Effects of Rock Avalanches on Glacier Behaviour and Moraine Formation
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Geomorphology 132 (2011) 327–338 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph 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.