Jökull - 01.01.2005, Side 33

Reviewed research article
The Late Miocene Tinná Central Volcano,
North Iceland
Árni Hjartarson 1,2
1 ISOR, Iceland Geosurvey, Grensásvegur 9, 108 Reykjavik, Iceland; ah@isor.is
2and Geological Museum, Øster Voldgade 5-7, DK-1350, Copenhagen K, Denmark
Abstract— The Tinná Central Volcano in the Skagafjörður Valleys belongs to the Neogene succession of N-
Iceland. It was active during the period 6–5 Ma. The total volume of the volcano is 210 km 3. Four rhyolite
lava domes are described. The largest one, the Skati Dome, was formed during an immense explosive eruption
accompanied by a heavy tephra fall 5.5 Ma. It is the most voluminous monogenetic rhyolite formation known
in Iceland both with respect to lava and tephra. The area of the dome is about 80 km 2 and its volume is 8 km3.
The tephra layer might additionally account for 10 km3 of dense rock, or 18 km3 altogether. The tephra layer
is correlated with 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. After its extinction it was buried by the younger lava pile while it drifted
away from the active rift zone.
INTRODUCTION
The Tinná Central Volcano is one of 40-50 known
central volcanoes in the Neogene regions of Iceland
(Jóhannesson and Sæmundsson 1998). The name was
introduced by Hjartarson et al. (1998) because the
Tinná river and the Tinná valley are situated near the
centre of the volcano. In Icelandic the word tinna (or
hrafntinna)means obsidian and the name Tinnámeans
“obsidian river”. Obsidian is the black and glossy
form of rhyolitic glass that is often found at the top
or the bottom of acid lavas and thus indicates a cen-
tral volcano. The first aim of this paper is to describe
the volcano and put it in context with other Icelandic
central volcanoes. The second aim is to introduce
the Skati rhyolite member and emphasise the unique
volume of this monogenetic eruptive formation. The
third aim is to correlate the Skati tephra layer with an
ash layer in the deep-sea sediment.
Regional and geological setting
The Tinná Volcano is located in the Skagafjörður Val-
leys, central North Iceland (N65◦15’, W18◦50’. Fig-
ure1). The district is mountainous terrain with the
highest peaks reaching over 1,000 m. Towards south
the glacier Hofsjökull covers an active but dormant
central volcano with an ice-filled caldera and a sum-
mit reaching over 1,800 m a.s.l. (Björnsson 1988).
The Northern Volcanic Zone lies 100 km to the east
and the centre of the Iceland Hot Spot is somewhat
farther to the southeast (Sæmundsson 1979). The
Tinná Central Volcano belongs to the Neogene suc-
cession of North Iceland and is assumed to originate
in the North Iceland Volcanic Zone. Exposures are
in general good in the deeply eroded landscape. Ex-
cellent natural cross sections are cut down through
the lava pile exposing the stratigraphy and tectonics
of the volcano’s surface formations but its roots and
magma chamber are still covered and hidden deeper
in the crust.
Another central volcano named Torfufell lies in
the Eyjafjörður District, 25 km east of the Skaga-
fjörður Valleys. It seems to be separated from the
Tinná volcano by the high mountains of Nýjabæjar-
fjall but in fact this might only be an apparent sep-
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