Jökull - 01.12.2006, Síða 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