Jökull

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Jökull - 01.12.2006, Qupperneq 60

Jökull - 01.12.2006, Qupperneq 60
David W. McGarvie et al. Figure 1. Map of Iceland showing the location of the Torfajökull central volcano. WRZ and ERZ refer to the Western and Eastern Rift Zones respectively, the on-land continuations of active spreading linked to the off- shore spreading ridges. SFZ is the Southern Flank Zone. The box indicates the location of Figure 2. – Einfaldað jarðfræðikort af gosbeltum landsins. tle active spreading takes place, and their volcanic systems are characterised by larger (overdeveloped) central volcanoes (e.g. Torfajökull, Tindfjallajökull, Ljósufjöll) that produce a higher proportion of silicic magma than their rift zone counterparts, and whose fissure swarms are either smaller or absent (Sæmunds- son, 1979). Basaltic magma compositions are transi- tional to alkaline and these are closely associated with alkaline to peralkaline silicic compositions such as comendites and pantellerites, although some trachytes and rare hawaiities, mugearites, benmoreites do occur (Sigurdsson, 1970, 1971; Hards et al., 2000). Available evidence suggests that rift zone central volcanoes have lifespans up to 1.5 Ma (Sæmundsson, 1979 and references therein). The geochemical evo- lution of rift zone central volcanoes is well known through the benchmark studies of extinct Tertiary vol- canic systems such as those of Carmichael (1964) and Wood (1978), whilst their morphological evolu- tion is well known via the work of Walker (1964) and Sæmundsson (1979) who utilised the stratigraphic re- lationships available due to both the deep erosion of the Tertiary volcanic and sub-volcanic pile, and to the presence of laterally-persistent stratigraphic mark- ers. However, no comparable studies have been under- taken of Tertiary flank zone central volcanoes (possi- bly because such central volcanoes are not presently exposed). Consequently, although active rift zone vol- canic systems are well understood because of the detailed studies that have been undertaken on their eroded Tertiary equivalents, the same is not true of the active flank zone volcanic systems. Consequently, remarkably little is known about the long-term geo- chemical and morphological evolution of flank zone central volcanoes such as Torfajökull, Tindfjalla- jökull, Hekla, Snæfellsjökull, and Öræfajökull, and yet it is from these volcanoes that the vast bulk of explosive silicic-intermediate eruptions has occurred during the Quaternary (Haflidason et al., 2000; La- casse and Garbe-Schonberg, 2001). A fuller understanding of the growth and evolu- tion of flank zone central volcanoes is hindered by the lack of deep erosion through these relatively-young volcanic complexes. An additional problem in Iceland is the abundance of glaciovolcanic edifices, where eruptions into ice have produced steep-sided and iso- 58 JÖKULL No. 56
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