Jökull - 01.12.2006, Side 61

Pleistocene rhyolitic volcanism at Torfajökull, Iceland
Figure 2. Simplified geological map of Torfajökull (after Sæmundsson and Friðleifsson, 2001), showing some
of the major eruptive units. The numbers indicate the localities of samples mentioned in the text, and for
clarity only the last two digits of sample names are given. Of the ring fracture rhyolites: Rauðfossafjöll com-
prises the three separate outcrops in the north-west of the area; Kirkjufell comprises the two outcrops erupted
along three linear fissures in the east-north-east; Illihnúkur comprises one large massif with an arcuate fissure
and two small outcrops in the east; Laufafell comprises one outcrop erupted along an arcuate fissure in the
south-west; whilst the scattered outcrops on the southern and eastern margins of the small Torfajökull ice cap
in the south-east corner have no formal name. – Einfaldað jarðfræðikort af Torfajökulssvæðinu, byggt á korti
Kristjáns Sæmundssonar og Guðmundar Ómars Friðleifssonar frá 2001. Kortið sýnir súrt berg (líparít) frá
síðustu ísöld (ljósgrá svæði), súr hraun sem runnið hafa eftir að ísöld lauk (dökkgrá svæði) og súr hraun (línu-
strikuð svæði) sem tengjast hringsprungum við brúnir Torfajökulsöskjunnar. Sýnatökustaðir eru einnig merktir
á kortið.
lated edifices (tuyas, ridges, etc.) which cannot be
correlated using normal stratigraphic methods (e.g.
Lescinsky and Fink, 2000; Smellie, 2000).
This latter aspect is a characteristic of the Qua-
ternary, where the dominance of cold conditions
(e.g. Lowe and Walker, 1997) has led to the re-
peated development of ice sheets which would have
fully or partly covered Icelandic central volcanoes at
glacial maxima. Furthermore as the central volca-
noes are themselves topographically-elevated areas,
they would have been amongst the first areas to be-
come ice-covered and also the last areas to become
ice free during deglaciation, thereby enhancing the
likelihood of erupting magmas encountering ice and
leading to accelerated construction of central volcano
superstructures. Consequently, flank zone central vol-
canoes which have been active during the Quaternary
contain a high proportion of glaciovolcanic products.
The stratigraphic problem posed by glaciovolcanism
is that if (for example) a central volcano erupted dur-
ing each of the past ten glacial periods (over a c. 1 Ma
time span), then ten different sets of isolated glacio-
volcanic edifices might exist with potentially no over-
lap between them to assist in establishing a relative
stratigraphy.
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