Jökull - 01.01.2005, Side 45

The Late Miocene Tinná Central Volcano, North Iceland
Figure 8. a) The tephra sector close to the eruptive site. The innermost ellipse is the estimated isoline for the 20
m tephra thickness based on measurements. The outer isolines are hypothetical. The tephra sector is projected
on the country after removing rocks younger than 5.5 Ma. b) The tephra sector is projected on the country as
it looks like today. The violet areas indicate rocks younger than 5.5 Ma. The regional distribution is illustrated
in Figure 4. – Gjóskugeirinn næst eldstöðinni. Innsta jafnþykktarlína ( 20 m) er byggð á mælingum, ytri línur
eru áætlaðar. a) Myndin sýnir útlínur landsins eftir að berg yngra en 5,5 milljón ára hefur verið fjarlægt. b)
Gjóskugeirinn teiknaður á landið eins og það lítur út í dag. Fjólubláu svæðin sýna berg sem er yngra en 5,5
milljón ára.
Tertiary woodlands.
Phase 2: The central phase started with the eruption
of the large Tinná olivine tholeiite lava. The relation-
ship between the lava and the central volcano is un-
clear. Shortly after the emission of the lava, violent
explosive volcanism took place with an immense py-
roclastic fall and the extrusion of a large new rhyo-
lite dome, building up at least 500 m high mountain,
the Skati Dome. The tephra is found both below and
on top of the dome, indicating a simultaneous emis-
sion of ash and lava. The total volume of eruptives
emitted was equivalent to at least 18 km3 of magma.
A thick and wide spread tephra sector was accumu-
lated. Westerly winds directed the tephra towards the
east but farther away (and higher up) the winds seem
to have been more southerly, turning the tephra sec-
tor towards the north (Figures 4 and 8). The Skati
eruption was followed by intermediate volcanism and
later on, the accumulation of the thin-layered lava pile
of the Tinná and Nýibær tholeiites which in the end
immersed the Skati Dome.
Phase 3: The final stage appears as an alternating
volcanic phase producing acid and intermediate lavas
with basaltic eruptions in between. The centre of the
activity was farther south than at the beginning, and
a caldera was formed. This is assumed to have hap-
pened about 5 Ma. After that the volcano became ex-
tinct and drifted to the west, away from the active vol-
canic zone of North Iceland, while younger lavas piled
up against it and covered it partly, though its highest
summits always seem to have extended above the en-
vironment.
DISCUSSION
Among the 40–50 known Neogene central volcanoes
in Iceland the Tinná Volcano is distinct in some re-
spects. This is not because of its overall size, the du-
ration of its activity or the great variety of rock types
formed in periods of explosive acid eruptions in be-
tween more quite periods of basic and intermediate
volcanism: all these topics seem fairly representative
for central volcanoes in general (Walker 1963, Jó-
hannesson 1975, Franzson 1978, Friðleifsson 1983).
It is the volume of the monogenetic Skati formation
that makes it unique, especially the Skati lava dome.
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