mixtures of ice, snow, water and minor amounts of clastic material, formed by the entrainment of snow and ice by ejected hot water. Chemical analyses of frozen deposit samples indicate that these flows contained less than 5% lake water by weight, and were composed of 65±90% snow and ice particles, highlighting the importance of large volumes of unstable snowpack in their generation. The character and size of syn-eruptive lahars declined over time (Cronin et al. 1997a). The earliest explosion-triggered lahars were snow rich, whereas later lake-displacement flows were hyperconcentrated (e.g. Beverage and Culbertson 1964; Costa 1988). As the volume of Crater Lake declined, largely due to lahar outflows, subsequent large phreatomagmatic eruptions (Table 1) generated progressively smaller lahars (Fig. 7). Relatively high sediment concentrations in these flows were apparently maintained by the cannibalisation of clastic material from fresh lahar deposits. The resumption of explosive volcanism on 17 June 1996 produced a small lahar that flowed ~8 km down the Whangaehu Valley. Hot ballistic blocks also caused sufficient melting of snow to generate very minor, nonhazardous lahars in the crater basin (Keys 1996).

Volcanic activity during the 1995±1996 eruptions directly remobilised seasonal snow and glacial ice on Ruapehu at least 20 times, mostly above 2000 m elevation and within 3 km of the vent (Keys 1996). The largest and potentially most hazardous syn-eruptive events were hard slab avalanches triggered by the impact of ballistic blocks. Six such avalanches occurred in the crater basin during 17±18 June 1996, including one class 3+ event, i.e. harmful to humans and structures (Owens and O©Loughlin 1979). Numerous minor point-release avalanches and snow sloughs with volumes of 1±10 m3 were triggered by ballistic block impact throughout the 1995±1996 eruptions. Other hard slab avalanches and ice falls were caused by syn-eruptive lahars undercutting unstable snow slopes. Airwaves and/or ground shaking caused by explosions triggered at least one slab avalanche and numerous point releases in the crater basin, beyond the range of ballistic ejecta, on 26 July 1996. Slabs up to 2 m thick slid on a buried 1996 ash layer on steep slopes below the basin rim 600 m from the vent (Keys 1996). Eolian remobilisation Syn- and immediately post-depositional reworking of primary tephra deposits by wind depended on their

285 Fig. 7 Whangaehu River hydrograph record (continuous line; gauged at Karioi, 56.5 km downstream of Crater Lake) and rainfall data for the Ruapehu area (grey bars; measured at Makotuku Trig, 24 km west of Ruapehu) September to December 1995. Flood peaks associated with early eruption-triggered lahars are decoupled from rainfall events, whereas later rain-triggered lahars show a close correlation to rainstorms. (Data courtesy of National Institute of Water and Atmospheric Research)

Fig. 8 Pit dug on the southern slopes of Ruapehu. The buried tephra layers were erupted in June and July 1996 during northerly wind conditions. Layer X consists of wind-blown tephra admixed with simultaneous snowfall. The most voluminous 1996 tephra, erupted on 17 June, is absent here because it was erupted during a strong southerly wind. Surface tephra erupted 6 h earlier has been extensively reworked over >50% of its area by winds of 30±45 km/h making it impossible to measure original thicknesses. Eolian action has moved the tephra into valleys and depressions leaving extensive areas of bare snow

grain size (i.e. cohesiveness) and the weather conditions. Fine-grained phreatomagmatic deposits were less affected than coarser-grained and better-sorted ªdryº fall deposits that were often extensively reworked within hours of eruption. The reworking had two effects: 1. Ash was often admixed with, and diluted by, granules of ice eroded from the snowfields. 2. Ash preferentially accumulated in hollows and valleys, enhancing its potential for subsequent removal by stream erosion, and was thin or absent on intervening ridges (Fig. 8). Post-eruptive interactions Post-eruptive interactions at Ruapehu were complex events, involving tephra, snow and ice, and water, in various combinations and proportions, on differing slope angles and aspects, and in an environment passing from winter through spring and into summer (Table 2). The timescales and magnitudes of these interacting processes covered a wide range.

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Tephra deposit thickness on snow and ice surfaces exerted a major control on subsequent remobilisation processes. Dark-coloured, low-albedo tephra absorbs incident solar radiation more effectively than reflective light-coloured snow and ice, causing increased surface heating (Driedger 1981; Tagborn and Lettenmaier 1982); thermal conduction through thin tephra layers then causes accelerated melting of the frozen substrate. At Ruapehu, tephra layers <5 mm thick caused the fastest melting, whereas the poor thermal conductivity of layers >20 mm thick inhibited melting by insulating the frozen substrate. This resulted in the development of an inverse topography on the snow surface. Depressions that accumulated slumped and wind-blown tephra lowered more slowly than thinly covered ridges, whereas ballistically emplaced scoria blocks became perched on ice pinnacles by differential melting. Snowmelt also resulted in the delayed juxtaposition of tephra layers deposited on different days and originally separated by intervening snow falls (Fig. 8). Insolation-generated meltwater contributed to ash remobilisation, particularly during spring, and caused diurnal spikes in the discharge (Fig. 7) and turbidity of streams draining Ruapehu. Tephra deposits also enhanced precipitation run-off, causing more sharply peaked stream hydrographs after rain than were the case before eruption.

Diurnal freeze-thaw, surface tension accretion, downslope creep, miniature mudflows Fine-grained tephra deposits <3 mm thick developed a distinctive ªcurd textureº by a variety of processes (Fig. 9a). Insolation-induced melting of the snow substrate dampened the tephra layer, causing it to accrete into centimetre-size globules of wet sticky ash due to surface tension (Driedger 1981). Downward migration of fine ash particles led to a diffuse tephra±snow contact. During the winters of 1995 and 1996, diurnal solifluction processes acted together with surface-tension accretion on slopes steeper than ~10 to cause downslope creep of tephra deposits. Fig. 9a±d Examples of TSW interactions on Ruapehu. a ªCurd textureº developed by surface tension accretion in a 5-mm-thick saturated ash layer, Tukino skifield, September 1995. Scale divisions 10 cm. b Miniature mudflows developed by meltwater-induced segregation within September 1995 ash deposits near Tukino. Spade 1 m long. c Mini-debris flow generated by rainfall saturation and remobilisation of 20-cm-thick October 1995 tephra deposits near Tukino. The small flow rests on the surface of a much larger debris flow deposit remobilised from the Mangatoetoenui Glacier. Ice axe is 80 cm long. d Young rill systems developed in 10-cm-thick tephra blanketing a low ridge near Tukino skifield. Note zone of sheetwash and/or no erosion near ridge crest, nucleation of rills below boulders, and incision of the deepest rill into the underlying regolith. Scale divisions 10 cm

287

More intense melting of the underlying snow pack completely saturated some thin fine-grained ash deposits, which flowed downslope as miniature mud flows (Fig. 9b). These displayed all the characteristics of large-scale debris flows, including U-shaped channels, lateral levees, a thin veneer deposit, low-shear centres, pulsing flow and lobate termini (Pierson and Costa 1987; Costa 1988); however, the smallest examples were only 1±2 cm wide and 1±2 m long, and the largest only a factor of 10 bigger (Fig. 9c). These flows caused size fractionation of the tephra deposits, transporting the fines and leaving lags of coarser particles. Flow ceased when channel slopes decreased, or when sediment concentrations became too high due to the entrainment of more material or the percolation loss of water into a permeable snow or regolith substrate. Eolian remobilisation Strong winds at altitude on Ruapehu in 1995 and 1996 modified tephra deposits, drying surfaces and locally removing sand-grade and finer ash by deflation. Mobile sandy ash was blown into small dunes and ripples in the lee of boulders, or piled against their upwind faces. Airborne ash plumes formed on several occasions during March 1997, extending up to 100 km from Ruapehu and becoming a hazard to aircraft. Sheetwash/rilling Sheetwash occurred on relatively impermeable finegrained tephra layers and on the surfaces of lahar deposits. A resistant crust developed on the surface of these deposits due to rain-beat compaction and the washing of fine ash into interstices. Cementation by the precipitation of volatile sulphur compounds and other leachates that migrated towards the surface during wetting and drying cycles contributed to crust formation. Sheetwash was generally confined to areas near the crests of ridges (Fig. 9d). Further downslope rills formed where erosion was focused by flow-concentrating irregularities, such as boulders or flow velocities, and depths became sufficient (Horton 1945). Rills commonly developed within a few days of tephra deposition. Once established, rills developed the standard pattern, becoming wider, deeper, rarer and more widely spaced downslope (Horton 1945; Schumm 1956). Run-off channelled through the largest rills eroded into the regolith, incorporating older preeruption material, and forming small fans at the toe of the slope. Stream erosion and reworking The impacts of the 1995±1996 eruptions varied considerably between different streams. Eastern catchments

were totally blanketed with tephra, resulting in high suspended-sediment loads, acidification, and contamination with potentially toxic ash leachates. Consequently, aquatic biotas were suppressed for over a year with significant fish kills. Streams on the west side of the mountain, although impacted by primary lahars, recovered much more rapidly since the total volume of pyroclastic material within the catchments was one to two orders of magnitude less. Large volumes of ash entered the Tongariro River via rain-triggered secondary lahars in the Mangatoetoenui and Wahianoa streams (Fig. 10). Lahars in the former (Hodgson and Manville 1999) entered upstream of the Rangipo hydroelectric facility. Stream reworking of this material repeatedly infilled the Rangipo intake basin, produced channel-bed aggradation and sand-bar formation, and caused substantial abrasion to the power station (Malcolm and van Rossen 1997). Further downstream, high suspended-sediment loads and migrating sandy bedforms affected the world-renowned trout fishery and resulted in a turbid inflow to Lake Taupo. Within Lake Taupo, tephra settling through the water column scavenged suspended organic matter and capped bottom sediments leading to a short-term improvement in water quality. Early lahars aggraded the channel of the highly acidic Whangaehu River on its fan, raising the possibility of an avulsion northwards into the Upper Waikato catchment and hence into the Tongariro, before re-incision stabilised the course of the river. Tephra-induced snow avalanches These complex remobilisation events were controlled by inter-relationships between eruption timing, nature of the tephra, stability and other characteristics of the underlying or overlying snow, slope angle and aspect, and meteorological conditions (Keys 1996). Possible avalanche mechanisms were evaluated using data obtained from 30 snow pits and weather and aspect comparisons. Thirty-one avalanche events were recorded, mostly during the 1996 eruption sequence. Almost all occurred above 2500 m altitude on Ruapehu, within 3 km of the active vent, and on north-facing slopes (Fig. 11); however, two slab avalanches occurred 11 km away on the cone of Ngauruhoe with two more small events on Tongariro volcano, following snowfall on tephra deposits from the 17 June 1996 eruption. Tephra-induced snow avalanches typically involved <10±1000 tonnes of snow and many failed in, on or just above buried tephra layers within 14 days of deposition. Shear strength tests indicated that such layers formed severe weaknesses in the snowpack. Weakening or softening of snow occurs by grain metamorphism and the disruption or weakening of bonds between grains (Conway and Raymond 1993). Depression of freezing points by soluble salts present in the

288 Fig. 10 Distribution, sources and timing of major rain-triggered lahars following the 1995 eruptions of Ruapehu. The 1995 tephra isopach map highlights the correlation between the thickness of fall material and the number and frequency of remobilisation events

tephra layers also weakened the snow pack. Freezing points of ±0.50.1C to ±1.30.5C were measured for some 1996 tephras (Keys 1996). Conversely, meltwater refreezing, rime formation, and grain metamorphism were stabilisation processes active during the 1996 winter. Rain-triggered lahars The scale and frequency of tephra/snow ice/water interactions and remobilisation events increased following the major tephra-producing eruptions at Ruapehu. This was due to (a) increased volumes of tephra available for erosion, and (b) climatic factors, i.e. rising air temperatures during spring causing

increased snowmelt and rainfall rather than snowfall. Many debris flows were initiated by the failure of water-saturated tephra deposits (c.f. Caine 1980; Neary and Swift 1987; Rodolfo and Arguden 1991), with ~34 rain-triggered lahars distributed across eight catchments in the period October 1995 to May 1996 (Fig. 10). The largest of these flows in the Whangaehu Valley were comparable in peak discharge and volume with earlier eruption-generated lahars (Fig. 7). The magnitude and frequency of rain-triggered lahars in a particular valley was directly related to the thickness of tephra in the source area (Fig. 3), with the largest flows occurring early in the sequence. Subsequent flows became progressively smaller and required longer and more intense rainfall episodes to generate them, particularly in catchments that had already

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Fig. 11 Tephra-induced avalanches generated in Crater Basin in July 1996, following burial of 17 June 1996 fall deposits by fresh snow. See Fig. 5 for explanation of triangle

experienced numerous flows. Some events were apparently caused by the delayed failure of tephra deposits saturated on the previous day (i.e. the 30 October 1995 Whangaehu lahar), possibly by the addition of meltwater from the snow and ice substrate. Initiation mechanisms Examination of the source areas of rain-triggered lahars indicates that remobilisation occurred by the sliding of tephra fall deposits above an internal detachment surface. This usually coincided with the contact between basal fine-grained cohesive September 1995 ash layers (which remained largely frozen to the substrate) and overlying coarser October 1995 tephra. A lag of wet and unsorted tephra slurry left on failed surfaces developed a secondary slope-parallel pattern of corrugations. Field tests reproduced this pattern of contact-zone failure in undisturbed tephra deposits adjacent to failed areas. After detachment, the tephra blanket moved slowly downslope as a translational slide sheet for 1±2 m before breaking up, liquefying and continuing as a debris flow. During rainfall, failure occurred when water infiltrated the tephra deposits, creating a saturated zone above the basal fine-grained September 1995 ashes. Eventually, failure was triggered by a combination of rising shear stresses (saturation-induced bulk density increase) and falling tephra strength (positive porefluid pressures, reduced surface-tension adhesion between grains, reduced solid:fluid ratio). During the main remobilisation events, discrete flows coalesced and entrained further material before entering channels and leaving the volcano via major stream valleys (Fig. 10). These lahars then diluted and transformed downstream to hyperconcentrated flows and sedimentladen streamflows (Pierson and Scott 1985; Smith and Lowe 1991; Hodgson and Manville 1999).

Fig. 12 Aerial view of the eastern crater rim area shows deep incision and gullying of the rock wall and summit glaciers by explosively ejected crater lake water and debris, 3 October 1995

Impacts on snowfields and glaciers The 1995±1996 eruptions had a significant impact on the summit ice cap and valley glaciers of Ruapehu. On its eastern side the crater rim was thinned and lowered by cascading water and other debris ejected from the lake, and the Whangaehu Glacier was gullied and pot-holed (Fig. 12). Lahars in the Whangaehu Valley unroofed the former channel through the glacier, cut a deep canyon, and caused retreat of the adjacent Tuwharetoa Glacier. Waves on Crater Lake undercut the Crater Basin Glacier, which now terminates in an ice cliff backed by a heavily crevassed zone. Thin tephra layers over the rest of the summit have caused enhanced melting of seasonal snowpack, reducing the size of all six valley glaciers, whose recovery has been hindered by poor 1997 and 1998 snowfalls.

Sediment yields Significant accumulation of tephra during the 1995±1996 Ruapehu eruptions was largely confined to areas underlain by seasonal and perennial snow and ice. Initial remobilisation of this material was mostly accomplished by large-scale slope failures which directly transferred huge volumes of material into permanent stream channels from where it could be rapidly transported away (Cronin et al. 1997a; Manville et al. 1998). The absence of pre-1995 tephra fall deposits on Ruapehu suggests that 100% of material deposited on seasonal and permanent snow and ice will be remobilised, compared with approximately 15% of material accumulated on other substrates (e.g. Collins et al. 1983; Collins and Dunne 1986; Leavesley et al. 1989); therefore, the volume of material avail-

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able for remobilisation constitutes 100% of the proximal tephra and 15% of the distal ash. Estimates of tephra erosion rates are based on stream discharge hydrographs from the Whangaehu River (Fig. 7) and data from the Tongariro hydroelectric power scheme. In the Whangaehu catchment, multiple rain-triggered lahars alone remobilised an estimated 7.0”105 m3 of tephra between 28 October and 1 November 1995, and ~1.6”105 m3 between 21 and 22 April 1996. The remobilisation of approximately 1”106 m3 of tephra over a 6-month period constitutes ~17% of the available material (15% of total tephra volume deposited in catchment). Fitting an exponential decay curve to these figures (e.g. Graf 1977; Schumm and Rea 1995) gives an estimated sediment yield half-life of just over 2 years. The sediment yield from the Mangatoetoenui catchment was gauged from the rate and frequency of infilling of a hydroelectric settling basin on the Tongariro River 21 km downstream (Fig. 1). Most tephra entering this basin was derived from the 28 October Mangatoetoenui rain-triggered lahar (Hodgson and Manville 1999), with only minor contributions from other catchments. The 0.35”105 m3 settling basin filled three times between September 1995 and May 1996. Allowing for material that bypassed the basin, either through the power station or while the intake was closed, the sediment yield from the catchment is estimated at ~2.4”105 m3 for the 6 months following the 1995 eruptions. Fitting an exponential decay curve to these values, i.e. 20% of the material available for remobilisation moved in 6 months, gives a sedimentyield half-life of ~1.5 years. Proximal tephra deposits from the 1996 eruptions are generally coarser, more permeable and harder to erode than those from 1995 (Fig. 4). Furthermore, the thickest (up to 1 m) and coarsest deposits accumulated on the relatively flat and protected northern summit plateau (Fig. 3). Only small lahars a few hundred metres long were produced in two catchments by heavy rain in January 1997, with minor events in two further streams in April 1997. This suggests that the 1996 tephra may have a longer residence time on Ruapehu.

Preservation potential of the products of the 1995±1996 eruptions The preservation potential of the 1995±1996 pyroclastics is considered to be low, except locally as a discrete millimetre- to centimetre-thick layer on the proximal ring plain (Donoghue et al. 1997) and in the sediments of Lake Taupo and adjacent water bodies, and as deposits in the Tongariro delta. On the upper slopes of the volcano scattered scoria bombs and tephra layers may be preserved in glacier ice; however, of the material erupted during the previous three largest Ruapehu eruptions this century, in 1945, 1969

and 1979 (Healy et al. 1978; Nairn et al. 1979; Hackett and Houghton 1989), only ballistic blocks are still identifiable. Further back in time, the post-181 A.D. eruption record comprises a sequence of approximately 19 andesitic tephra layers (Palmer et al. 1993; Donoghue et al. 1995), apparently from considerably larger eruptions than the historical ones. Lahar deposits are also unlikely to be preserved. Eruption-generated ªsnow-slurryº lahar deposits melted during the 1995±1996 summer, leaving porous ashy deposits a few centimetres thick that were rapidly washed away by rain. Lahars in eastern catchments resulted from much smaller eruptions than those associated with lahar deposits in the stratigraphic record (e.g. Hodgson 1993; Cronin and Neall 1997), and their channel-confined deposits were rapidly reworked (Cronin et al. 1997b).

Discussion The sedimentary responses to eruptions at snow-covered volcanic cones reflect the complex interplay of a variety of factors. These include eruption duration and intensity, physical properties and grain size of the tephra, meteorological conditions, and the architecture and hydrology of the volcano. The number and frequency of remobilisation events at Ruapehu was strongly correlated with primary tephra thicknesses, which were partly a function of syn-eruptive weather conditions. The contrast in grain size and hence physical properties between early fine-ash-rich Surtseyan ejecta and later subplinian ashes also exerted a strong influence on subsequent remobilisation processes (Fig. 4). Environmental factors associated with the volcano included slope angle, aspect, and nature of the substrate. Climatic parameters included such variables as mean temperature, diurnal temperature range, insolation and variations in the intensity and nature (i.e. rain vs snow) of precipitation. Some processes only became active with time as tephra thicknesses increased, or with the changing seasons (Fig. 13). Estimates of sediment yield from streams draining the volcano suggest that rates are declining rapidly, allowing for the complication of summer±winter climatic cycles. Evidence from the most strongly affected catchments indicates that nearly all 1995 tephra will be rapidly removed from perennial ice and seasonal snow near the summit, mostly by large-scale processes such as rain-triggered lahars. Sediment-yield half-lives for whole catchments are in the range 1.5±2 years, with remobilisation 90% completed within 5±7 years. The different physical properties and distribution of 1996 tephra apparently give it a longer residence time on the volcano.

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Fig. 13 Summary diagram for seasonal/geographical variations in tephra±snow±water interactions at Ruapehu including tephra remobilisation processes

Hazardous sedimentary responses to volcanism Previous assessments of potential hazard at snow-clad volcanoes have focused on the primary hazards associated with syn-eruptive melting of summit ice caps (e.g. Major and Newhall 1989; Pierson and Janda 1994; Pierson 1995). At Ruapehu, hazard mitigation was predicated around the hazards arising from phreatomagmatic eruptions through the crater lake (Houghton et al. 1987); hence, the development of warning schemes for primary, eruption-triggered lahars. The events of 1995±1996 show that significant additional hazards exist during, and following, prolonged eruption episodes on composite volcanoes. We consider these as delayed or ªstoredº hazards which do not coincide with specific eruptions events, and/or as disjoint hazards that extend beyond the range of primary eruption hazards. An extreme example is from late Pleistocene lahar deposits in central Mexico (Siebe et al. 1999). There, remobilisation of pyroclastic deposits from the slopes of Cerro Tlµloc was delayed until the onset of deglaciation, approximately 3000 years after their eruption from PopocatØptl 40 km to the south. Delayed hazards Rain-triggered lahars were a previously unrecognised class of hazard at Ruapehu, and yet, in 1995±1996, the cost of damage associated with such delayed events was far larger than for eruption-triggered lahars. Remobilisation of tephra by rain-triggered lahars and during the 1995 spring melt cost the hydroelectric industry in excess of $US 9 million in equipment damage and lost generation (Malcolm and van Rossen

1997). Lahars impacted the Whangaehu River channel, degrading the channel by 5 m in some places and aggrading it by up to 2 m in others. Track footbridges on the mountain were destroyed on four separate occasions, once by an eruption-triggered lahar and three times by rain-triggered events. Future releases of tephra during heavy rainfall will continue to be a problem. Tephra-induced avalanches added to alpine hazards on Ruapehu, due to the destabilising influence of ash layers buried in the seasonal snowpack. In the future, such hazards could arise following an early season eruption whose products are buried by later snowfall, or even during the following winter. Avalanches triggered by seismic activity are a particular risk: the severity and longevity of this hazard will depend on the original ash thickness, and the preceding and subsequent snowfall, rainfall and insolation history. Avalanche hazards will persist until the snowpack becomes stabilised or the tephra layer is removed. Tephra-contaminated water is a hazard to downstream infrastructure and populations. Remobilisation of tephra from normal substrates is generally rapidly accomplished, but melting of snowpack containing buried tephra layers may cause problems for years to decades after the initial eruption. Disjoint hazards The sedimentary responses outlined in this paper relate to the remobilisation of tephra deposits; therefore, they can extend into areas unaffected by eruption-triggered lahars or proximal hazards such as ballistic blocks or explosions. For example, distal ashfall may destabilise or contaminate snowpack over a huge area downwind of an erupting volcano, or result in rain-triggered lahars on mountains other than the active volcano. Eolian remobilisation of fine-grained tephra can be hazardous to aviation. The area affected by disjoint hazards increases with eruption magnitude

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as tephra deposits become thicker and more widespread. Acknowledgements The authors acknowledge the contributions of other members of the Institute of Geological and Nuclear Sciences Ruapehu Surveillance Group. Funding was by the Foundation for Research Science and Technology, New Zealand, contracts C05516 (Manville, White and Houghton) and C05430 (Hodgson). The Electricity Corporation of New Zealand and B. Waugh and G. Wilson (National Institute of Water and Atmospheric Research) supplied rainfall and hydrograph data. The paper benefitted from reviews by R. Waitt Jr, J.-C. Thouret, T. Pierson and N. Riggs, and from GNS Scientific Contribution No. 1970.

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