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

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Chapter 10. Glaciovolcanism: a 21St Century Proxy for Palaeo-Ice 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 distinc- tive 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 sub- glacially 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 Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00010-5 © 2018 Elsevier Ltd. All rights reserved. 335 336 CHAPTER 10 GLACIOVOLCANISM 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, glacio- volcanic sequences are characteristically thick (tens or hundreds of metres, to .1 km) and are usu- ally 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 num- ber 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 earli- est 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 volca- nic 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 dur- ing 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.,
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