Geomorphology 132 (2011) 327–338

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Geomorphology

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Effects of rock avalanches on glacier behaviour and moraine formation

Natalya V. Reznichenko a,⁎, Tim R.H. Davies a, David J. Alexander b a Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand b School of Geography, Planning and Environmental Management, University of Queensland, QLD 4072, Brisbane, Australia article info abstract

Article history: Although large rock avalanches are infrequent, sediment production in active orogens is dominated by such Received 10 January 2011 events, which may strongly influence geomorphic processes. Where rock avalanches fall onto glaciers, they Received in revised form 24 May 2011 may affect glacier behaviour and moraine formation. We outline the processes of rock avalanche initiation and Accepted 25 May 2011 motion, and show that supraglacial deposits of rock avalanche debris have distinct sedimentological and Available online 1 June 2011 thermal properties. Laboratory experiments on the effects of such debris on ice ablation are supplemented by field data from two rock avalanches in the Southern Alps, New Zealand. Their effects are compared with those Keywords: Rock avalanches of the thinner supraglacial debris that results from small rockfalls and melt-out of englacial debris. Supraglacial debris Implications of rock–avalanche debris cover for glacier behaviour are explored using a mass-balance model of Ice-surface ablation the Franz Josef Glacier in New Zealand, demonstrating a likely supraglacial rock avalanche origin for the Glacier advances Waiho Loop moraine, and considering the potential hazard of a large rock avalanche onto the present-day Terminal moraines glacier. Paleoclimatology and hazards © 2011 Elsevier B.V. All rights reserved.

1. Introduction established the importance of such events in the sediment delivery and geomorphology of active orogens. Where glaciers are present in In this paper we examine the effects of the deposits of large rock mountain valleys, some rock avalanches inevitably fall onto them; avalanches on glacier behaviour and moraine formation. Supraglacial however, supraglacial rock avalanche deposits appear to be less rock avalanche deposits on the glacier ablation zone reduce ice- frequently reported than those that do not fall onto glaciers. This is surface ablation, and this can significantly alter the overall glacier partly because of the limited extent of present-day glaciers and also, mass balance. With a sufficient proportion of the ablation zone significantly, because supraglacial rock avalanche deposits are rapidly covered by rock avalanche debris the glacier mass balance will reworked by glaciers and deposited as moraines whose potential become significantly less negative, and, as a result, the terminus may rock–avalanche origins have rarely been considered in the past. By advance. This advance may generate a terminal moraine that has no contrast, a rock avalanche depositing into a glacier-free river valley, climate-related cause. The much-debated Late-Glacial Waiho Loop though progressively removed by river erosion, often leaves long- moraine, the Franz Josef Glacier, New Zealand (Barrows et al., 2007; term recognisable traces of its initial extent in the form of remnant Tovar et al., 2008), appears to be the result of such a sequence of large angular boulders and vestigial high terraces (Chevalier et al., events and we use it as an example to demonstrate how a large rock 2009; Lee et al., 2009). Thus, more likely that the frequency of avalanche deposit can reduce ablation causing a glacier to advance supraglacial rock avalanches and, hence, their significance to glacier and form a prominent frontal moraine. dynamics and behaviour have in the past been underestimated. Rock avalanche deposits on glaciers and their effects have received Notably, many recently identified rock avalanche deposits, for particular attention only recently. The increasing frequency of example those studied by Hewitt (2009) in the Karakoram Himalaya, recognised historical and prehistorical catastrophic rock avalanche had earlier been interpreted as moraines and corresponding (incor- deposits in high mountains areas worldwide (e.g., Post, 1967; rect) paleoclimatic inferences drawn (see also McColl and Davies, Whitehouse and Griffiths, 1983; Hewitt, 1998, 2009; Hermanns 2011; Kariya et al., 2011). One of the conclusions of the present work et al., 2001; Geertsema et al., 2006; Allen et al., 2011), together with is that prominent moraines can be generated by rock–avalanche- well-defined magnitude–frequency relationships that indicate that driven glacier advances; these cannot presently be distinguished from very large events contribute more volume over time than do medium- moraines reflecting climatically driven advances, and their inclusion sized ones (Malamud et al., 2004; Korup and Clague, 2009) has in moraine age data may also have led to incorrect paleoclimate interpretations. In this work we highlight recently-published information about ⁎ Corresponding author. Tel.: +64 3 364 2987x7697; fax: +64 3 364 2769. the properties of rock avalanche deposit, their effects on glacier E-mail address: [email protected] (N.V. Reznichenko). ablation, and glacier response to reduced ablation. We report an

0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.05.019 328 N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 investigation of the current state and effects of two recent rock submicron diameter; large quantities of fine particles are expelled avalanche deposits on glaciers in Southern Alps of New Zealand, and from the surface to form a growing aerial dust cloud. By the time it develop a conceptual picture of the response of mountain valley reaches the glacier surface, the mass is pervasively fragmenting and glaciers to emplacement of extensive rock avalanche debris on the travelling at high velocity. The debris then moves rapidly across the ablation zone. Mass-balance modelling of the Franz Josef Glacier ice surface, spreading both laterally and longitudinally as it does so response to severely reduced ablation also demonstrates the potential and becoming thinner in the process. A rock avalanche travelling of a large rock avalanche to substantially advance the present-day across a glacier continues to comminute until it halts, as demonstrated terminus of this glacier, with following significant disruption to by the frequent presence of distally located, shattered but undisag- society and tourism. gregated clasts; these are the result of fragmentation and, if formed more proximal, would have been disaggregated by the shear strain 2. Rock avalanches onto glaciers field in the avalanche (e.g., McSaveney, 1978). Often evidence is found that the ice-surface has been eroded by the rock avalanche passage, 2.1. Causes which contributes mass to the emplaced deposit (e.g., McSaveney, 1978, 2002). A rock avalanche is a large-scale rock slope failure (of the order of a million cubic metres or more) that leads to a mass of fragmenting rock 2.3. Deposits travelling across a landscape at velocities up to 100 m/s (Voight and Pariseau, 1978; Fort et al., 2009; Hewitt, 2009). Failures of this scale Supraglacial rock avalanche deposits are usually much thinner are frequently triggered by major earthquakes, but a seismic trigger is relative to their volume than their nonsupraglacial counterparts, and not necessary; for example, the 1991 Aoraki/Mt. Cook, 1991 Mt. thus larger in area. This is the result of the lower basal friction of rock Fletcher, 1999 Mt. Adams, and 2007 Young River landslides in the avalanches that travel over snow and ice (Post, 1967; McSaveney, Southern Alps of New Zealand between 1991 and 2007 were all about 1975, 2002; Gordon et al., 1978; Evans and Clague, 1988; Jibson et al., 107 m3 in volume and neither seismically- nor rainfall-triggered 2004, 2006; Lipovsky et al., 2008; Deline, 2009; Hewitt, 2009), which (McSaveney, 2002; Hancox et al., 2005; Allen et al., 2011). Relatively allows a given volume to spread over a greater area. For example, the few large-scale rock-slope failures appear to be triggered by rainfall average thickness of the 40×106 m3 Sherman Glacier rock avalanche (Melosh, 1987; Soldati et al., 2004). Contrary to convention, recent of 1964 (Fig. 1) was about 1.5 m (McSaveney, 1978), whereas the work suggests that loss of ice buttressing of valley slopes during and thickness of the majority of reported nonsupraglacial rock avalanches following deglaciation may have been overemphasised as a cause of thicknesses varies between 10 m and 60 m or more (Smith et al., the apparently high frequency of large rock avalanches in the early 2006). This characteristic allows a rock avalanche to cover a very large Holocene (McColl et al., 2010). area of glacier with a debris carpet some metres thick. As a result of the intense fragmentation that occurs during fall and 2.2. Processes runout, the debris of rock avalanches is predominantly extremely fine. For example, McSaveney and Davies (2007) found that 90% by weight A rock avalanche usually initiates as the detachment of a large (N99% by number) of the surface debris of the 1991 Aoraki/Mt. Cook mass of rock from the upper part of a mountain edifice by completion rock avalanche, New Zealand, was b10−5 m in diameter. The coarser of a through-going failure surface. To generate the highly-fragmented (metre-scale) “carapace” layer is usually at the debris surface (caused debris described below, the volume needs to be in the order of 106 m3 by less intense fragmentation at the surface because of lower or more (Davies and McSaveney, 2009). The detached mass initially confining pressure); so the surface appearance is often blocky, but slides under gravity down the mountainside, progressively collapsing most of the underlying debris is extremely fine, with larger clasts of all as it does so into joint-controlled blocks. However, at an early stage, sizes forming a fractal grain size distribution (Fig. 2). The bulk density progressively smaller intact (i.e. uncracked) rocks will start to of the subsurface debris is high caused by the wide grain size comminute (or fragment; e.g., Davies and McSaveney, 2009) under distribution, and the voids ratio and permeability are correspondingly the high stresses of the fall, generating fragments of all sizes to low, compared to those of other types of supraglacial debris. As a

Fig. 1. The Sherman Glacier, Alaska, as seen on 27 August 1963 (left) and with rock avalanche deposit on 24 August 1964 (Post, 1965). N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 329

Fig. 2. Examples of rock avalanche deposits: (A) basal exposure of the nonsupraglacial 1929 Falling Mountain Rock avalanche, NZ; and (B) rock avalanche debris about 2 m thick on the terminal surface of the Mueller Glacier, Southern Alps, NZ. result, rock avalanche debris has high thermal mass and inertia that is, 3.1. Melt-out debris it requires a large quantity of heat to increase its temperature, so it heats up slowly compared to debris with large volumes of air-filled The ablation zones of many mountain valley glaciers are covered voids. This gives rock avalanche debris high insulation capacity, with extensive supraglacial debris. This results mostly from the melt-out compared to the thin melt-out debris common on valley glaciers, as of englacially or subglacially transported debris originating from we now describe. subglacial or extraglacial sources (Rogerson et al., 1986; Winkler, 2009); it is commonly several to a few tens of centimeters in thickness and characterized by angular, coarse clasts with a small proportion of 3. Supraglacial debris: Effects on ablation fines and, as a result, high permeability (Drewry, 1986), low bulk density and relatively low thermal inertia. Melt-out debris increases in The major direct effect of a rock avalanche deposit on a glacier is to thickness toward the terminus from lateral displacement and continued protect the ice surface beneath the debris from solar radiation, emergence of englacial debris and if it achieves sufficient thickness can resulting in reduced surface ablation beneath the deposit (Østrem, reverse the increasing rate of clear-ice ablation toward the terminus 1965; McSaveney, 1975; Lundstrom et al., 1993; Mattson et al., 1993; caused by reduction in glacier surface elevation. The high debris porosity Nakawo and Rana, 1999; Brock et al., 2010; Reid and Brock, 2010; favours air and water circulation, reducing the insulation effect of the Reznichenko et al., 2010). The heat flux through the debris layer debris; for example, Sharp (1949) found that coarse debris 15 cm deep depends on its thickness and physical properties (lithology, bulk has the same insulating effect on ice as sand 3 cm deep. Debris-covered density or porosity, albedo, moisture content, and permeability; e.g., glacier snouts usually are very rough, so a several-centimetres-thick Nakawo and Young, 1981, 1982; Kayastha et al., 2000; Nicholson and debris layer forms a mosaic with thin layers of debris, clear patches of Benn, 2006), and determines the ice-surface melt rate. Where a large the ice, and exposed ice cliffs. Differential melting amplifies the rock avalanche deposit covers a sufficient proportion of the ablation topographic unevenness of the glacier and complicates the estimation zone with debris of the order of metres thick, surface-ice ablation will of total melting rates under this cover (Fig. 3). almost completely cease and, as a result, the dynamics of the covered glacier will be significantly affected (Shulmeister et al., 2009). 3.2. Rock avalanche debris Ablation within or beneath the ice will be unaffected by surficial debris, but this is usually a small component of total ablation (of the The effect of rock avalanche debris on underlying glacier ice is order of 10%; Alexander et al., in review). distinctly different from that of melt-out debris owing to its greater

Fig. 3. The debris-covered lower Tasman Glacier, Southern Alps, NZ. (A) The rough topography of the debris-covered glacier about 8 km from the modern terminus with well- developed glacier ‘karst’; the Hochstetter icefall is the steep tributary glacier in the background. (B) Differential ablation from discontinuous debris cover where the Hochstetter ice stream flows into the Tasman Glacier. Note the general thinness of the debris cover. 330 N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 thickness (meters) and thermal inertia (because of the fractal grain Our study showed that sandy supraglacial cover 5 cm thick size distribution and high proportion of very fine grains). The effect of (representing melt-out debris) reduced melting to 50% of that with high thermal mass is to require a large quantity of heat to be input bare ice under same conditions, and 9 cm of sand reduced ablation to before the mass can achieve steady-state heat transfer, so this takes a 25% (Fig. 5A). Using rock avalanche debris instead of sand under long time. If this time exceeds the diurnal radiation cycle time, steady- diurnally-cyclic radiation (Reznichenko et al., 2010) increased the state heat transfer to the underlying ice-surface is never achieved insulating effect substantially. The presence of rock avalanche debris (Reznichenko et al., 2010). The major effect of a thick rock avalanche also significantly reduced rainfall-induced ice-surface ablation, deposit on a glacier ablation zone is thus to dramatically reduce the whereas this effect did not occur with sand (Fig. 5B). surface ablation beneath the debris by comparison with adjacent bare In summary, rock avalanche debris cover on a glacier ablation zone or thinly-debris-covered ice (see next section). This leads to dramatically reduces total ablation beneath the debris; the reduction thickening of ice (in fact, a reduction of ice thinning) under the is much less under normal (melt-out) debris cover. deposit relative to neighbouring clear ice, which begins immediately and is often clearly evident a year after emplacement of the deposit. If 3.4. Field studies the deposit thickness exceeds 2–3 m, the underlying ice-surface ablation reduces effectively to zero (Bozhinskiy et al., 1986). In this In order to compare the laboratory results of Reznichenko et al. situation the rate of relative ice thickening beneath the deposit equals (2010) with effects in the field, we surveyed rock avalanche deposits the surface ablation rate of clear ice at that location. from 1991 Aoraki/Mt. Cook and 2004 Mt. Beatrice on the Tasman and A rock avalanche deposit also contributes a large mass of sediment Hooker Glaciers, respectively (Southern Alps of New Zealand, Fig. 6). to the debris transportation system of the glacier almost instanta- Both avalanches were typical of aseismic slope mass failures in the neously, whereas the gradual exhumation of melt-out debris cover Southern Alps (McSaveney, 2002; Cox et al., 2008). takes place slowly and quasicontinuously without adding mass to the Ground penetrating radar (GPR) surveys of these deposits were system. carried out to measure the effect of rock avalanche debris on the If the glacier surface is crevassed, part of the rock avalanche surface-ice ablation. GPR images of two perpendicular profiles along deposit may enter the sub- and englacial transportation systems of the rock avalanche deposit and adjacent clear or debris-covered ice the glacier (Hewitt, 2009). However, most of the debris remains provided information on the thickness of the rock avalanche deposits, supraglacial and will eventually be carried to the terminus by glacier the elevation of the underlying ice surfaces, the modification of the flow (e.g., the Sherman Glacier, Alaska; Figs. 1 and 4). deposits since emplacement, and the interaction of the rock avalanche deposits with adjacent clear or debris-covered ice. 3.3. Insulation mechanisms 3.4.1. Mt. Beatrice rock avalanche deposit In order to study the role of the debris cover in insulating the The 2004 rock avalanche from Mt. Beatrice (2528 m) was underlying ice from melting, we conducted laboratory experiments in emplaced onto the debris-free part of the ablation zone of the Hooker which ice blocks with clear surfaces and identical sizes covered with Glacier, close to an earlier deposit that presumably originated from either medium sand (representing typical highly permeable melt-out the same source (Fig. 6A). Satellite images show that between 2004 debris) or rock avalanche debris were exposed to identical radiation, and 2009 the deposit moved about 800 m down the valley, indicating both steady and diurnally-cyclic (Reznichenko et al., 2010). These an average ice flow velocity of about 160 m/y at that location, which is experiments showed that the insulating effect of debris on ice similar to previously estimated velocities at the same altitudes depends on the occurrence of cyclic radiation input. For example, (higher than 1200 msl.) on the Tasman Glacier (Quincey and Glasser, under specific laboratory conditions with continuous, steady-state 2009). radiation, sand cover greater than 5 cm thick delays the onset of ice- In December 2009 we obtained the longitudinal and cross-profiles surface melting by more than 12 h as the sand layer warms up, but of the Mt. Beatrice deposit shown in Fig. 6A. Profiles revealed that five after that heat transfer through the debris is steady and ablation rates years after emplacement an elevated ice platform, varying between 20 become similar to those for bare ice. By contrast, when realistic and 30 m above the adjacent upstream and downstream ice surfaces, diurnal cycles of radiation are imposed the debris cover continues to respectively, was present under the 3–7 m thick rock avalanche reduce the rate of ice melt over the long term (Fig. 5A). deposit (Figs. 6 and 7), indicating an average clear-ice ablation rate of

Fig. 4. The 1964 Sherman Glacier rock avalanche deposit 44 years after emplacement on 2008: (A) the rock–avalanche cover travels supraglacially; and (B) and is carried to the terminus by glacier flow, where it is dumped to form a moraine (photos by M.J. McSaveney). N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 331

Fig. 5. (A) Example of surface lowering of bare ice and ice under 1, 5, 9, and 13 cm of debris cover under diurnal-cycle conditions. (B) Comparison of the surface lowering of the bare ice, ice under debris cover of sand and rock avalanche material 9 cm deep with diurnal-cycle conditions and diurnal rainfall effect (Reznichenko et al., 2010).

Fig. 6. Location of the two rock avalanche deposits where GPR studies were conducted (red lines indicate GPR profiles, black arrows indicate ice flow directions, satellite image of 2001, LandSat): (A) Mt. Beatrice (2004) rock avalanche deposit on the Hooker Glacier, 2007 (aerial photo by S. Winkler); and (B) Aoraki/Mt. Cook rock avalanche (1991) deposit on the Tasman Glacier with Hochstetter and Tasman ice indicated (satellite image 2009, ASTER). 332 N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338

Fig. 7. GPR profile along the 2004 Mt. Beatrice rock avalanche in December 2009; the rock avalanche deposit thickness reaches about 7 m, and relative ice thickening is about 20 and 35 m at the upstream and downstream sides of the deposit, respectively. (A) The Mt. Beatrice rock avalanche on the Hooker Glacier deposit 5 years after emplacement; and (B) The~20-m-high upvalley slope of the deposit looking downvalley.

4–6 m/y. The average debris thickness we estimated to be 3–5m, 4. Effects of rock avalanches on glacier dynamics giving a volume of 1.4–2×105 m3. However, as the debris is carried downglacier, the increasing relative ice thickness causes steep slopes 4.1. Mass balance to form along the sides of the deposit. These slopes grow in height and backwaste from melting over time, redistributing the deposit on the The net mass balance of a glacier is determined by ice-mass gain in edges. As a result, the elevated area of the avalanche deposit cover the accumulation zone and ice-mass loss in the ablation zone. A reduces with time, and the estimated volume decreases as material is positive net mass balance over a certain period will cause glacier shed to the adjacent glacier surface. The GPR data also indicate thickening and frontal advance, while negative net mass balance (i.e. shearing of the thickened underlying ice owing to the differential ice ablation exceeds accumulation) will usually cause the glacier to thin flow (Fig. 7). along its profile and the terminus may retreat. Both advance and retreat have response times specific to each glacier. However, in some circumstances (e.g., a long, low-gradient glacier tongue with 3.4.2. Aoraki/Mt. Cook rock avalanche deposit extensive debris cover), negative net mass balance can cause vertical The 1991 Aoraki/Mt. Cook rock avalanche, whose volume was surface lowering while the terminus remains stationary, as with the originally estimated at 11.8±2.4×106 m3 (McSaveney, 2002), trav- downwasting Tasman Glacier (Kirkbride, 1993). elled across the Grand Plateau, down the Hochstetter icefall, and Two main changes in the glacier system result from the spread onto and across the Tasman Glacier (Fig. 6B). The motion of the emplacement of a rock avalanche deposit on the ablation zone avalanche deposit following emplacement was complicated. Shortly surface: after deposition, the Hochstetter ice stream deformed the original (i) the net mass balance becomes less negative or more positive by avalanche deposit into two main parts: that on the Hochstetter ice had reduction of ice-surface ablation in the ablation zone, assuming moved more than 2200 m down the valley by 2009, whereas that on there is no change in accumulation rate; and Tasman ice had moved only 500 m (Fig. 6B). On the latter deposit we (ii) additional mass is provided (a) immediately, by the rock obtained two perpendicular GPR profiles through the avalanche and avalanche deposit, and (b) gradually, by the increasing relative the adjacent upglacier, debris-covered ice. ice thickness caused by reduced ablation under the rock The Aoraki/Mt. Cook rock avalanche deposit (Fig. 8)isupto10m avalanche deposit. in thickness and a 25-m-high debris-covered ice ridge has formed at the upstream edge of the deposit; this was not present immediately Rock avalanche debris deposited in the glacier accumulation zone after emplacement (McSaveney, 2002). The GPR survey showed rapidly becomes buried by snow and ice (and entrained in the meltwater channels under the deposit, which can cause local englacial transport pathway), so it only affects the ablation rate for a collapses. Therefore, caused by differential ice flow velocities in the short time. It will most likely emerge in the ablation zone as melt-out deposition area, complications from tributary ice flow and these englacial or subglacial debris after being transported and dispersed in meltwater channels, the relative ice thickening from reduced ablation the en/subglacial transport pathways. is hard to estimate. The reduction of total ablation under a rock avalanche deposit can Similar ice thickening has been reported on other glaciers, with the be large (e.g., Reid, 1969; McSaveney, 1975; Reznichenko et al., 2010; formation of platforms up to 30 m above surrounding ice surfaces Alexander et al., 2011). Ice-surface ablation usually reduces to close to from reduced ice-surface ablation (Post, 1968; Molnia, 2008). Hewitt zero beneath a rock avalanche deposit. While sub- and englacial (2009) reported that the rock avalanche deposit onto the Bualtar ablation by meltwater heat flow, heat conductivity and other Glacier, Karakoram, increased the surface elevation by 5–15 m in one processes will be unaffected by the surface debris; they contribute year by comparison with adjacent ice. Field data, thus, confirm the only in a minor way (~ 10–20%; Alexander et al., in review) to total experimental result that rock avalanche debris dramatically reduces ablation. Thus, if a sufficient area of the ablation zone is covered, the ice-surface ablation. glacier net mass balance can be significantly altered. Because the N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 333

Fig. 8. GPR profile along the 1991 Aoraki/Mt. Cook rock avalanche and adjacent debris-covered glacier surface in February 2009, the Tasman Glacier, Southern Alps, NZ. The thickness of the rock avalanche deposit is up to 10 m, and the relative ice thickening at the upglacier edge of the deposit is about 25 m. (A) The ridge formed at the upvalley edge of the deposit with the Hochstetter icefall in the left background; and (B) GPR measurements on the upvalley slope of the Aoraki/Mt. Cook rock avalanche deposit. clear-ice ablation rate increases toward the terminus (Schytt, 1967) (Shulmeister et al., 2009). Secondly, the relative thickening of ice and the rock avalanche deposit will be progressively carried down- under the deposit is added to the normal (ice) mass of the glacier. For glacier, the reduction in surface ablation per unit area will increase as example, the area of glacier covered by the Mt. Beatrice rock the debris approaches the terminus. At the same, time the upglacier avalanche 5 years after emplacement has the equivalent of about limit of the debris cover is moving downvalley and clear-ice ablation 2×(1.4–2×105)m3 additional ice weight from the rock avalanche thereby increasing, so the net variation of mass balance with time is deposit plus about 13×105 m3 of ice (if the average additional ice not simple. For the present we note only that when the rear of the thickness is 27 m), which is increasing annually. Totally it is debris cover has advected downvalley of the original terminus equivalent of an extra 17×105 m3 of ice on an area of about 5 ha, or position, the glacier mass balance is re-established and the debris- 34 m of ice depth. This additional mass must alter the glacier covered ice downvalley thereafter behaves independently of the dynamics by increasing basal sliding, and this will occur in proportion glacier. to the percentage of the ablation zone affected and in inverse proportion to the original ice depth. 4.2. Glacier response to positive mass balance The sliding velocity of a warm-based valley glacier is proportional to the square of the bed shear stress, which depends on the ice-surface A rock avalanche covered 50% of the ablation zone of the Sherman slope and the mass of the ice per unit area of bed (Paterson, 1994). Glacier to an average depth of 1.5 m during the Alaska earthquake of Thus 10% additional ice depth should increase flow velocity by 20% at 1964, and ablation beneath the deposit was reduced by about 80% a given location. In addition, extra sediment finding its way to the (McSaveney, 1975). The resulting alteration in the net mass balance glacier base via crevasses and moulins may affect the subglacial changed previous glacier retreat to steady-state conditions and, after topography and drainage and hence the basal water pressure, which some years, a slow advance became evident (Post, 1968; McSaveney, in turn affects sliding (Turnbull and Davies, 2002; Davies and Smart, 1975), which still continued in 2009 (M.J. McSaveney, GNS Science, 2007); however, the magnitude of this effect is difficult to determine. PO Box 30368 Lower Hutt, NZ, pers. comm. 2009). A rock avalanche This effect was first observed after the St. Elias earthquake by Tarr and onto the Bualtar Glacier, Karakoram, in 1986, caused substantial Martin (1914), who proposed the “earthquake advance theory” when relative ice thickening under rock avalanche debris covering only 15% seven glaciers were surging and advancing 10 years after the event. of the ablation zone, but the positive impact on mass balance was The advances were attributed to added mass from rock, snow, and ice equivalent to a 20% increase in annual accumulation (Hewitt, 2009). avalanches, and also to the uplift of the St. Elias Mountains by 14 m As a result, the Bualtar Glacier (which had been retreating for several during the earthquake and the corresponding increase in glacier decades) surged during first 2 years after the rock avalanche and gradients (McSaveney, 1975). continued to advance during the following 12 years. The addition of sufficient rock avalanche debris to a glacier The mass added to the glacier both immediately (rock avalanche evidently can have significant effects on both mass balance and glacier debris) and gradually (relative ice thickening) will tend to increase motion (Shulmeister et al., 2009). An equilibrium glacier may the glacier flow velocity. Firstly, the rock avalanche material itself is advance; a retreating glacier may retreat more slowly, or come into equivalent to about double the weight of the same volume of ice equilibrium, or advance; and an already advancing glacier may 334 N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 advance more rapidly. A surge-type glacier may surge prematurely Therefore, these two deposits are not expected to noticeably affect the (Hewitt, 1988). overall behaviour of their glaciers. In case of the Mt. Beatrice rock avalanche onto the Hooker Glacier, no changes in glacier behaviour have been observed, because the area 4.3. Modelling covered by the deposit was small proportionally to the ablation area. Shulmeister et al. (2009) estimated that glacier behaviour would be The effects of rock avalanche debris on glacier behaviour can be altered by a rock avalanche deposit that covered more than about 10% predicted quantitatively. Alexander et al. (2011) developed a simple of the ablation zone area. With smaller deposits, the glacier may steady-state mass balance model, calibrated on the modern (1999– experience local effects from the additional supraglacial load, but the 2010) quasisteady-state conditions of the Franz Josef Glacier and overall mass balance will not be significantly affected. The Mt. Beatrice successfully proven against its Little Ice Age terminal moraine rock avalanche covers about 0.05 km2 of the roughly 7 km2 of the positions and trim lines. This was used to test the hypothesis of ablation zone for the Hooker Glacier, which is proportionallyb1% of Tovar et al. (2008) that a catastrophic rock avalanche could have the area (Fig. 6A). The Aoraki/Mt. Cook avalanche deposit covered caused the advance of the Franz Josef Glacier (western Southern Alps, about 1.7 km2, b4% of the ablation zone of the Tasman Glacier (about New Zealand; Fig. 9) to the location of the Late Pleistocene/Early 45 km2; Kirkbride, 1989). The deposit has since been redistributed Holocene Waiho Loop moraine; and thus have formed the moraine. and compressed and now covers an even smaller area (Fig. 6B). The modelling shows that a steady-state terminus position at the

Fig. 9. An example of the modelled rock avalanche debris cover on the Franz Josef Glacier, NZ, showing percentage of ablation reduction, which is staggered below the ELA to a terminus position at the Waiho Loop, assuming temperatures 2 °C less than the present-day, with an ELA of 1480 masl (satellite image of 2001, LandSat). The staggered levels of ablation reflect the accumulation of large quantities of debris toward the terminus, while in the upper-ablation zone, ablation rates will return to those of clean ice once the debris has moved downvalley. Location of the Franz Josef névé is also shown in Fig. 6. N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 335

Waiho Loop requires mean temperatures about 4–5 °C cooler than present-day. Anderson and Mackintosh (2006), using an ice flow/mass balance model, found that an advance to the Waiho Loop required 3–4 °C cooling from present-day. The results from both studies contradict proxy data, which indicate temperatures no more than ~2.5 °C cooler than present-day during the last glacial-interglacial transition (LGIT) about 12,000–13,000 YBP (Carter et al., 2003; Turney et al., 2003; Barrows et al., 2007). Direct dating of the Waiho Loop moraine by Barrows et al. (2007) yielded an age of about 10,500 YBP, corresponding to significantly warmer temperatures than those that occurred during the LGIT and making a climatically driven advance even less likely. To explain the advance of the Franz Josef Glacier from an equilibrium position close to Canavan's Knob to the Waiho Loop position during a time without any cooling (but ~2° cooler than present), Tovar et al. (2008) suggested, based on the sedimentology of the Loop moraine, that the ablation zone of the glacier was covered with debris by a rock avalanche. Alexander et al. (2011) showed that 65% reduction in total ablation (or a gradual reduction from 20 to 80% along the glacier) is needed to cause the required 3 km of advance from near Canavan's Knob to an equilibrium position at the Waiho Loop moraine (Fig. 9), assuming constant climate; based on the above data this appears reasonable. A coseismic rock avalanche of the required volume in the Waiho valley is certainly possible; Tovar et al. (2008) provided evidence of sufficiently large deep-seated, source- area scars in the correct location to match the dominant Loop Fig. 10. Response of a glacier to rock avalanche debris covering the ablation zone: lithology. Similar events have been reported elsewhere (e.g., Post, (A) glacier in equilibrium prior to rock avalanche; (B) rock avalanche emplaced; total 1967; McSaveney, 1975, 2002; Gordon et al., 1978; Evans and Clague, ablation reduced by~70%; (C) lack of melting increases ice depth and sliding velocity; 1988; Orombelli and Porter, 1988; Gardner and Hewitt, 1990; Jibson rock-covered reach reduces in length caused by relative velocity and ice thickens; (D) glacier now debris-free to original terminus; debris-covered ice continues to flow et al., 2004, 2006; Lipovsky et al., 2008; Deline, 2009; Hewitt, 2009). and slowly melt, forming terminal moraine by dumping RA deposit; and (E) all ice Vacco et al. (2010) recently presented a one-dimensional melted, terminal moraine remains with ablation moraine. numerical model of the Franz Josef Glacier. Though approximate, it confirmed that a debris driven advance could reach the Waiho Loop under certain circumstances and, significantly, that the resulting deposition would have a prominent, high end moraine with an Eventually the original glacier ablation zone is debris-free because all extensive blanket of ablation moraine to its rear, as appears to be the the debris has been carried beyond the original terminus. The debris- case at the Waiho Loop based on available evidence (Shulmeister covered ice downvalley is now independent of the glacier and et al., 2010). continues to flow slowly under its own surface gradient while slowly In summary, there is substantial evidence that a rock avalanche melting, forming a terminal moraine; it eventually becomes stagnant deposit covering a sufficient proportion of an ablation zone can cause and slowly melts to form a sheet of ablation moraine to the rear of the glaciers mass balance fluctuations and its nonclimatically driven terminal moraine. This form is similar to that generated by the model advance. of Vacco et al. (2010), and is strikingly similar to that displayed by the small Classen terminal moraine in New Zealand shown in Fig. 11. 5. Consequences Supraglacial rock avalanche debris is normally between 1 and 10 m thick; this allows estimation of the size of a rock avalanche 5.1. Terminal moraine formation required to cover a specified proportion of the ablation zone of a given glacier and suppress ablation accordingly. For example, the Franz Josef Here we describe theoretically how a rock avalanche may cause Glacier ablation zone area is about 6 km2, so a rock avalanche of the glacial advance and frontal moraine formation without any hin- order of 106 m3 in volume would be needed to cover 10% of the drances. A rock–avalanche-driven glacier advance will be short-lived; ablation zone to a depth of N1 m and cause a significant change in perhaps in the order of decades, compared to centuries to a glacier behaviour. Such a volume of rock avalanche deposit arriving at millennium in the case of an advance driven by climate such as the the approximately 0.4 km wide terminus could form a moraine of Younger Dryas or the Little Ice Age. As the rear of the debris deposit is cross-sectional area ~103–104 m2; this would certainly be a promi- carried downvalley by ice flow, ice-surface ablation will increase and nent feature in the landscape. mass balance will gradually become less positive. It will return to How moraines are formed by climatic glacier advances is also normal conditions, or the mass balance as before the event, when all incompletely understood. Bulldozing is able to generate small “push” the debris has been carried past the original terminus. Then the moraines of the order of several metres high (Molnia, 2008; Winkler advance of the detached debris-covered ice will eventually slow and and Matthews, 2010), while sluicing of debris by subglacial water stop, during which time substantial quantities of rock avalanche flow generates characteristic stratified “outwash” moraines (Hyatt, debris will reach the new terminus and form a moraine. The sequence 2010); however, “dumping” of supraglacial debris, which can build of events is illustrated conceptually in Fig. 10; ice covered with rock very high and steep lateral moraines (Benn and Evans, 2010) seems to avalanche deposit reduces in elevation less slowly because of be the most likely potential cause of very large, steep-fronted terminal suppressed melting than the previous clean or thinly-covered ice, so moraines such as the Waiho Loop (Vacco et al., 2010), and the it flows more rapidly. In addition, ice at the terminus has compressive requirement for large volumes of supraglacial debris to do this flow which is less rapid than that farther upvalley, so the length of suggests that a rock avalanche may be the most likely source of such debris-covered ice reduces and its thickness increases accordingly. moraines. 336 N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338

5.3. Effects on society

The potential for large rock avalanches to affect some present-day glaciers may resulted in significant impacts on society and business, because of the increasing level of tourism-related human activity in the vicinity of accessible glaciers (which are often the larger ones that extend to low altitudes). A good example is the Franz Josef Glacier because of its location in a tectonically active setting that frequently experiences intense seismicity (Shulmeister et al., 2009) and because of its popularity as a tourist destination owing to its easily accessible terminus (at ~300 masl). Catastrophic landslides have previously been caused by earth- quakes on the Alpine fault, a 700-km-long plate boundary fault running along the western rangefront of the Southern Alps of New Zealand (e.g., Chevalier et al., 2009; Dufresne et al., 2010). This fault generates earthquakes of M~8 several times per millennium on average, and the last one was in 1717 according to paleoseismic Fig. 11. Classen moraines, Godley Valley, NZ, 2009; the upper terminal moraine has the information (Rhoades and Van Dissen, 2003). The large number of form indicated by Vacco et al. (2010) and by Fig. 10 (Photo by S. Winkler). deep-seated landslide source-area depressions are observed in the Southern Alps, whose morphology indicates that their origin was most likely coseismic (Tovar et al., 2008). Shulmeister et al. (2009) Given the potential for rock avalanches to generate terminal suggested that a large rock avalanche could be expected in the order moraines, improved understanding of moraine formation processes of once per millennium on average in the Franz Josef Glacier by rock–avalanche-driven advances is an urgent requirement. catchment. However, given that since 1999 six rock avalanches have occurred in the Southern Alps, all of the order of 107 m3, and none 5.2. Paleoclimatology were triggered by either earthquake or rain (Aoraki/Mt. Cook and Mt. Fletcher I and II; McSaveney, 2002; Mt. Adams; Hancox et al., 2005; Terminal moraines are commonly used as paleoclimatic indicators. Young River; unreferenced; Joe River; unreferenced), the probability This method is based on the assumption that all moraines owe their of any kind of rock avalanche in any specific mountain valley is clearly existence to climatically driven advance-retreat sequences, but our nontrivial. work above clearly shows this assumption to be potentially in error. The ever-present and immediate hazard from a rock avalanche Anypurelyrock–avalanche-generated moraine whose date is onto the Franz Josef Glacier (and perhaps running out along the assigned paleoclimatic significance generates an error in the inferred proglacial upper Waiho valley) is to the lives of people on the glacier paleoclimate record. Of course, a rock avalanche can fall onto a glacier and in the valley (several hundred visitors at any one time on a fine that is in the course of a climatically driven advance-retreat sequence, day), since warning of such an event may only be a minute or so for in which case the moraine age will have paleoclimatic relevance. people at the terminus and less for people on the glacier. This Terminal moraines form when a glacier reaches an advanced therefore represents a not insignificant threat to peoples’ safety, given position; when it retreats again the moraine records the previous the ~250,000 tourists that visit the valley, terminus, and ice surface maximum extent (Benn and Evans, 2010). If a glacier terminus annually (Corbett, 2001). undergoes an advance-retreat sequence and is supplied with To estimate the potential effect of a rock avalanche on the present- sediment by any process, a terminal moraine is likely to form that day glacier, Alexander et al. (2011) assumed that debris from a rock will remain after retreat has begun. The greater the volume of avalanche in the mid-upper catchment is voluminous enough to cover sediment able to be delivered to the moraine, the larger it will be. the entire present-day ablation zone (area about 6 km2) to metre- Thus, a rock–avalanche-driven advance will be capable of generating a scale depth . This would require a volume of about 107 m3. If total substantial terminal moraine because it involves a very large volume ablation were reduced by 80% as a result, the modelling indicates that of debris. Such a moraine will have no necessary association with any the Franz Josef Glacier would advance almost as far as the township climatic change (Larsen et al., 2005; Tovar et al., 2008; Shulmeister et (about 6 km; Fig. 9) if the present climate conditions and accumu- al., 2009). Even if a rock avalanche deposit does not cause a glacier to lation rates prevailed. The speed of a rock–avalanche-driven terminus advance, the eventual arrival of a large volume of supraglacial debris advance can be estimated from the known rate of ice accumulation in at the terminus can form a large moraine in a short time, whereas to the catchment, and appears to be of the order of 1 km/y (Tovar et al., form the same size of moraine without rock avalanche debris would 2008). Apparently, the glacier could possibly advance to the vicinity of require the terminus to remain in one position for a relatively long Franz Josef township in perhaps a decade; human life would not be time. threatened by such an advance, but the main highway along the west In the same way, moraines left by surge-type and tidewater coast of South Island would be. The consequent effects on the glaciers may record nonclimatic ice-front fluctuations (Yde and behaviour of the already troublesome Waiho and Callery rivers Paasche, 2010). Therefore the terminal moraines that are used as (McSaveney and Davies, 1998; Davies and McSaveney, 2001) would paleoclimatic indicators must be known to be generated by climatic also need to be investigated—when the Franz Josef Glacier advanced variation rather than by nonclimatic processes. To date, however, no about 1.5 km between 1982 and 1999, the proglacial area aggraded of such investigations of this topic have been reported, and all terminal the order of tens of metres. Clearly, there is the potential for major moraines are assumed de facto to result from climatic variations. disruption of tourism and other business in this iconic tourist Given the crucial contemporary importance of paleoclimatology in destination. understanding climate change, this is potentially a serious deficiency. This scenario has not to date been part of any assessment carried The various difficulties outlined herein can only be resolved when the out for the area, and, therefore, no management strategies are processes of formation of both climatically driven and rock– developed. A similar threat could exist for the locality of the Fox avalanche-driven moraines are much better understood, so that Glacier, 20 km south of the Franz Josef Glacier, whose glacial and criteria can be developed for distinguishing them in the field. geomorphological characteristics are similar. Evidently, if a rock– N.V. Reznichenko et al. / Geomorphology 132 (2011) 327–338 337 avalanche-generated glacier advance were to approach Franz Josef Deline, P., 2009. Interactions between rock avalanches and glaciers in the Mont Blanc massif during the late Holocene. Quaternary Science Reviews 28, 1070–1083. township, the carefully planned, ordered, and structured relocation of Drewry, D., 1986. Glacial Geologic Processes. Edward Arnold, London. 276 pp. the settlement and of the highway are required. No active mitigation Dufresne, A.M., Davies, T.R.H., McSaveney, M.J., 2010. Influence of runout path material methods appear to be practicable. The requirement for further on the emplacement of the Round Top rock avalanche, New Zealand. Earth Surface Processes Landforms 35, 190–201. doi:10.1002/esp. 1900. investigation of this scenario is clear. Evans, S.G., Clague, J.J., 1988. Catastrophic rock avalanches in glacial environments. Proc. Fifth Int. Symp. on Landslides 2, 1153–1158. 6. Conclusions Fort, M., Cossart, E., Delin, P., Dzikowski, M., Nicoud, G., Ravanel, L., Schoeneich, P., Wassmer, P., 2009. Geomorphic impact of large and rapid mass movements: a review. Geomorphology: relief, processus, environment 1, 47–64. Gardner, J.S., Hewitt, K., 1990. A surge of Bualtar Glacier, Karakoram Range, Pakistan: a (i) Large rock avalanches falling onto glaciers can affect glacier possible landslide trigger. Journal of Glaciology 36 (123), 159–162. behaviour significantly by reducing the total ablation beneath Geertsema, M., Clague, J.J., Schwab, J.W., Evans, S.G., 2006. An overview of recent large the deposit by up to 90%, causing substantial changes in glacier catastrophic landslides in northern British Columbia, Canada. Engineering Geology – behaviour such as advances and surging. 83, 120 143. Gordon, J.E., Birne, R.V., Timmis, R., 1978. A major rockfall and debris slide on the Lyell (ii) Rock–avalanche-driven glacier advances can form prominent Glacier, South Georgia. Arctic and Alpine Research 10 (1), 49–60. terminal moraines. Hancox, G.T., McSaveney, M.J., Manville, V.R., Davies, T.R.H., 2005. The October 1999 Mt. fl (iii) Terminal moraines generated by rock–avalanche-driven ad- Adams rock avalanche and subsequent landslide dam-break ood and effects in Poerua River, Westland, New Zealand. NZ Journal of Geology & Geophysics 48, vances have no necessary climatic association; therefore, the 683–705. validity of paleoclimatic inferences from moraines depends on Hermanns, R.L., Niedermann, S., Garcia, A.V., Gomez, J.S., Strecker, M.R., 2001. identifying the origin of the moraine. Neotectonics and catastrophic failure of mountain fronts in the southern intra- Andean Puna Plateau, Argentina. Geology 29, 619–622. (iv) Further investigation is required into the processes of terminal Hewitt, K., 1988. Catastrophic landslide deposits in the Karakoram Himalaya. Science moraine formation caused by rock–avalanche-driven and 242 (4875), 64–67. climatically driven advances and into criteria for distinguishing Hewitt, K., 1998. Catastrophic landslides and the Upper Indus streams, Karakoram Himalaya, northern Pakistan. Geomorphology 26, 47–80. moraines formed by these processes. Hewitt, K., 2009. 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