Jökull - 01.01.2005, Page 43

The Late Miocene Tinná Central Volcano, North Iceland
data indicate a volcano from an axial rift zone and
not an off-rift volcano (Lacasse and Grabe-Schönberg
2001).
Size and age
The total volume of the Tinná Central Volcano can be
roughly estimated. Its base at the bottom of the Ágúll
Dome is about 30 km wide and the average thickness
300 m. If it is assumed to be semicircular, its area
is 700 km2. Hence the volume would be 0.3 km x
700 km2 = 210 km3. Here the 300 m thick Fjóslækur
basaltic formation below the volcano is not taken into
account. For comparison, the Breiðdalur Central Vol-
cano, East Iceland, has been estimated to be 400 km3
(Walker 1963). (If the rhyolites of Torfufell belong to
the Tinná Central Volcano, it would be≈500 km3 and
among the greatest ones known in Iceland).
The age of the Tinná Central Volcano is based on
an Ar/Ar-date from the Skati rhyolite dome (Hjartar-
son 2003). This gives 5.212±0.016Ma. There seems
to be some disharmony between this age and the mag-
netic polarity. The polarity of the rock is reverse but
at 5.212 Ma the Earth’s magnetic field is supposed to
have been normal, representing the Thverá subchron
(C3n4n), the lowest normal interval of the Gilbert po-
larity chron. Before the Thverá subchron a rather
long reverse subchron occurred, representing the low-
est part of the Gilbert polarity chron, C3r, 5.89–5.23
Ma (Cande and Kent 1995). The Skati Dome is as-
sumed to belong to the central or the lower part of this
subchron (Figure 7) and its Ar/Ar-age might be there-
fore slightly too low. A more credible age would be
5.5 Ma.
The lifetime of an ordinary central volcano is 0.5–
1 Ma (Guðmundsson 2000). After that it cools down
and is buried by younger lavas while it drifts out of
the active volcanic zone. The Tinná volcano seems
to be typical in this respect, since it remained active
for at least three polarity subchrons (C3An.1n–C3r–
C3n.4n), from c. 6 Ma to 5 Ma. This was the end of
the Miocene and beginning of the Pliocene (Harland
1989).
The Ar/Ar-age of the Tinná Volcano indicates its
origin in the North Iceland Volcanic Zone that took
over the spreading in North Iceland some 6–7 mil-
lion years ago (Sæmundsson 1979). Since then it has
drifted out of the zone towards west and belongs to the
North American crustal plate, located some 100 km
from the axis.
Figure 7. Stratigraphy and history of the Tinná Cen-
tral Volcano. A simplified geological column. The
thickness of the formations is given in m on the left. –
Jarðlagaskipan og saga Tinnáreldstöðvarinnar (ein-
földuð mynd). Þykktir helstu myndana eru gefnar á
kvarðanum til vinstri.
Distant correlation
The Skati tephra might be recognizable in ODP cores
from the ocean floor around Iceland. Lacasse and
Garbe-Schönberg (2001) studied the explosive vol-
canism in Iceland and the Jan Mayen area during the
last 6 million years from tephra layers found in the
deep-sea cores. They recreated a composite marine
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