Jökull - 01.01.2005, Page 46

Árni Hjartarson
Rhyolite domes such as those described here (Table 4)
are rare in Iceland. Only a few domes are known in the
Neogene areas. G.P.L. Walker (1963) e.g. described
small domes in the Breiðdalur Central Volcano. In
the Quaternary areas the Hágöngur mountains in Cen-
tral Iceland are the most prominent examples, but they
might be subglacial formations. No such dome is
known to have been formed in the Holocene time.
The Skati lava dome is of an order of magnitude
larger than other known rhyolite domes in the country.
The reason for this is unknown. The magma cham-
ber might have been located at an unusually shallow
depth in the crust, allowing acid lavas an easy ac-
cess to the surface. The low degree of alteration and
lack of a geothermal aureole around the volcano sup-
port this suggestion. But the absence of caldera for-
mation along with the eruption, and the absence of
a high-temperature area seem to argue against it. A
shallow magma chamber, however, with a thin roof
might be harder to identify than a deep-seated one. It
might have collapsed contemporarily with the extru-
sion of the dome, leaving the caldera unexposed be-
low it. (The caldera near the Fossá river (Figure 1)
was formed later, during the final stages of the vol-
cano).
Table 4: The acid domes of the Tinná Volcano – Súrir
gúlar Tinnáreldstöðvar
Name Thickness Volume SiO2
m km3 %
Ágúll dome 100 0.5
Skati dome 100 8 75
Hvítárdalir dome 75 <0.1 75
Keldudalur dome 100 0.5 68
Geothermal activity and high-temperature areas
are commonly associated with most of the rift-related
central volcanoes of Iceland (Sæmundsson 1979,
Guðmundsson 2000). No indications of such a high-
temperature area are found near the Tinná Volcano.
CONCLUSION
The Tinná Central Volcano in the Skagafjörður Val-
leys belongs to the Neogene succession of North Ice-
land. It originated inside the North Iceland Volcanic
Zone and was active during the period 6–5 Ma. The
total volume of the volcano is at least 210 km3, com-
prising four rhyolite lava domes (Table 4). Its volcanic
products belong to the tholeiitic rock series. The ma-
jor eruption of the Tinná volcano, the Skati eruption
5.5 Ma producing 18 km3 DRE, with a high fraction
of lava, is outstanding among Icelandic acid eruptions.
The tephra layer is correlated to an acid ash layer
found at ODP site 907, 500 km NNE off Iceland’s
coast and can serve as an important marker horizon
for the late Miocene in the deep-sea sediments. A
collapse caldera was formed during the final stage of
the volcano. The Tinná Volcano was not a stratovol-
cano but rather an irregularmassif of heaps and domes
without anymajor summit crater. At times it rose high
above the environment but while it was being formed,
flood-basalts were issuing from fissures throughout its
surroundings. Finally it was buried, or nearly buried,
by them as the volcano drifted westwards, away from
the active rift zone of North Iceland.
Acknowledgements
A thorough and constructive review by Asger Ken
Pedersen, docent at the Geological museum in Copen-
hagen and Kristján Sæmundsson, a geologist at ISOR
(Iceland GeoSurvey) is gratefully acknowledged. I
also wish to thank Guðmundur Ómar Friðleifsson
and Þórólfur H. Hafstað, of ISOR, for their cooper-
ation and companionship in the field work and Jeffrey
Cosser for good advice, comments and corrections re-
garding the English language. I wish to thank BHM
(Association of University Graduates) and Hagþenkir
for sponsoring travel expenses.
ÁGRIP
Tinnáreldstöðin í Skagafirði er ein af 40–50 meg-
ineldstöðvum sem vitað er um í hinum tertíera berg-
grunni landsins. Nafnið er nýnefni og er valiðmeð til-
liti til þess að Tinná og mynni Tinnárdals eru í grennd
viðmiðju eldstöðvarinnar. Tinnan í fyrsta lið þess vís-
ar til hins súra bergs sem jafnan einkennir megineld-
stöðvar. Eldstöðin var virk fyrir 5–6 milljónum ára.
Hún einkennist af allstórum líparítgúlum sem orðið
46 JÖKULL No. 55