Jökull


Jökull - 01.12.2006, Page 68

Jökull - 01.12.2006, Page 68
David W. McGarvie et al. lar relevance to the model in Figure 4 are steep-sided chimneys that have been observed in the deeper ice beneath the shallow-angled basin of the ice cauldron (for example at 1996 Gjálp eruption, Guðmundsson et al., 2004; Guðmundsson, 2005). A scaled-up ver- sion would provide the two forms of receptacle that could explain the two tiers of lavas seen at the best- developed tuyas, with the (confined) lower lava tier being erupted within the steep-sided vertical chim- ney (Figure 4a) and the (unconfined) upper tier be- ing erupted above this (Figure 4b). Given that the bot- toms of recently-observed ice cauldrons in Vatnajök- ull have been up to 200 m below the original ice sheet surface (Guðmundsson, 2005), and that lava B at rhy- olite tuyas is only a few tens of metres thick at most (Tuffen, 2001; Tuffen et al., 2002), it is reasonable to suggest that the original initial ice sheet elevation at rhyolite tuya eruptions was at least 100 m higher than the summit of the tuyas, and possibly as much as 200 m higher (see Figure 4b). Consequently, it can be con- cluded that the height of a rhyolite tuya summit will represent only the minimum ice sheet thickness at the time of eruption. As well as enabling estimates of ice thicknesses to be made for each of the three locations where these tuyas were erupted, if corrections due to Holocene re- bound are taken into account (so that the likely origi- nal elevation of the tuya can be estimated) then an es- timate of the minimum elevation of the ice sheet sur- faces can also be made. Using reasonable densities of 920 kgm−3 for ice and 3100 kgm−3 for the Icelandic mantle (Sigmundsson, 1991), and assuming a simple linear relationship between ice thickness and depres- sion of the lithospheric, a value of c. 0.3 the ice thick- ness is derived. This simple relationship (see Table 2) indicates that during the eruption of the ring fracture rhyolites of Illihnúkur and SW Rauðfossafjöll the ice sheet elevation at each tuya was 1093 m and 1087 m respectively, which is consistent with the idea that the ice sheet surface may be relatively insensitive to local bedrock topography, which has long been recognised at the Vatnajökull and Mýrdalsjökull ice caps (Björns- son, 1988). The older tuya of Gvendarhyrna provides less re- liable information on ice thickness. An Ar-Ar age of 278 ka suggests that Gvendarhyrna has been eroded during three glacial maxima (the Drenthe, Warthe, and Weichselian), with an unknown thickness of ma- terial removed from its summit area (e.g. pumiceous carapaces are absent). Nevertheless the height differ- ence between the base of the tuya (580 m) and the summit (879 m) indicates a minimum ice thickness of 300 m. Given that the tuya was eroded by three ice sheets, it is possible that the original summit altitude may have been higher than the present one. Figure 4. A schematic diagram showing one pos- sible relationship between rhyolite tuya morphology and ice cauldron morphology, after Tuffen (2001): (a) shows a cone-shaped ice cauldron developed in an ice sheet, with confined rhyolite lava A being erupted into a steep-sided deep chimney; (b) shows further upward development of the tuya lava cap, with the emplace- ment of the relatively unconfined and thin lava B. Not drawn to scale. – Skýringarmynd sem sýnir hvernig höfundar sjá fyrir sér súrt gos undir jökli. 66 JÖKULL No. 56
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