Marie Byrd Land

Marie Byrd Land contains the largest Cenozoic volcanic field in Antarctica (LeMasurier and Thomson, 1990). It is also the most remote region and is among the least visited and least well known of all Antarctic volcanoes. Numerous reports published in the 1970s and 1980s suggested a subglacial origin for many centres (summarized by LeMasurier and Rocchi, 2005). While agreeing generally that the eruptive environment was probably glacial, the published criteria used most often (e.g. mainly presence of hyaloclastite; less often interbedded tillite and/or glacially scoured surfaces) are too simplistic and permit only limited interpretation of the eruptive environment, except in rare outcrops where more modern detailed studies have been carried out. In addition, most Marie Byrd Land volcanoes <c. 12 Ma in age have undergone only minimal dissection, probably as a result of a change to a polar thermal regime (LeMasurier and Rocchi, 2005; cf. Armienti and Baroni, 1999), so only the latest and topographically highest erupted products are available for examination. As those units were usually erupted subaerially, they provide little of environmental use except a limiting elevation for any coeval ice sheet. However, caldera and sector collapses have led to a few inland volcanoes being well exposed (e.g. Crary Mountains, Mount Sidley: Wilch and McIntosh, 2002), and many of the coastal volcanoes are deeply dissected (e.g. Mount Murphy: LeMasurier et al., 1994; Smellie, 2000a,b, 2001a,b; LeMasurier, 2002; Wilch and McIntosh, 2002; LeMasurier and Rocchi, 2005). Only the better exposed and described examples are considered here.

Mount Murphy is situated on the coast in eastern Marie Byrd Land. It is a complex polygenetic edifice constructed from at least three centres, of which only the westernmost is well exposed. The basal section, up to c. 1,400 m a.s.l., is basaltic and it has a thinner capping sequence of mainly trachyte lavas. The basaltic section is dominated by multiple units individually several tens of metres thick and composed of stratified hyalotuff, hyaloclastite breccia and sheet or pillow lava. Each unit is separated by a glacial unconformity that is often draped by a thin diamict (tillite), and a few units are capped by strombolian scoria. Although LeMasurier (2002) interpreted the Mount Murphy succession in terms of lava-fed deltas (and therefore, by implication, eruptions beneath ''thick ice''), the sequences are much more similar to those associated with a thin glacial cover, i.e. composed mainly of snow, firm and/or fractured ice that was never more than tens of metres thick (Smellie, 2000a,b, unpublished). The sequences extend down to below 400 m a.s.l., and they reflect the local dynamics of a small ice cap probably similar in appearance to that centred on Mount Murphy today. Conversely, several satellite centres at Icefall Nunatak, Turtle Rock and Hedin Nunatak have younger ages of 6.8-5.95 Ma (Table 10.2). They were associated with a low-elevation (<600m a.s.l.) ice sheet composed of relatively thick ice (at least several hundred metres). That ice was thicker than today and had a surface elevation that fluctuated through 200 m during the period (Smellie, 2000a,b, 2001a,b). A wet-based thermal regime (temperate or subpolar ice) characterized the glacial cover associated with the main (8-9 Ma) Mount Murphy edifice, but its nature is unknown for the younger satellite centres. Finally, Mount Murphy is overlain by a thin laminated mudstone interpreted as a glacial lake sequence related to a significant but undated overriding event (< 3.5 Ma) by an extraordinarily thick former ice sheet that was at least 1,500 m thicker than at present (LeMasurier et al., 1994).

Crary Mountains (9.13-1.81 Ma; Table 10.2) are situated 250 km inland of Mount Murphy, where the surrounding WAIS surface elevation is 1,600 m a.s.l. Only the two older northern centres (Mount Rees and Mount Steere) are well exposed and have ages in the range between 8 and 9 Ma, i.e. coeval with Mount Murphy. The mafic to intermediate successions are individually <100m thick and composed of alternating ''dry'' lavadominated and ''wet'' hyaloclastite-rich lithofacies, interpreted as lavas interacting with local slope ice and snow (Wilch and Mcintosh, 2002). Thus, the coeval Crary Mountains and Mount Murphy successions are broadly similar, being products of interaction with a thin glacial cover. They differ, however, in that the former lack the interbedded tillites and glacial surfaces that are so prominent at Mount Murphy, a difference that was tentatively interpreted to be due to the Crary Mountains ''ice'' being cold based.

Finally, sector collapse at Mount Sidley has exposed a magnificent 1,300 m-high caldera wall section that extends from c. 2,700 to >4,000 m a.s.l. The outer slopes are essentially undissected and expose only subaerially erupted volcanic products, including a widespread ''dry'' trachyte-phonolite fall deposit. By contrast, the caldera wall section is mainly formed of interbedded phonolite and trachyte lavas, domes and ignimbrite ranging in age from 5.69 to 4.18 Ma. The sequence is essentially subaerial. Evidence for glacial conditions is absent, apart from minor signs of water interaction (stratified hyalotuffs and thin vitroclastic lava bases) in one of the subsequences dated as 5.36-4.81 Ma. The latter was interpreted as due to limited glaciovolcanic interaction, presumably local slope ice/snow. However, the basal 1.5 km of the volcano is obscured by the present-day WAIS and the earlier (and lower elevation) environmental history is therefore unseen.

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