Chapter 10. Glaciovolcanism: a 21St Century Proxy for Palaeo-Ice

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CHAPTER GLACIOVOLCANISM: A 21ST CENTURYPROXYFORPALAEO-ICE10 J.L. Smellie University of Leicester, Leicester, United Kingdom

10.1 INTRODUCTION: WHAT IS GLACIOVOLCANISM AND WHY IS IT IMPORTANT? Volcanoes that erupt in association with snow or ice have the power to construct their landscape, preserve the past, and inform the future. This is true whether the associated ice is an extensive ice sheet or an ice cap confined to the volcano itself. The volcanic products are commonly distinctive and they reflect the presence and physical characteristics of any coexisting ice. Until recently, eruptions in association with ice were referred to empirically as subglacial but they also include examples where lithofacies have simply abutted against ice (e.g., when subaerial lavas are but- tressed by a valley glacier; the ‘ice-marginal flows’ of Kelman et al., 2002; see also Lescinsky and Sisson, 1998). Thus, a description as subglacial is inappropriate and the deposits are more appropri- ately called ‘glaciovolcanic’. ‘Glaciovolcanism’, a word first used by Kelman et al. (2002),is defined as the interactions between magma and ice in all its forms, including snow, firn and any meltwater resulting from those interactions (Smellie, 2006). The topic is important for several rea- sons. Firstly, it is a research area in its own right, and a very young one. Although studies of subglacially erupted volcanoes extend back to the early 20th century (e.g., Russell et al., 2014; Smellie and Edwards, 2016, and references therein), publications were few and sporadic until the mid-late 1990s, after which the publication rate underwent an exponential increase (Russell et al., 2014, fig. 1). Secondly, it is a boundary condition for ice sheets. Volcanoes represent geothermal heat, and geothermal heat is an important attribute affecting the basal thermal regime and, ultimately, ice sheet or glacier stability. Thirdly, it is a geological hazard. For example, a major distinctive hazard associated with glaciovolcanic eruptions is the generation and rapid release of enormous quantities of meltwater. The comparatively small eruption of Gjalp´ volcano beneath Vatnajo¨kull in Iceland in 1996 released a jo¨kulhlaup (meltwater flood) with a discharge that peaked at c. 50,000 m3 s21. For a few hours it was one of the largest flows of freshwater on Earth, about three times that of the Mississippi River. Moreover, the regional economic impact can be considerable. Ash from the 2010 eruption of Eyjafjallajo¨ll (Iceland) grounded aircraft operating throughout much of Europe and caused substantial costs to the airline industry estimated at c. 1.3 billion euros (d1.1 billion, US$1.7 billion, CAD$2.1 billion). Glaciovolcanism is also an increasingly important environmental proxy. Although historically it has been largely underutilized, it has been developed intensively since the early 1990s until it is now the most powerful and most holistic method for determining

BOX 10.1 PARAMETERS THAT CAN ROUTINELY BE DETERMINED FROM MOST GLACIOVOLCANIC SEQUENCES 1. Whether ice was present during eruptions; 2. The age of that ice; determined by isotopic dating of associated usually very fresh lavas; 3. Ice thickness; this is a unique and quantifiable property derived from glaciovolcanic sequences, for any geological period; 4. The elevation of the coeval ice surface; this is also quantifiable and unique to glaciovolcanic studies (for pre-LGM periods); it can be determined in an absolute sense (i.e., metres above sea level) if the tectonic history of a region is well-enough known, or else relative to a local datum, e.g., compared with ice elevations associated with previous eruptions of a volcano; 5. Basal thermal regime; this property (designated as either cold-based or wet-based ice) can be derived from features of the glaciovolcanic sequences themselves but is more often deduced from characteristics of the geological surfaces (often unconformities) separating eruptive units and the presence and types of any associated glacial sediments. Of these parameters, ice thickness and thermal regime are probably the most important for reconstructing past ice conditions.

quantitatively the widest range of critical parameters of past ice sheets compared with any other methodology (see Box 10.1). It is especially useful for reconstructing characteristics of ice sheets for periods prior to the last glacial. Finally, the recognition of likely glaciovolcanic edifices on Mars has significantly influenced our understanding that Mars has a water-rich inventory (e.g., Ghatan and Head, 2002; Fassett and Head, 2006, 2007).

10.2 ADVANTAGES AND DISADVANTAGES OF VOLCANIC VERSUS SEDIMENTARY ROCKS AS PALAEOENVIRONMENTAL TOOLS Terrestrial tills are normally almost impossible to date directly, whereas the ability of glaciovolcanic sequences to be dated isotopically, usually by the 40Ar/39Ar method, is a great advantage. However, the precision of 40Ar/39Ar dating (2-sigma values usually 40À60 kiloyears) is poor compared with the duration of glacial cycles (41À100 kiloyears) although it is improving particularly for K-rich minerals in felsic lavas (Flude et al., 2008; Martin et al., 2011). Tills are also often thin (a few metres) and patchy, and they are largely removed by each successive glaciation. By contrast, glaciovolcanic sequences are characteristically thick (tens or hundreds of metres, to .1 km) and are usually protected by lavas that are highly resistant to erosion. They are thus highly robust features capable of surviving multiple glaciations extending over many millions of years (e.g., glaciovolcanic sequences in Antarctica extend back in time to 29 Ma; LeMasurier and Thomson, 1990; Wilch and McIntosh, 2000; Haywood et al., 2009) (see chapters: Quaternary Glaciations and Chronology; Subglacial Processes and Sediments). Compared with higher-resolution marine sedimentary deposits, volcanic sequences are a low-resolution record of past ice sheets. However, it is extremely difficult to deduce the thermal regime of ice from marine studies unambiguously, whereas it can be derived routinely from glaciovolcanic sequences (cf. Hambrey and Glasser, 2012; Smellie et al., 2014). Volcanic eruptions may take place at intervals of hundreds of years to a few hundred thousand years, resulting in large gaps in the record (Smellie et al., 2008) but gaps are also frequent in the marine 10.2 ADVANTAGES AND DISADVANTAGES OF VOLCANIC VERSUS 337

FIGURE 10.1 Comparison of geological records obtained by drilling onshore and offshore in the Ross Sea region and the record contained by volcanic outcrops (Hallett Coast volcanics) in northern Victoria Land, Antarctica. The red box highlights how almost the entire mid-late Miocene sedimentary record has been removed by glacial erosion across the region but is preserved in the volcanic outcrops. Adapted from Smellie, J.L., Rocchi, S., Gemelli, M., Di Vincenzo, G., Armienti, P., 2011b. Late Miocene East Antarctic ice sheet characteristics deduced from terrestrial glaciovolcanic sequences in northern Victoria Land, Antarctica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 307, 129À149.

record, and many millions of years of sediment may be erased by overriding ice (Fig. 10.1; Smellie et al., 2011b). Finally, volcanism clearly needs to occur in a glacial environment in order to preserve a record, and not all glacierized regions contain active volcanism, which is why it will never be used as a proxy methodology to study the development of the Greenland Ice Sheet (since Greenland lacks appropriate-age volcanic rocks). The most geographically and temporally extensive glaciovolcanic regions are Antarctica (29 Ma to present; the largest and longest-lived glaciovolcanic province), Iceland (c. 4.5 Ma to present; a compositionally diverse glaciovolcanic region with the greatest number of glaciovolcanic centres in a comparatively small area), and British Columbia (Canada; c. 3 Ma to present; also compositionally diverse, the location of some of the earliest descriptions and 338 CHAPTER 10 GLACIOVOLCANISM

innovative environmental interpretations of glaciovolcanic products, and source of some of the earliest specialist terminology; Fig. 10.2). Volcanoes are known to have interacted with ice on practically all major continental landmasses apart from Australia. However, even nonglacially emplaced volcanic units can sometimes be used to deconvolve regional glacial histories, for example 40Ar/39Ar and K-Ar dating of subaerial lavas in Patagonia used to bracket the ages of interbedded tills formed during glacial advances in Patagonia (e.g., Singer et al., 2004).

10.3 RELATIONSHIP BETWEEN VOLCANISM AND CLIMATE Links between glaciovolcanic activity and the presence and thickness of overlying ice have been postulated for more than a century (e.g., Pjetursson, 1900; Peacock, 1926; Grove, 1974; Hardarson and Fitton, 1991; Jull and McKenzie, 1996; Sigmundsson et al., 2010). Whilst explanations for a link are still debated, there is a broad consensus that the weight of a growing (thickening) overlying or geographically adjacent ice mass will transmit stresses to crustal fractures (potential volcanic conduits) and increase the pressure on crustal and mantle-derived magmas. Thus eruptions may be suppressed by ice that is thickening. It is even possible that quite subtle variations in ice thickness may be capable of influencing eruptions (e.g., Pagli and Sigmundsson, 2008).Conversely,adecayingicemassmayhelptodestabilizemagmas,withthe reduction in effective lithostatic/glaciostatic pressure reducing stresses on crustal fractures and helping to release volatiles in the magmas, thus potentially triggering eruptions. In a climate context, glacials should be associated with fewer glaciovolcanic eruptions, whilst eruptions during interglacials should be enhanced. Because of the asymmetry of Milankovitch cycles, which have a ‘saw tooth’ shape characterized by slow gradual glacial build-ups, followed by much more rapid ice loss during deglaciation, there should be a prominent peak in eruptive activity during the few thousand years when global temperatures rise rapidly following glacials. A pattern of significantly increased eruptions in the early Holocene has been demonstrated convincingly for Iceland (e.g., Slater et al., 1998; Sigvaldason et al., 1992; Maclennan et al., 2002; Sinton et al., 2005). Thus, by extension, it might be expected that information about contemporaneous ice conditions contained in glaciovolcanic sequences of any age will be strongly biased towards the periods of rapid ice thinning following glacial maxima. However, eruptions also occur during glacials and they are not restricted to the early interglacial period. With improving precision in isotopic dating methods, this is now being demonstrated by, admittedly few, studies directly dating glaciovolcanic outcrops (e.g., McGarvie et al., 2006; Licciardietal.,2007; Flude et al., 2010; Hall et al., 2011; Fig. 10.3). Additional relevant information can be gleaned for volcanic outcrops situated in nearfield positions—i.e., in geographically proximal locations relative to Pleistocene ice and subject to transmitted glaciostatic stresses (Glazner et al., 1999). One of the best datasets with abundant precise dating consists of Quaternary volcanic outcrops in Europe, which were situated between the major European ice masses during the Pleistocene and therefore were in a nearfield position. It is an intensively dated volcanic field, through a combination of C14, thermoluminescence, K-Ar, and 40Ar/39Ar dating methods. When the data for the last 450 kiloyears (when age precision is greatest, i.e., typically a few kiloyears) are plotted on a composite deep-sea oxygen FIGURE 10.2 Views of representative volcanoes in the three principal glaciovolcanic regions on Earth. (A) Mount Melbourne, an ice-shrouded active stratovolcano in northern Victoria Land, Antarctica. (B) View of O¨ raefajo¨kull, Iceland’s largest active stratovolcano, showing its prominent ice-filled summit caldera 4À5 km in diameter. Although currently only ice-capped, features of the rock sequences indicate that it erupted frequently during glacial periods when it was entirely covered by ice. Image: Tom Pfeiffer (www.volcanodiscovery.com). (C) View of Hoodoo Mountain, a polygenetic subaerial and subglacially erupted stratovolcano in British Columbia, Canada. The steep flanks are caused by impounding of lavas by surrounding ice (now melted). Image: Ben Edwards. 340 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.3 Diagram showing that glaciovolcanic eruptions also occur in glacial periods and not just during the rapid transitions to interglacials. The Milankovitch curve is after Lisiecki, L.E., Raymo, M.E., 2005. A Pliocene- Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography 20, PA 1003, doi:10.1029/ 2004PA001071 and the numbered peaks and troughs are Marine Isotopic Stages. Age data from Flude, S., McGarvie, D.W., Burgess, R., Tindle, A.G., 2010. Rhyolites at Kerlingarfjo¨ll, Iceland: the evolution and lifespan of silicic central volcanoes. Bull. Volcanol. 72, 523À538 (Kerlingarfjo¨ll, Iceland; white symbols) and McGarvie, D.W., Burgess, R., Tindle, A.G., Tuffen, H., Stevenson, J.A., 2006. Pleistocene rhyolitic volcanism at Torfajo¨kull, Iceland: eruption ages, glaciovolcanism, and geochemical evolution. Jo¨kull 56, 57À75 (Torfajo¨kull, Iceland; black symbols). Precision of the ages (1-sigma) is indicated by horizontal error bars through each data point. Absence of a bar indicates the error is within the diameter of the displayed data point.

isotope curve, a clear pattern emerges. There are well-defined peaks in eruption frequency dur- ingtheHoloceneandalsointheperiodofwarming temperatures between Marine Isotope Stages (MIS) 8 and 7 (and possibly but less convincingly between MIS 12 and 11), which implies that the volcanoes are reacting to glacially influenced crustal stresses. However, a major- ity of eruptions took place in glacials, during periods in which global temperatures were progressively decreasing and ice thicknesses presumably increasing (Fig. 10.4). Because the European volcanic field was subaerial rather than subglacial, it does not preserve evidence of ice parameters (thickness, thermal regime, etc.). By extension, the implication is that in ice-covered areas undergoing volcanism there is a possibility that even conditions of maximum ice thicknesses (peak glacials) may be represented by eruptions. However, given that many volcanoes erupt at intervals of several kiloyears to tens of kiloyears, it is likely that evidence for peak ice thicknesses will only rarely and fortuitously be preserved in glaciovolcanic sequences. Thus, the ice thicknesses recorded by glaciovolcanic sequences are probably best regarded as ‘typical’ for each glacial and not maximal (Smellie et al., 2009, 2011b). Improvements in isotopic dating now make it possible to date glaciovolcanic sequences with precisions of ,10 kiloyears, i.e., well within the duration of glacial cycles (McGarvie et al., 2006; Flude et al., 2008, 2010; Singer et al., 2008; Guillou et al., 2010) and it is hoped the situation will continue to improve. Thus, with enough eruptions in a glaciovolcanic field, it may become possible to reconstruct the fluctuations in ice thicknesses as they vary through a single glacial cycle. 10.4 A TYPICAL BASALTIC GLACIOVOLCANIC ERUPTION 341

FIGURE 10.4 Diagram showing dated eruptions in the subaerial European Quaternary volcanic field. Data are jittered vertically to aid display. Two prominent peaks in activity correspond to transitions to interglacials (between Marine Isotope Stages 2À1 and 8À7; blue arrows), which implies that the volcanoes are reacting to glacially influenced crustal stresses, but most of the eruptions occurred within glacial periods of thickening ice conditions. Modified after Nowell, D.A.G., Jones, M.C.,Pyle, D.M., 2006. Episodic Quaternary volcanism in France and Germany. Journal of Quaternary Science, 21, 645-675. 1-sigma errors are too small to display.

10.4 A TYPICAL BASALTIC GLACIOVOLCANIC ERUPTION Basaltic volcanoes are the commonest volcanoes on Earth and many of the events and processes that occur during a subglacial eruption can be simply illustrated by basaltic eruptions that result in the construction of tuyas. Tuyas are morphologically distinctive flat-topped glaciovolcanic edifices with steep sides (Mathews, 1947). A tuya eruption usually commences with the creation of a subglacial cavity or vault filled by meltwater. Sagging of the ice roof creates a conspicuous collapse cauldron on the surface. If the ice thicknesses are sufficient to suppress significant volatile release from the magma, the eruption commences with passive effusion of pillow lava to form a pillow mound or pillow volcano around a central vent or else it creates a pillow ridge for eruptions along a fissure (Jones, 1969; Werner and Schmincke, 1999). During the upward growth of the pillow mound, the overlying ice becomes thinner and volatiles are released more vigorously from the magma until the eruption becomes explosive (phreatomagmatic). Large quantities of lapilli ash are erupted that create a subaqueous tephra mound (tuff cone) or ridge (tindar; Smellie, 2001; Schopka et al., 2006). The phreatomagmatic period is one of rapid vertical aggradation (Smellie, 2001). Ultimately the ice roof either collapses or is melted through and the vault becomes an englacial lake that surrounds the growing volcanic edifice. When the tuff cone emerges (Fig. 10.5) eruptions become fully subaerial. The vent dries out and the eruption becomes magmatic, with the accumulation of abundant scoria, usually forming a small pyroclastic cone, lava effusion, and construction of a small lava shield. Lava flowing into the meltwater lake forms a lava-fed delta (the volcanic equivalent of a Gilbert-type sedimentary delta), which extends the edifice laterally and the eruption becomes dominated by progradation (Smellie, 2001). It is the delta phase of the eruption that gives tuyas their distinctive flat tops and steep flanks. The presence of a protective cap of erosion-resistant lavas is what helps to preserve tuyas from being removed by multiple overriding glaciations. 342 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.5 View of glaciovolcanic tuff cone emerging from a meltwater lake; 2004 eruption of Gr´ımsvo¨tn, Iceland. Note how small and protected the lake is and how the lake surface is lower than the surrounding ice surface. The hole in the ice is c. 1 km in diameter. Image: Snaebjo¨rn Gudbjo¨rnsson.

It is the melting of the surrounding ice by volcanic heat that causes meltwater to accumulate in the subglacial vault, subaqueous clastic lithofacies to be deposited and lavas to be cooled rapidly. Since ice is effectively impermeable on the timescale of volcanic eruptions (see Section 10.5), the depth of water represented by the cumulative thickness of subaqueous or water-cooled lithofacies is an approximation for the thickness of coeval ice. That relationship is very important. It forms the principal basis for deriving past ice thicknesses and is why glaciovolcanic studies have become an important, yet still largely underutilized, palaeoenvironmental tool.

10.5 PHYSICAL PROPERTIES OF ICE IMPORTANT FOR GLACIOVOLCANIC ERUPTIONS It is the presence of ice, its thickness, and the flow of associated meltwater that strongly influence the types of glaciovolcanic lithofacies present and the relative order in which they occur (i.e., the sequence architecture; Smellie and Edwards, 2016). Before we review the different types, characteristics, and palaeoenvironmental utility of glaciovolcanic sequences, it is important to understand the properties of ice that influence their distinctive features, namely physical structure, thermal regime, rheology, and hydraulics. They are discussed in detail in glaciological textbooks (e.g., Paterson, 1994). Ice rheology principally concerns the speed with which an ice mass deforms to an applied stress. Thermal regime has been used to categorize ice mass types, i.e., cold ice (below pressure-melting point throughout), warm ice (at pressure-melting point throughout except in a cold 10.5 PHYSICAL PROPERTIES OF ICE IMPORTANT 343

surface layer), and polythermal ice (an intermediate category); both warm and polythermal ice are collectively called wet-based. The thermal regime broadly describes how stable or dynamic an ice mass is. It is a crude indication of how rapidly it will deliver ice to the oceans and thus influence sea levels. The most obvious influence of ice rheology and thermal regime on glaciovolcanism is probably on the morphometry of the resulting landforms, with taller, narrower edifices formed in colder, stronger ice (Smellie, 2013). However, the most important properties for their effect on glaciovolcanic eruptions are physical structure and hydraulics, which determine and describe how meltwater is transmitted through the enclosing ice masses. They are described in more detail below.

10.5.1 PHYSICAL STRUCTURE: THE TRANSFORMATION OF SNOW TO ICE Snow is a particulate substance that transforms into ice by a process of progressive densification during compaction. The density increases from c. 50À400kgm23 for snow to 800À917kgm23 for ice. In ice, the interconnecting air passages between the ice crystals and grains have been sealed off and, apart from very slow migration of water along capillary channels at three-grain boundaries, intergranular veins, and microcracks (Nye and Frank, 1973), permeability is extremely low. The capillarity paths are tortuous and very limited in their effect (at any time the local permeability probably only extends for a distance of several grains) and the whole glacier cannot be modelled as a porous medium for which Darcy’s law applies. On the timescale of a volcanic eruption, ice is effectively an aquiclude (Smellie, 2006). These comments clearly apply only to warm ice and not to cold ice since free water is absent in the latter. By contrast, capillary and intercrystal channels are very abundant in firn and snow and they have permeabilities of 10210À1025 m2, respectively (Colbeck and Anderson, 1982; Fountain, 1989). Although similar to the permeability of sand, they are still low values corresponding more to aquicludes (or at least aquitards) than to aquifers (but see below). The effect of densification of snow to ice is that the uppermost level of an ice sheet or glacier is stratified in the accumulation zone (in the ablation zone ice is exposed at the surface and the entire glacier is formed of ice). A threefold partition is the result (snow, firn, ice, with grada- tional boundaries), which, together with the presence of a fractured ice layer (crevasses), has pro- found consequences for the hydraulics of the system (see Section 10.5.2). There are major differences between the thickness of the upper permeable layers between temperate and cold-based ice. Maximum thicknesses of firn and snow up to c. 125 m can occur on cold-based ice, but may only be a few tens of metres for temperate ice (Paterson, 1994).

10.5.2 HYDRAULICS Hydraulics (the description of the flow of water through a glacier) is a fundamentally important property that has a greater effect on the course of glaciovolcanic eruptions than any other property of ice. As described earlier, water is transmitted only very slowly in firn and snow, and effectively not at all in ice (apart from along microfractures). However, because ice particles in snow and firn melt, the permeability changes constantly (i.e., increases) due to seepage of volcanically heated warm meltwater. This will happen much more rapidly for firn or snow than for ice, leading to water in a vault overflowing along the firn:ice surface if the conditions are suitable (Fig. 10.6; Gore, 1992; Gudmundsson et al., 1997; Smellie, 2000, 2001, 2006). Conversely, if present, a fracture layer (i.e., zone of crevasses) will dominate water flow and will drain any overlying firn and act as 344 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.6 Meltwater gushing out from portals situated at the contact between firn and superimposed ice (dashed line), illustrating how meltwater is channelled along firn:ice interfaces due to permeability differences. The height of the cliff is c. 16 m. See Gore (1992). Image: Damian Gore. an aquifer (Fountain, 1989). Because crevasses wedge out downward over 20À30 m, the layer of fractures is largely confined to the surface of a glacier. In wet-based ice, basal water migrates along the ice:bedrock interface either as distributed or channelized flow (Fountain and Walder, 1998). Distributed flow is relatively slow (but significant), whereas channelized flow is much faster and occurs in two main channel types, named Nye and Ro¨thlisberger channels, that are cut down into bedrock or up into the ice, respectively. The presence of Nye and Ro¨thlisberger channels is important for understanding glaciovolcanism. Neglect of their influence led to early models for subglacial volcanic eruptions predicting that basal melting caused by the eruption would lead to all meltwater ponding in a sealed englacial cavity (Bjo¨rnsson, 1988). In fact, the cavities or vaults formed during glaciovolcanic eruptions are inherently leaky (at least associated with wet-based ice), as was spectacularly observed during the 1996 eruption at Gjalp,´ Iceland (Fig. 10.7; Gudmundsson et al., 1997). The early-formed meltwater will immediately run into any active Nye or Ro¨thlisberger channels. Because it is volcanically heated (Gudmundsson et al., 2004), the channel dimensions rapidly enlarge due to viscous heat dissipation and the vault is prevented from sealing despite the modification to the hydraulic equipotentials directing englacial water flow into the vault from the enclosing ice mass (cf. Bjo¨rnsson, 1988, fig. 1.3A).

10.6 CLASSIFICATION OF GLACIOVOLCANIC SEQUENCES AND LANDFORMS Tuyas are the most morphologically distinctive type of glaciovolcanic edifice but other types occur and have many different forms. Most glaciovolcanic successions occur as edifices but some occur simply as sequences, either because the nature of the associated edifices is unknown or they 10.6 CLASSIFICATION OF GLACIOVOLCANIC SEQUENCES AND LANDFORMS 345

FIGURE 10.7 Schematic cross-sections showing subglacial and supraglacial meltwater flow during different stages of the 1996 eruption of Gjalp,´ Iceland. Note also the depression of the ice surface over the eruption site. From Gudmundsson, M.T., Sigmundsson, F., Bjo¨rnsson, H., 1997. Ice—volcano interaction in the 1996 Gjalp´ eruption, Vatnajo¨kull, Iceland. Nature, 389, 954À957. are not distinctive. At least eight different glaciovolcanic types have been recognized so far and are listed in Table 10.1 together with a summary of their characteristics and their principal lithofacies and architecture are shown in Fig. 10.8. The figure is based on monogenetic examples, i.e., they were constructed during a single eruptive phase, and for largely unmodified landforms. Although 346 CHAPTER 10 GLACIOVOLCANISM

Table 10.1 Classification and Major Characteristics of Monogenetic Glaciovolcanic Landforms and Sequences Felsic Tuya (2): Felsic Tuya (1): Lava Flow Felsic Domes and Landform Mafic Tuya Tephra-Dominated Dominated Lobes Composition Basalt Andesite, dacite, Andesite, dacite, Andesite, dacite, rhyolite, trachyte, rhyolite, trachyte, rhyolite, trachyte, phonolite phonolite phonolite Morphology Table mountain, mesa Table mountain, mesa Flat-topped column, Tall steep-sided bladed ridge domes, lobes, or irregular Characteristic Subaerial lava, glassy Sheet lava, tephra, Sheet lava, tephra; Lava, glassy lithologies breccia, tephra pumice; minor minor polymict breccias polymict sediments sediments Architecture Lava-fed delta on Lava-capped Lava column Lava mound (mainly precursor tephra cone table mountain; ash endogenous?) (tindar) and/or pillow core mound Deposit type Vent edifice Vent edifice Vent edifice Vent edifice Eruptive Ice sheet; glacial lake Ice sheet Ice sheet Alpine glacier, environment mountain ice cap, ice sheet Known Iceland, Canada, Iceland Canada Iceland, Canada distribution Antarctica Tephra Mound/ Sheet-Like Landform Ridge (Tindar) Pillow Mound/Ridge Sequencesa Esker-Like Lavas Composition Basalt, andesite, Basalt Basalt; rare trachyte, Basalt dacite, rhyolite phonolite, rhyolite Morphology Steep cone or ridge Low oblate smooth Thin sheets, sinuous Low straight to mounds, subdued ribbons and lobes; sinuous ridge; steep- ridges may be widespread sided and voluminous Characteristic Tephra; basal pillow Pillow lava Sheet lava, local Lava, pillow lava; lithologies lava pillow lava, glassy minor hyaloclastite tephra Architecture Tall tephra mound/ Low mound/ridge of Ribbon- or sheet-like; Low lava ridge ridge with steep sides; pillow lava interbedded lava and some with pillow lava glassy tephra base Deposit type Vent edifice Vent edifice Outflow facies Outflow facies Eruptive Ice sheet; glacial Ice sheet Alpine glacier, Not well known; environment vault/lake mountain ice cap; ice probably Alpine sheet glacier, mountain ice cap; ice sheet Known Iceland, Canada, Iceland, Canada, Iceland, Canada, Canada distribution Antarctica Antarctica Antarctica

The table includes all currently known glaciovolcanic types that erupted beneath ice. It excludes those that are simply ice-impounded and otherwise subaerial. aTwo types of sheet-like sequences formerly recognized (Smellie, 2008) have now been combined into a single group (Smellie and Edwards, 2016). Modified after Smellie, J.L., 2013. Quaternary volcanism: subglacial landforms. In: Elias S.A. (Ed.), The Encyclopedia of Quaternary Sciences, vol. 1, second ed. Elsevier, Amsterdam, pp. 780À802. 10.6 CLASSIFICATION OF GLACIOVOLCANIC SEQUENCES AND LANDFORMS 347

FIGURE 10.8 Classification of mafic and felsic glaciovolcanic landforms and their principal constituent lithofacies (modified after Smellie, J.L., 2013. Quaternary volcanism: subglacial landforms. In: Elias, S.A. (Ed.) The Encyclopedia of Quaternary Sciences, vol. 1, second ed. Elsevier, Amsterdam, pp. 780À802). There is a hierarchy of the landforms with an evolutionary progression indicated by the arrows, e.g., pillow mounds evolving up into tindars, then into tuyas. The scale is only indicative. From Smellie, J.L., Edwards, B.E., 2016. Glaciovolcanism on Earth and Mars. Products, Processes and Palaeoenvironmental Significance. Cambridge University Press, Cambridge, 487 pp. glaciovolcanic polygenetic edifices are common, they are not morphologically distinctive (Smellie, 2013; Smellie and Edwards, 2016). Some of the monogenetic edifices are morphologically distinct, mainly the different types of tuyas (mafic and felsic, broadly flat-topped and encircled by steep sides), but most are not and a glaciovolcanic origin can only be determined from examining the constituent lithofacies (Fig. 10.9). A hierarchy of landforms is also evident, whereby some 348 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.9 Montage showing different types of glaciovolcanic edifices. (A) Intermediate-composition subglacial dome; Ember Ridge, British Columbia. The rock face is c. 150 m high. Image: Paul Adam. (B) Transverse section through a subglacial esker-like basalt lava, Cheakamos River valley, British Columbia. The outcrops shown in (A) and (B) contain radially orientated (fanning) columnar joints around their surfaces, indicating their original shapes (dashed lines). (C) Felsic tuya; Laufafell, Torfajo¨kull, Iceland. The tuya is tephra-dominated and capped by two subhorizontal rhyolite lavas. It rises about 400 m above the surrounding plain. (D) Small polygenetic basaltic tuya about 1.5 km in diameter; Shield Nunatak, Victoria Land, Antarctica. (E) Flank of a basaltic tindar (tephra ridge) showing its pillow lava and pillow breccia core draped by explosively erupted tuffs in which the bedding becomes less steep upward. (F) Two tephra-dominated rhyolitic tuyas with flat lava caps towering above a rhyolitic tephra mound (tindar) in the foreground; O¨ gmundur, Kerlingarfjo¨ll, Iceland. 10.7 GLACIOVOLCANIC SEQUENCES AS PALAEOENVIRONMENTAL PROXIES 349

glaciovolcanic edifices can evolve into others (Fig. 10.8). By contrast, a recent alternative classification regards many of the glaciovolcanic landforms as different types of tuyas (Russell et al., 2014). Thus, tuyas, as used following the original description by Mathews (1947), i.e., flat-topped volcanic landforms with steep sides, are regarded as simply one of several categories of tuya in the alternative classification and are called flat-topped tuyas. Similarly, tephra mounds (tuff cones) and ridges (tindars) are renamed conical and linear tuyas, respectively. Pillow lava ridges are called pillow-dominated linear tuyas and large polygenetic glaciovolcanoes are included in a group called complex tuyas even though the edifices may show no distinctive tuya-like morphology. The new classification excludes important glaciovolcanic categories such as sheet-like sequences, esker-like lavas (the ‘subglacial flows’ of Kelman et al., 2002), and felsic domes/lobes. It is thus not comprehensive, creates two parallel and thus potentially confusing extant classifications and has no clear overriding advantage over the well-established existing classification of glaciovolcanic landforms (see Smellie, 2007, 2009, 2013). It is not adopted in this chapter (see Table 10.1; Fig. 10.8).

10.7 GLACIOVOLCANIC SEQUENCES AS PALAEOENVIRONMENTAL PROXIES Once a glacial setting has been reasonably proven (see Box 10.2), volcanic sequences are major sources of environmental information relating to past ice sheets. The most useful sources are the large long-lived polygenetic volcanoes, which often have lifetimes .1 million years. They contain volcanic sequences formed during multiple dateable eruptions and, thus, it is possible to track the temporal changes that occur in any associated ice sheet during the life of the host volcano, including periods when an ice cover was absent. However, with periods between eruptions amounting to centuries, thousands of years, or tens of thousands of years (sometimes $ 100 kiloyears; Smellie et al., 2008), the volcanic record contains many gaps. Some monogenetic volcanic fields also contain products erupted over similar timescales as the polygenetic edifices (e.g., abundant monogenetic edifices in Iceland associated with enhanced magma production at a spreading centre affected by a mantle plume) and their temporal environmental record may compare well with that of the polygenetic volcanoes. However, the glaciovolcanic records are of low resolution compared with those contained in many marine sequences and they are herein called punctuated. In rare instances, eruptions may be frequent enough to be regarded as essentially continuous, e.g., the nonglacial Quaternary European volcanic field (Fig. 10.4 and see later). The use of glaciovolcanic sequences as proxies for past ice sheets arises because many retain unique evidence for important ice sheet parameters (Box 10.1). Recognizing those parameters relies on identifying the distinctive lithofacies and, in particular, the sequence architecture. One of the most important aspects of the sequence architecture consists of subaerial:subaqueous transitions, which reflect the elevation of the meltwater lake that is created during most glaciovolcanic eruptions (Fig. 10.5). During a subglacial eruption, the abundant meltwater created by geothermal heat accumulates around the vent. The initial lithofacies are therefore erupted and emplaced subaqueously. In most instances, the meltwater depth (as measured by the total thickness of subaqueous lithofacies) is a generally reliable but approximate proxy for the thickness of the surrounding impermeable ice. An additional modification needs to be made to allow for ice surface sagging 350 CHAPTER 10 GLACIOVOLCANISM

BOX 10.2 EVIDENCE FOR A COEVAL GLACIAL SETTING FOR VOLCANIC SEQUENCES 1. A known history of glaciations in a region; such evidence can be ambiguous and may simply indicate that ice was present before or after eruptions took place; 2. Relationships between the volcanic and glacial sedimentary rocks that indicate contemporaneity, e.g., breccias or lavas sunk into till and till back-injected up into the overlying volcanic rocks; 3. Presence of subaqueous lithofacies (e.g., pillow lava, hyaloclastite) at elevations far above contemporaneous sea level, and absence of a palaeotopography that might have ponded surface water; together these observations suggest the water was probably meltwater impounded by ice; 4. Lavas and intrusions showing evidence (as distinctive sets of cooling fractures) for rapid water chilling in a terrestrial setting (Fig. 10.9D); 5. Presence of horizontal prismatic columns; this indicates chilling against a vertical ice barrier (Fig. 10.10D); 6. Beds of tephra at high elevations in tuff cones or tuff ridges that are either shallow-dipping (much less than stable angle of repose) or back-tilted towards source (Fig. 10.10C); these relationships signify banking of subaqueously deposited tephra against an ice wall of an englacial vault filled by meltwater; 7. An absence of marine fossils in subaqueous tephra sequences (tuff cones and tuff ridges); marine tephra deposits are rapidly colonized by organisms and plants (Fig. 10.10F) and their absence is consistent with a freshwater environment and possibly meltwater; 8. An absence of erosional surfaces together with evidence for traction currents with varied transport directions, perhaps bidirectional, wave bedforms, and an absence of strongly abraded clasts; these features are common in shallow marine sequences and their absence (together with (7)) signifies an exceptionally protected low-energy setting, such as in an englacial vault or lake (Fig. 10.5); 9. Presence of abundant large-scale (hundred metres extent) concave-up surfaces within subaqueous tephra successions (Fig. 10.10A) sometimes associated with large displaced blocks of the tephra sequence (e.g., Smellie, 2001); these features are attributed to multiple sector collapses of unstable piles of tephra, with frequent collapses triggered by retreat (by melting) of the enclosing ice walls against which the tephra was banked; such features also occur in marine-erupted tephra cones but they are much less common; 10. Presence of contemporaneous fossil cliff-lines within the subaerial lavas of the capping units in lava-fed deltas; such cliffs are common in marine-emplaced lava-fed deltas due to their much more exposed situations and erosive effects of storms and strong wave action, which are absent in very protected englacial vaults/lakes (Fig. 10.5); 11. Passage zones (a type of subaqueous:subaerial transition; see main text) in lava-fed deltas that vary in elevation (up and down) by up to several tens of metres; some passage zones also rise progressively (Fig. 10.11); lava-fed deltas advancing into the sea will result in horizontal passage zones only; 12. Passage zones that are orientated parallel to the slope of the underlying bedrock surface and which dip radially away from the source vent; this feature, impossible in a marine-emplaced delta (in which passage zones will be horizontal), signifies that the delta advanced down the ice-draped slopes of the volcano (Fig. 10.10B); 13. Distinctive compositions of authigenic minerals (zeolites) indicating growth in fresh water rather than seawater and therefore possibly glacial meltwater (Johnson & Smellie, 2007); whilst the methodology has been successfully applied (Smellie, 2008; Nawrocki et al., 2011), care has to be taken as zeolite compositions can be altered by the subsequent passage of porewater fluids with compositions unrelated to the original eruptive setting (Johnson and Smellie, 2007; Antibus et al., 2014).

towards the erupting volcano (well illustrated by Gudmundsson et al., 2004 for the Gjalp´ subglacial eruption in Iceland, 1996; Fig. 10.7) and for the thickness of permeable firn, snow, and any fractured ice (Smellie et al., 2011b, Supplementary Information; Fig. 10.5). However meltwater depth can be quite variable at times. It will accumulate until (1) the ice is floated and the meltwater escapes in a sudden rush as a jo¨kulhlaup and the vault is emptied; (2) it escapes subglacially contin- uously in a leaky system; or (3) it overflows supraglacially via permeable layers (snow, firn, 10.7 GLACIOVOLCANIC SEQUENCES AS PALAEOENVIRONMENTAL PROXIES 351

fractured ice; cf. Bjo¨rnsson, 1988; Smellie, 2001, 2006; Magnu´sson et al., 2012) leading to a maximum elevation for the meltwater lake surface. For eruptions in warm ice, the hydraulics are particularly dynamic and in some situations the subaerial:subaqueous transition can vary in elevation by several tens of metres during a single eruption depending on the balance achieved between volcano-induced meltwater generation and subglacial meltwater escape (Smellie, 2006). Some examples are spectacular, with transitions falling abruptly or rising steeply through tens of metres (Figs. 10.10E and 10.11). The most conspicuous subaerial:subaqueous transitions occur in mafic p¯ahoehoe lava-fed deltas because of the strong structural (bedding orientation) and colour contrasts: the capping subaerial lithofacies are subhorizontal grey lavas and the underlying subaqueous lithofacies are large-scale homo- clinal foreset beds composed of orange-brown (palagonite altered) tuff breccia that dip at ca. 25À40 degrees (angle of repose). Five different types of subaqueous:subaerial transitions in lava-fed deltas have been defined so far (Fig. 10.12). Most are diagnostic of glacial eruptive conditions. Probably because of rheological differences (higher viscosity), felsic lava-fed deltas do not occur and intermediate-composition examples are relatively uncommon. The latter include mugearite and tephri- phonolite ‘a‘¯a lava-fed deltas in northern Victoria Land, Antarctica. The lithofacies in ‘a‘¯alava-feddel- tas differ in several respects from those present in p¯ahoehoe-fed deltas and the transition zone, also called a passage zone (Jones, 1969), is a more coarsely defined feature (Smellie et al., 2011a,b, 2013a). The presence of passage zones in tephra mounds and ridges has also been postulated, together with elevation variations analogous to those seen in lava-fed deltas (Russell et al., 2013). Criteria that may be used to distinguish subaerial from subaqueous tephra include the following: (1) abun- dance of accretionary and armored (ash-coated) lapilli (subaerial); (2) presence of low-angle cross- stratified sandwaves and antidunes (subaerial); (3) small-scale ripple bedding (subaqueous); (4) marine fossils (subaqueous); and (5) presence of pyroclastic fall beds and impact structures associated with ballistic bombs and blocks (subaerial) (Smellie and Edwards, 2016). However, most of the criteria are difficult to apply unambiguously for environmental discrimination and, as there are also no visible colour or structural contrasts (unlike lava-fed deltas), any pyroclastic passage zone will be a very ill-defined feature and exceptionally hard to recognize. In practical terms, it is effectively invisible and therefore probably impossible to use as a palaeoenvironmental criterion. Moreover, for meltwater lake fluctuations to occur during the eruption probably implies that the tuff cone formed over a relatively prolonged period (months, years, decades), whereas the principal construction of glaciovolcanic tuff cones or tindars (representing mainly vertical aggradation) takes place in the early stages, switching to lava-fed delta progradation soon after subaerial emergence of the tephra cone. The period of emergence of the tuff cone is thus short (days, weeks) before the explosivity is replaced by lava effusion and in most cases it is unlikely that there is enough time to develop the meltwater lake fluctuations seen in the longer-lived lava-fed deltas (Smellie, 2006). Therefore, the existence and geometrical characteristics of pyroclastic passage zones, as postulated by Russell et al. (2014), are very uncertain and their general recognition is unlikely. Subaerial:subaqueous transitions are much less common in other glaciovolcanic sequences. They do not occur in pillow lava piles (wholly subaqueous) and are probably unlikely in esker-like lavas and mafic sheet-like sequences (generally wholly subglacial) but they can be present in some glaciovolcanic felsic domes and felsic sheet-like sequences. Examples of the latter have been described that show some combination of (1) basal glass-rich and aphanitic breccias, fine-scale jointing, spiracles, and perlitization (a product of pervasive devitrification in a warm wet 352 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.10 Montage of selected glaciovolcanic features showing evidence for coeval ice and volcanism. (A) Multiple slump scar surfaces (yellow lines) caused by sector collapses as enclosing ice walls melted back; Kalfstindar, Iceland. (B) Sketch showing how passage zones in lava-fed deltas advancing through slope ice are parallel to the underlying bedrock surface, a geometry diagnostic of a glacial setting (from Smellie, J.L., Rocchi, S., Gemelli, M., (Continued) 10.7 GLACIOVOLCANIC SEQUENCES AS PALAEOENVIRONMENTAL PROXIES 353

FIGURE 10.11 Example of a rising passage zone in a basaltic lava-fed delta; James Ross Island, Antarctica. The passage zone is horizontal between A and B. Between B and C the passage zone rises as the elevation of the meltwater lake surface rose progressively during delta advancement (from right to left). At times the meltwater flooded over the delta surface, resulting in the two prominent beds of massive breccia seen, which formed when the delta readvanced. The delta is c. 150 m thick.

environment) (i.e., wet conditions); and (2) fully crystalline massive lava with coarsely spaced joints higher up draped by crystalline carapace breccia that may show oxidation (i.e., subaerial conditions; Smellie et al., 2011a,b). In these cases, although the dome base was clearly chilled by water and the top was subaerial, it is hard to define a precise position for the subaerial:subaqueous transition and its elevation can thus only be inferred approximately. However, most felsic domes

L Di Vincenzo, G., Armienti, P., 2011b. Late Miocene East Antarctic ice sheet characteristics deduced from terrestrial glaciovolcanic sequences in northern Victoria Land, Antarctica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 307, 129À149). (C) Flank exposure of a basaltic tindar showing bedding (white lines) that becomes more horizontal upward due to banking against enclosing ice walls during eruption; the outcrop is c. 200 m above the surrounding lava plain; Herdubreidarto¨gl, Iceland. Image: Stephanie Tucknott. (D) Horizontal polygonal columns at the margin of an intermediate-composition ice-impounded lava, caused by cooling against steep ice walls; Mt Rainier, USA. (E) Delta front overlap structure caused by rapid subglacial drainage of meltwater during the emplacement of a basaltic lava-fed delta; James Ross Island, Antarctica (after Smellie, J.L., 2006. The relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions: an important unresolved conundrum. Earth-Sci. Rev. 74, 241À268). The drop in water level has resulted in subaerial lavas being banked against subaqueous delta foreset breccias. (F) Impression of an asterozoan in tuffs of a marine-emplaced tuff cone; James Ross Island, Antarctica. Marine-erupted tuff cones are rapidly colonized, even during the eruptive period (Williams et al., 2006). 354 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.12 Sketch summarizing the five different types of subaqueous:subaerial transitions currently recognized. From Smellie, J.L., 2006. The relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions: an important unresolved conundrum. Earth-Sci. Rev. 74, 241À268. (A) Cone surface overlap structure; (B) delta front overlap structure; (C) horizontal passage zone; (D) rising passage zone; (E) falling passage zone. Types (B), (D), and (E) are diagnostic of glacial conditions. are emplaced subglacially and their outcrops can yield only minimum estimates for coeval ice thicknesses (Kelman, 2005). The value of glaciovolcanic sequences lacking clear passage zones lies principally in their ability to identify glacial conditions, without being able to yield other criteria of the coeval ice other than the age of that ice (through isotopic dating of the lavas).

10.7.1 DETERMINING BASAL THERMAL REGIME The thermal regime is important for the interpretation of glaciovolcanic investigations in a palaeoenvironmental context. It is an accurate measure of the relative stability or dynamism of an ice cover. Simply stated, cold ice is stiffer and harder to deform internally and its presence effectively pins an ice sheet whilst, depending on the permeability of the bed, wet-based ice (i.e., warm or polythermal) has a basal surface lubricated by a thin film of meltwater that aids much more rapid movement mainly by sliding and internal deformation. The thermal regime can be deduced from two principal lines of evidence: (1) geometry of the passage zone in lava-fed deltas; and (2) characteristics of the surfaces separating units from different eruptions. Under cold ice conditions, meltwater is unable to escape basally. The meltwater 10.7 GLACIOVOLCANIC SEQUENCES AS PALAEOENVIRONMENTAL PROXIES 355

simply accumulates until it overflows, leading to a more or less stable passage zone elevation that will reflect the elevation of the surrounding ice (there will be a slow progressive lowering of the lake surface (and thus passage zone elevation) caused by thermal erosion at the spillway; Smellie, 2006). Under wet-based ice conditions, the hydraulics are much more dynamic and passage zone elevation will fluctuate according to a balance between meltwater generation and escape. This is generally true for long-lived eruptions with voluminous lava-fed deltas. However, eruptions of most small monogenetic tuyas are short-lived and much less voluminous and meltwater elevations are probably essentially stable during the short eruptive period whether the eruption is within wet- based or cold ice. By contrast, the physical characteristics of inter-eruption surfaces are a much more reliable criterion for determining thermal regime. The surfaces represent time gaps, i.e., periods when no eruption took place. They will show evidence for erosion (striations, ice moulding) and subglacial sediments (tills, fluvial sediments) if the ice was wet-based, but will be largely unmodified beneath cold ice because of the essentially protective effects of that ice (notwithstanding the caveats described by Atkins et al., 2002 and Atkins, 2013). Moreover, because of the ability of a draping ice sheet to reform after each eruption, the units provide a succession of freshly created surfaces with which to interact with the reformed ice. Thus, by isotopically dating successive erupted units and observing the physical features present on each surface, it is possible to map the temporal evolution of basal thermal regime at a single geographical locality. Only one study of this type has been undertaken so far (Smellie et al., 2014).

10.7.2 INFLUENCE OF VOLCANIC HEAT ON BASAL THERMAL REGIME Intuitively, volcanic heat may strongly affect the basal thermal regime of any overlying ice but the effects have not yet been examined in detail for terrestrial examples. In cases where the ambient ice is cold-based, the thermal regime may be changed to wet-based and it thus will not be representative of the local climate regime. For wet-based ice influenced by volcanic heat, no change will be detectable apart, possibly, from an increase in the volumes of meltwater present. Volcano heating effects on basal regime have been modelled for two cases using Mars examples and environmental conditions (Gulick, 1998; Fassett and Head, 2006, 2007). A first attempt for terrestrial glaciovolcanism was made by Smellie and Edwards (2016) and it is further developed here because of its potential importance for the use of glaciovolcanism as a palaeoenviromental proxy. The influence of volcanic heat on thermal regime is only relevant to studies of large polygenetic glaciovolcanoes. The short-lived monogenetic volcanoes will probably always accurately preserve a snapshot of the local climate-related basal regime as they are not generally preceded by long periods of enhanced volcanic heat flux. Basal thermal regime is influenced by a combination of variables, including ambient tempera- ture, precipitation, ice thickness, and geothermal heat flux. Apart from the latter, they are all climate-related and they vary spatially and temporally. Volcanic heat is a form of geothermal heat and it is a conservative property (i.e., it adds to the regional heat flux). The two Mars studies used different boundary conditions: transient conductive heat flow under ‘dry’ conditions (Fassett and Head, 2006, 2007); and hydrothermalism associated with a continually replenished groundwater system (Gulick, 1998). Of the two, the study by Gulick (1998) is probably closer to terrestrial conditions but, nonetheless, the net results of both sets of studies are remarkably similar. Both suggested that the volcanic-induced additional heat flux is focused in the summit region of the volcano, within a concentric zone with a diameter approximately three times that of the underlying 356 CHAPTER 10 GLACIOVOLCANISM

cooling magma chamber (the heat source). Furthermore, the hydrothermally driven upward heat flow is probably more narrowly restricted than that modelled by Gulick (1998) because of a combination of (1) the effect of radially outward-dipping impermeable beds of the volcano deflecting and focusing the groundwater flow more centrally (Smellie and Edwards, 2016; Gulick’s models assumed horizontal bedding, so a centroclinal heat flow was not an outcome); and (2) the continual influx of cold groundwater that will constrict the outward diffusion of heat (Gulick, 1998; Fig. 10.13). Thus, much of the surface of any large volcanic edifice will probably be situated out- side of the summit zone of enhanced heating and evidence for the contemporaneous basal thermal regime preserved in those rocks will accurately reflect the climate-related conditions. That volcanic heat flux does not always override the local climate-related thermal regime is demonstrated by the preservation of evidence for polar ice conditions during eruptions (Wilch and McIntosh, 2002; Smellie et al., 2011a,b, 2014).

FIGURE 10.13 Cartoon showing heat flux in a polygenetic glaciovolcano. The surface effects of the zone of enhanced heating (grey colouration) are limited to a relatively narrow zone at the summit of the volcano, caused by the centroclinal focusing effects of outward-dipping impermeable beds. In the model, the climate-related basal thermal regime of overlying ice is affected by the volcanic heat only in the summit region. 10.8 CASE STUDIES USING GLACIOVOLCANISM 357

Knowing approximately how far an outcrop is situated from its source vents (crater/caldera) of a glaciovolcano is thus important. For example, it is easily deduced for well-preserved shield volcanoes in northern Victoria Land, Antarctica, where the Late MioceneÀEarly Pliocene thermal regime varied spatially and temporally between cold and wet-based (i.e., a polythermal ice sheet; Smellie et al., 2011a,b, 2014). Conversely, vent locations are not known for glaciovolcanic sequences at Minna Bluff, in southern Victoria Land, where the original volcano shapes are much more degraded and the source vent locations unknown. The thermal regime at the latter locality is unvaryingly wet-based between c. 11 and 9 Ma (Smellie et al., 2014). However, Antibus et al. (2014) suggested intuitively that the thermal regime was an artefact of an enhanced local volcanic heat flow. Until the positions of the sequences with respect to their source vents are determined, it is probably impossible to distinguish between the two hypotheses.

10.8 CASE STUDIES USING GLACIOVOLCANISM TO RECONSTRUCT PAST ICE CONDITIONS 10.8.1 ANTARCTICA—1: ANTARCTIC PENINSULA ICE SHEET (APIS) Glaciovolcanic rocks in the Antarctic Peninsula extend back to 7.7 Ma (Smellie et al., 1988) and the region includes the large volcanic field known as the James Ross Island Volcanic Group (JRIVG), which extends over an area of 7000 km2 and is dominated by the Mt Haddington shield volcano. The JRIVG contains evidence for at least 50 discrete eruptive episodes, almost all of which were glaciovolcanic, and there is a published detailed geological map (Skilling, 1994; Smellie et al., 2006b, 2008, 2013b). It has also been the source of discoveries important for understanding the formation of glaciovolcanic sequences generally, especially lava-fed deltas and their palaeoenvironmental significance (Nelson, 1975; Jones and Nelson, 1970; Smellie and Skilling, 1994; Smellie and Hole, 1997; Skilling, 2002; Smellie, 2006; Smellie et al., 2006a, 2008; Johnson and Smellie, 2007; Calabozo et al., 2015; Nehyba and Ny´vlt, 2015) and it has been the subject of the only published glaciovolcanic investigation linked to climate modelling (Smellie et al., 2009). The Antarctic Peninsula also contains the sequence holotypes for small-volume sheet-like sequences (known as Mt Pinafore type: Smellie et al., 1993; Smellie, 2008; but see Smellie and Edwards, 2016). Two principal generic types of glaciovolcanic sequences occur in the Antarctic Peninsula: (1) lava-fed deltas; and (2) sheet-like sequences; all examples are mafic (Smellie et al., 2006c, 2009; Fig. 10.14). Lava-fed deltas are the commonest because they dominate the long-lived JRIVG (Smellie et al., 2008), but elsewhere sheet-like sequences are prevalent although fewer in number (Smellie et al., 1993). Using a dataset of c. 100 isotopic ages (most by 40Ar/39Ar; the JRIVG is the most intensively dated volcanic field in Antarctica), it was demonstrated that the APIS was wet- based throughout the period (probably polythermal) and ice thicknesses varied from c. 100 to 850 m (Hambrey and Smellie, 2006; Smellie et al., 2008, 2009; Hambrey et al., 2008; Nelson et al., 2009). Ice thicknesses may also have increased generally in the younger periods, possibly in response to overall globally cooler temperatures and its impact on Antarctic ice (colder, stiffer ice, less able to move dynamically, therefore thicker) but the information is not definitive (Fig. 10.15). The dataset includes .50 estimates for ice thickness and thermal regime. Volcanic features situated 358 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.14 Cartoons showing generic glaciovolcanic sequence types present in the Antarctic Peninsula (A and B) and northern Victoria Land, Antarctica (C and D). In both regions, the two types are lava-fed deltas and sheet-like sequences but they differ in the lithofacies present principally due to different magma compositions and rheologies (see text for explanation). Modified after Smellie, J.L., Haywood, A.M., Hillenbrand, C.-D., Lunt, D.J., Valdes, P.J., 2009. Nature of the Antarctic Peninsula Ice Sheet during the Pliocene: geological evidence & modelling results compared. Earth-Sci. Rev. 94, 79À94; Smellie, J.L., Rocchi, S., Gemelli, M., Di Vincenzo, G., Armienti, P., 2011b. Late Miocene East Antarctic ice sheet characteristics deduced from terrestrial glaciovolcanic sequences in northern Victoria Land, Antarctica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 307, 129À149. at high elevations in the JRIVG are essentially unmodified by ice. This observation and others in the region suggest a maximum limit for the thickness of the (erosive) APIS. It was probably always a relatively thin draping ice cover (,750À850 m; Smellie et al., 2008, 2009) and was never the very thick (2À2.5 km) ice sheet postulated by some modelling studies (e.g., Denton and Hughes, 1981). It is the most detailed temporally constrained dataset for any part of the Antarctic Ice Sheet, covering the period between 7.7 Ma and the present. In addition, the prevalence of glaciovolcanic eruptions during the period (only a few indicated subaerial or marine conditions; e.g., Johnson and Smellie, 2007; Smellie et al., 2008) together with many periods characterized by unusually thin ice suggest that not only glacial but interglacial periods are represented and that the interglacials were 10.8 CASE STUDIES USING GLACIOVOLCANISM 359

FIGURE 10.15 Diagram summarizing ice thicknesses deduced from glaciovolcanic sequences in the Antarctic Peninsula. From Smellie, J.L., Haywood, A.M., Hillenbrand, C.-D., Lunt, D.J., Valdes, P.J., 2009. Nature of the Antarctic Peninsula Ice Sheet during the Pliocene: geological evidence & modelling results compared. Earth-Sci. Rev. 94, 79À94. This is the largest dataset of ice thicknesses determined for any palaeo-ice sheet in Antarctica. ice-poor rather than ice-free. That suggestion was supported by (1) climate modelling (Smellie et al., 2009); (2) cosmogenic surface dating (Johnson et al., 2009); and (3) palynology of associated till interbeds (Salzmann et al., 2011). Isotopic and physical studies of shelly fossils in tills interbedded in the JRIVG also provided related Neogene climate information (i.e., near-shore sea surface temperatures, sea ice extent, and evidence for seasonality; Clark et al., 2010, 2013; Williams et al., 2010). The important environmental information deduced from the associated nonglaciovol- canic units on James Ross Island demonstrate that, for the most holistic results, all avenues for research should be pursued and not just a single strand. In the present context, it is important to note that, as a direct result of mainly glaciovolcanic studies, the pre-Last Glacial Maximum (LGM) Antarctic Peninsula Ice Sheet and its surrounding physical environment (presence of sea ice; sea surface temperatures; seasonality) is now one of the best-studied parts of the palaeo-Antarctic Ice Sheet. Notwithstanding the valuable insights gained from several drilling campaigns that recovered marine shelfal sediments in the Ross Sea region (e.g., Hambrey et al., 2002; Naish et al., 2009), the Antarctic Peninsula Ice Sheet has one of the most comprehensive temporal datasets of critical ice sheet parameters of any pre-Quaternary ice sheet in the world. 360 CHAPTER 10 GLACIOVOLCANISM

10.8.2 ANTARCTICA—2: WEST ANTARCTIC ICE SHEET Despite the importance of knowing the history of stability of the climatically sensitive West Antarctic Ice Sheet (WAIS), the region has largely been studied only at reconnaissance level and most of the studies predate the many major advances in understanding the origins and palaeoenvironmental uses of glaciovolcanic sequences made since 2000 (e.g., LeMasurier, 1972a,b; LeMasurer and Rex, 1982; LeMasurier et al., 1994). This is largely because the numerous large long-lived polygenetic volcanoes are little eroded and are also largely buried in ice .2 km thick and, for some, only the caldera rim is exposed (e.g., Fig. 10.16; LeMasurier and Thomson, 1990; LeMasurier and Rocchi, 2005; Rocchi et al., 2006). The record is therefore patchily exposed and has large gaps. However, the region is important historically because it provided the first unequivo- cal evidence for a pre-Quaternary Antarctic Ice Sheet (Miocene tills and glaciovolcanic sequences in the Jones Mountains; Craddock et al., 1964; Rutford et al., 1968) a decade prior to the much better-known discoveries based on marine sedimentary sequences obtained by ocean-floor drilling in the Ross Sea (Hayes and Frakes, 1975; Hambrey et al., 2002; acknowledged by Barrett, 2009). The oldest terrestrial outcrop evidence for ice in West Antarctica consists of several glaciovolcanic sequences at Mt Petras, central Marie Byrd Land, which were initially regarded, incorrectly, as remnants of a volcanic table mountain erupted beneath a thick regional ice sheet (LeMasurier and Thomson, 1990). However, Wilch and McIntosh (2000) demonstrated that the outcrops were constructed mainly during explosive hydromagmatic eruptions that formed several tuff cones at different elevations and at different times (27À29 Ma). The results indicate the former presence of a mountain ice cap and a regional ice sheet may have been absent at the time. Middle Miocene (c. 8À9 Ma) glaciovolcanic rocks crop out extensively in Marie Byrd Land at Mt Murphy and the Crary Mountains and have been interpreted in context of a complicated history of WAIS variations. The Mt Murphy outcrops (Fig. 10.17) were interpreted as evidence for a thick

FIGURE 10.16 Satellite views of two Marie Byrd Land volcanoes with prominent ice-filled calderas, showing how they are comparatively uneroded despite ages of c. 14À10 Ma. The volcanoes are largely buried in ice c. 2 km thick. See Rocchi et al. (2006). 10.8 CASE STUDIES USING GLACIOVOLCANISM 361

FIGURE 10.17 Oblique aerial view of Mt Murphy, Marie Byrd Land, a Late Miocene polygenetic glaciovolcano (LeMasurier et al.,1994), showing the distribution of basement (purple) and volcanic (blue) outcrops. The three small satellite nunataks in the foreground are also volcanic but younger than the main massif. Coastal volcanoes like Mt Murphy are often much more eroded than those inland in West Antarctica (cf. Fig. 10.16).

regional ice sheet whose surface elevation fluctuated through a few hundred metres resulting in the formation of an unusual ‘passage zone’ composed of alternating subaerial and subaqueous rocks (LeMasurier et al., 1994; Wilch and McIntosh, 2002). Despite the huge difference in scale, a broad comparison was made (probably incorrectly) with Icelandic tuyas. Mt Murphy is also important for the presence of a small outcrop of ice-marginal (glaciolacustrine?) sedimentary rocks at an elevation of 2200 m above sea level (a.s.l.) (c. 1300 m above the present ice surface) which contain recycled marine microfossils with multiple early to late Neogene ages. It was inferred that the coeval ice sheet thus had a much higher elevation than present-day ice. The presence of the microfossils was interpreted to indicate intervals of near-complete deglaciation in West Antarctica at several times between c. 24 and 3.5 Ma. Small volcanic satellite nunataks near Mt Murphy, with Late Miocene ages (c. 5À7 Ma), were interpreted as tuyas but their location, situated on a postulated glacial interfluve, potentially invalidated using their passage zone elevations as an indication of the coeval ice surfaces (Wilch and McIntosh, 2002). Finally, a tuff cone on Mt Murphy interpreted as glaciovolcanic, with an age of 0.6 Ma and at an elevation of ca. 1300 m a.s.l. (c. 500 m above the present ice surface), was cited as further evidence for a thicker Pleistocene regional ice sheet. Comparisons were also made with the glaciovolcanic sequences at the Crary Mountains. Although the volcanic rocks are broadly similar in appearance and of the same age as the main Mt Murphy shield (i.e., 8À9 Ma), the evidence suggests that the basal thermal regime of the associated ice sheet was very different: possible cold-based ice at Crary Mountains (well-preserved surfaces between eruptive units; absence of glacial tills and glaciofluvial sediments) compared with wet- based ice at Mt Murphy (prominent erosional surfaces, tills, and water-deposited sediments; Wilch and McIntosh, 2002). The Crary Mountains sequences were also interpreted as having interacted 362 CHAPTER 10 GLACIOVOLCANISM

with slope ice (i.e., an ice dome or ice cap) rather than a regional ice sheet because the Crary Mountain sequences dip radially away from their summit crater (diagnostic of slope ice; Fig. 10.10B). Although the Crary Mountains units were compared with sheet-like sequences, the sequences there and at Mt Murphy are probably products of ‘a‘¯a lava-fed deltas (unpublished information of the author, cf. Smellie, 2013). Finally, glaciovolcanic studies of several small, isolated outcrops on the Hobbs Coast of Marie Byrd Land suggested that they were eroded remnants of at least seven monogenetic volcanoes with a wide range of ages (,10.7 Ma; mostly latest MioceneÀPliocene), not all of which were glaciovolcanic (Wilch and McIntosh, 2008). Like the satellite nunataks at Mt Murphy, reconstructing the coeval ice conditions was deemed problematical owing to their occurrence on interfluves. An important con- clusion of the study was that there is no compelling evidence that regional palaeo-ice elevations were substantially higher than present (cf. see Section 10.8.3; Smellie et al., 2011a,b, 2014).

10.8.3 ANTARCTICA—3: EAST ANTARCTIC ICE SHEET Glaciovolcanism was widespread in northern Victoria Land during Miocene and Pliocene times and is represented by several fissure-erupted shield volcanoes situated along 400 km of the Ross Sea coast and fewer scattered stratovolcanoes in inland locations (LeMasurier and Thomson, 1990). The volcanic record largely fills a large gap in the regional environmental record, obtained by drilling (Fig. 10.1). Moreover, the region is historically important for the publication of a major monograph that conclusively proved, ahead of the Ross Sea ocean-floor drilling, the presence of a pre-Pleistocene ice sheet in Antarctica (Hamilton, 1972). The early study proposed a model for eruptions in which the coastal volcanoes grew up progressively through a very thick drowning East Antarctic Ice Sheet (EAIS), broadly similar to the model deduced for Mt Murphy and similarly flawed (Hamilton, 1972; LeMasurier et al., 1994; cf. Smellie et al., 2011a,b). Subsequent work identified two generic glaciovolcanic sequence types: lava-fed deltas and sheet-like lavas (Fig. 10.14). Although this appears similar to glaciovolcanic sequences identified in the Antarctic Peninsula (see Section 10.8.2), the lava-fed deltas in northern Victoria Land are different. The deltas were not fed by p¯ahoehoe but by ‘a‘¯a lavas (the first examples of glaciovolcanic ‘a‘¯a lava-fed delta lithofacies to be described anywhere) and the sheet-like sequences include unique felsic examples (Smellie et al., 2011a,b). Other volcanic sequence types include uncommon scoria cones, tuff cones, and a single glaciolacustrine sequence. The sequences also formed under contrasting glacial conditions. Whereas the lava-fed deltas show evidence for eruption in association with cold ice (uneroded sequence boundaries, lack of evidence for flowing water; no sedimentary glacial deposits), the sheet-like sequences were erupted in wet-based ice (erosional surfaces, fluvial and glacial sediments, including till). The sequence thicknesses also indicate that the ice cover was thin (,300 m) and formed a draping (rather than drowning) ice cover similar to today but, unlike the Antarctic Peninsula, there is no evidence for maximum past ice thicknesses, which may at times have been much thicker (Smellie et al., 2014). Moreover, an assessment of glaciovolcanic outcrops along 800 km of coastal Victoria Land was able to demonstrate that the basal glacial regime varied both temporally and spatially between c. 11 and 1.5 Ma and that the prevailing paradigm for the evolution of the EAIS, from a dynamic to a stable condition in a single step at either 14 Ma or c. 2.5 Ma (cf. Wilson, 1995; Lewis et al., 2007), was incorrect. The ice cover was polythermal throughout the period and probably consisted of a patchwork mosaic of cold- and wet-based ice at 10.8 CASE STUDIES USING GLACIOVOLCANISM 363

FIGURE 10.18 Diagram summarizing variations in basal thermal regime of East Antarctic ice along 800 km of Victoria Land margin, Antarctica. Also shown are estimates of thermal regime obtained from drilling offshore sedimentary successions. Note how the glaciovolcanic data are much more time-specific, enabling quite subtle temporal and geographical variations to be identified that are consistent with a polythermal (subpolar) basal regime. From Smellie, J.L., Rocchi, S., Wilch, T.I., Gemelli, M., Di Vincenzo, G., McIntosh, W., et al., 2014. Glaciovolcanic evidence for a polythermal Neogene East Antarctic Ice Sheet. Geology, 42, 39À41.

different times and in different places (Smellie et al., 2014; Fig. 10.18). Glaciovolcanic studies have thus helped partly to resolve an important long-standing ( . 30 years) controversy known as the ‘Sirius debate’ (Barrett, 2013; Smellie, 2014). Further studies are now underway to map out the scale and extent of the cold- and wet-based basal ice conditions and to seek evidence for any ice-free periods. 364 CHAPTER 10 GLACIOVOLCANISM

10.8.4 BRITISH COLUMBIA Although some of the earliest and most intuitive studies of glaciovolcanic successions took place in Canada (e.g., Mathews, 1947, 1951, 1952), only a few recent palaeoenvironmental studies have been conducted. The evidence for glacials only goes back to c. 3 Ma and eight or nine glacial periods are currently recognized from studies of glacial sediments (e.g., Duk-Rodkin et al., 2004). The associated glaciovolcanic record is important for potentially filling in some of the gaps in the patchily preserved glacial sedimentary evidence but problems distinguishing whether the ice associated with the volcanic rocks was of regional extent (ice sheet) or local (mountain ice cap) have not always been resolved. Three volcanic areas have been investigated. Hoodoo Mountain, with mainly phonoliteÀtrachyte volcanic activity spanning the past 100 kiloyears, is a small polygenetic volcano bounded partly by tall cliffs of overthickened phonolite lavas that were impounded by encircling ice (Fig. 10.2C). It contains the most detailed glaciovolcanic environmental record obtained so far in the region (Edwards and Russell, 2002; Edwards et al., 2002). Activity commenced during MIS 6 at 157 ka when the coeval ice surface was at least 100 m above that at present. Glaciovolcanic deposits are also present at the transition between MIS 5 and MIS 4 (85 ka) and indicate ice thicknesses 200À400 m greater than today. Evidence for younger glacial erosion and two till units indicate that ice was again present on the mountain during MIS 4 and MIS 2. During that period (i.e., between 80 and 54 ka), two discrete units composed of lapilli tuffs and lavas were erupted. The tuffs are not environmentally diagnostic but the lavas are subaerial and signify either ice-free conditions or they were erupted on the volcano summit ( . 1300 m a.s.l.) above any surrounding ice. Between 54 and 30 ka, the erupted rocks comprise glaciovolcanic lavas and tuff breccia coincident with MIS 2 that indicate maximum coeval ice thicknesses exceeding 1 km occurred, with an ice surface c1700 m a.s.l. The presence of tills overlying the latter indicates that ice subsequently reformed and has probably persisted at least sporadically and is represented by the present small ice cap. Mt Edziza is a much larger polygenetic volcano with a wide range of mafic to felsic compositions and an age that extends back to 7À10 Ma (Souther, 1992; Fig. 10.19). There is evidence as tills and glacial erosion for glacial activity during much of the volcanic period, but glaciovolcanism is only represented in a few of the eruptive units (,3 Ma in age) although they are geographically widespread. Most have not been investigated in detail but those forming Pillow Ridge and Tennena Cone formed under ice that was much thicker ( . 400 m) than today (Edwards et al., 2009; Hungerford et al., 2014). The ice may be related to expansion of the current ice cap. Further work is required to improve the precision of isotopic ages of the multiple nonglacial and glaciovolcanic eruptive units and associated glacial deposits on Mt Edziza. Lastly, 40Ar/39Ar isotopic dating of multiple mainly monogenetic volcanic sites in the KawdyÀTuya region has established the age of the oldest regional ice sheet so far known in British Columbia, at 2.8 Ma, together with multiple younger periods in which the Cordilleran Ice Sheet was present, with the youngest at 66 ka (Edwards, in Smellie and Edwards, 2016). The region is otherwise devoid of sedimentary evidence for glacial conditions. Isotopic dating at one locality, Mathews Tuya, yielded an age of 0.718À0.742 Ma, which provided the first firm evidence for thick ( . 700 m) glaciers in north-central British Columbia and coincident with large global ice volumes during MIS 18. 10.8 CASE STUDIES USING GLACIOVOLCANISM 365

FIGURE 10.19 View of Mt Edziza, British Columbia (Canada), a particularly long-lived polygenetic volcano with glaciovolcanic eruptive phases. The view shows well-formed Eve Cone in the foreground, which is a postglacial subaerial scoria cone set in an extensive subaerial lava field, and the ice-covered summit of Mt Edziza. Mt Edziza has a 3-km- wide ice-filled crater or caldera formed in nonglacial volcanic rocks. Pillow Ridge, situated just below the skyline and trending to the right, is glaciovolcanic and formed under much thicker ice than present. Image: John Scurlock.

10.8.5 ICELAND Like British Columbia, the use of glaciovolcanic sequences to reconstruct past ice conditions in Iceland has lagged far behind comparable studies in Antarctica. This is largely because most of the studies have focused on the abundant small monogenetic edifices which, by definition, give only an essentially instant snapshot of the coeval ice conditions and there have been few attempts to place the ice conditions in a regional context (Licciardi et al., 2007; see also Bourgeois et al., 1998). Moreover because of their youth (last Glacial) and the low-K compositions of the mainly basaltic lavas, there have been few attempts to date those edifices, those by Flude et al. (2008, 2010, for felsic glaciovolcanic sequences) being the most notable exceptions. In addition, few of the large Icelandic polygenetic volcanic complexes have been investigated environmentally and some are surprisingly short-lived (tens of kiloyears rather than hundreds of kiloyears; e.g., Askja: McGarvie et al., 2013). Others are longer-lived (e.g., Eyjafjallajo¨kull: ,800 kiloyears: Loughlin, 1995) but the lifetimes of most are poorly known. However, the potential for longer-term documentation of the Icelandic glacial environment was demonstrated by Helgason and Duncan (2001) in their study of volcanic sequences and associated glacial deposits at Skaftafell, southern Iceland. Using a vari- ety of dating and correlating methods, including palaeomagnetism and K-Ar isotopes, they noted 366 CHAPTER 10 GLACIOVOLCANISM

the interbedding of subaerial (mainly sheet lavas) and subglacial (pillow lavas and hyaloclastite) units, which they ascribed to alternating glacial and interglacial conditions (Fig. 10.20). They identified at least 16 glacial and interglacial intervals that occurred over a period of c. 5 million years. Despite the presence of some substantial gaps in the sequence, it is the most detailed environmental history documented for any part of Iceland so far. Eruptions during the glacial periods constructed pillow lava ridges, breccias, and sedimentary rocks, including tills, whereas the interglacial periods were characterized by subaerial lavas and thin sedimentary interbeds. A high level of stratigraphical complexity was evident (also clear from the author’s experience of the polygenetic Eyjafjallajo¨kull and O¨ ræfajo¨kull volcanoes), and lateral correlations between outcrop ridges sometimes proved to be very difficult due to glacial erosion between the eruptive phases. The large number of glacial and interglacial periods represented compared to their paucity elsewhere in Iceland (cf. Fig. 10.21) was ascribed to the proximal location of the Skaftafell sequences relative to the evolving coeval Iceland ice dome. However, the physical characteristics of the volcanic units were not well described by Helgason and Duncan (2001). It is highly likely that a reinvestigation of the region, and other localities in Iceland, will yield important new information on late PlioceneÀPleistocene ice parameters as well as some surprises.

10.9 SUMMARY The study of glaciovolcanism is a young science, with many rapid advances in knowledge made principally during the past 15 years. It is defined as the interaction between magma and ice, firn, and snow and any meltwater derived from that interaction. Ice exerts a fundamental control on any volcanoes that attempt to erupt subglacially, principally by confining the eruptive products and the meltwater created, and suppressing or permitting magmatic explosivity. Thereafter, ice hydraulics determine the course of an eruption and, specifically, the kinds of lithofacies present (e.g., pillow lava or tephra) and the order in which they are formed (e.g., pillow lava before tephra before lava-fed deltas). Because of these intimate relationships, glaciovolcanoes are remarkable palaeoenvironmental repositories, preserving information specifically relating to past ice conditions. Unlike most terrestrial glacigenic deposits, which are typically thin and easily erased by successive glaciations, glaciovolcanic successions are characteristically much thicker and mantled by erosion-resistant lavas. They are thus durable and normally survive multiple glaciations. A range of critical parameters can be deduced routinely from glaciovolcanic successions, including establishing the presence of ice; its age (by isotopic dating of associated lavas); thickness; surface elevation; and thermal regime. This is the widest and thus the most holistic range of parameters of any proxy method currently used to investigate past ice masses. Glaciovolcanic palaeoenvironmental studies have been applied successfully to reconstructing past ice conditions principally in Antarctica for which the longest record exists, with fewer examples for Iceland and Canada (British Columbia). As a result of glaciovolcanic studies, multiple critical parameters of several major parts of the Antarctic ice sheet are now known in greater detail than any other pre-LGM ice masses on Earth. However, considerable work remains to be done, not only in Antarctica but particularly in Iceland and British Columbia where extensive ice conditions were also formerly present but remain poorly known. 10.9 SUMMARY 367

FIGURE 10.20 Summary of volcanic successions at Skaftafell and Hafrafell, Iceland, showing interpreted eruptive setting. The glaciovolcanic sequences are the most detailed terrestrial geological record of glacial/interglacial periods for Iceland. From Helgason, J., Duncan, R.A., 2001. Glacial-interglacial history of the Skaftafell region, southeast Iceland, 0-5 Ma. Geology 29, 179À182. 368 CHAPTER 10 GLACIOVOLCANISM

FIGURE 10.21 Stratigraphical succession at (A) Fljo´tsdalur and (B) Jo¨kuldalur, east Iceland, showing volcanic and interbedded glacial and interglacial sedimentary rocks. Glaciovolcanic rocks occur mainly toward the top of the section and interglacial sediments are present to base. The protective effects of the abundant lavas have helped to preserve a more detailed environmental history than otherwise would be expected (cf. Fig. 10.20). From Geirsdo´ttir, A´., Miller, G.F., Andrews, J.T., 2007. Glaciation, erosion, and landscape evolution of Iceland. J. Geodyn. 43, 170À186 (Geirsdo´ttir et al., 2007).

ACKNOWLEDGEMENTS

This chapter summarizes much research carried out principally by the author over the past 30 years. The author is very grateful to Ben Edwards, Snaebjo¨rn Gudbjo¨rnsson, Tom Pfeiffer, Damian Gore, Stephanie Tucknott, John Scurlock, and Paul Adam for permission to publish their images in this chapter. The author also gratefully acknowledges conversations on glaciovolcanism over many years with colleagues too numerous to mention. However, particular thanks are extended to Sergio Rocchi, Joanne Johnson, Bill McIntosh, Mike Hambrey, Dave McGarvie, Ian Skilling, Magnus Gudmundsson, and Malcolm Hole.

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