Jökull - 01.01.2005, Síða 37

The Late Miocene Tinná Central Volcano, North Iceland
Figure 3. The Skati Rhyolite Dome. The Skati Dome, rhyolite lava is denoted by a yellow, hatched color. Yel-
low dotted = Skati rhyolite tephra above and below the dome. Blue = Gently sloping pile of lavas above and
below the dome. Orange = Sediment with lignite seams. The Austurdalur Pleistocene Volcano (green colour)
and its feeder dyke are shown on the right. The estimated location of the feeder dyke of the Skati Dome also
shown. (Vertical/horizontal = 1/10). – Þversnið um Skatastaðafjall. Gult = Skatagúllinn (líparíthraun). Gult
punktað = Gjóska frá Skatagosinu undir og ofan á gúlnum. Blátt = Hallandi basalthraunastafli undir og ofan
á gúlnum. Rauðgult = Setlag með surtarbrandi. Austurdalseldstöðin (í grænum lit) og aðfærslugangur hennar
eru sýnd til hægri. Áætluð aðfærsluæð Skatagúlsins er einnig sýnd. (Hlutfall lóðrétt/lárétt = 1/10).
place to place. The thickness of the lignite, on the
other hand, is only 1-30 cm. The thickest lignite lay-
ers occur at 300 m a.s.l. in the Goðdaladalur canyon.
Five seams can be seen, consisting of black, com-
pressed wooden trunks and branches, inside a 4.4 m
thick light-coloured layer of siltstone. Imprints of
leaves or seeds are rarely seen. No investigation has
been made of the flora or possible fauna in the layers
in Skagafjörður. The Tinná lignite sediment indicates
a time of quiescence before increased volcanic activ-
ity in the following ages. A reversal of the magnetic
field occurred during the accumulation of the sedi-
ment.
Tinná olivine tholeiite. The deposition of the
widespread Tinná sediment was stopped by an erup-
tion of thick and extensive lava, the Tinná olivine
tholeiite. This is the most voluminous basalt lava in
the whole area. It varies in thickness from place to
place between 20 and 60 m, and in some localities
it is absent. In most places the layer consists of one
thick lava unit, unlike the more common composite
olivine tholeiites that are made up of several mono-
genetic lava bands. Its extent is at least 25 km x
10 km, i.e. 250 km2. Its average thickness has been
estimated at 25 m, thus its volume is 6 km3. This is
not very large compared to the great Holocene lavas,
but it is still the most voluminous reported Neogene
basalt lava of Iceland.
The Skati rhyolite formation consists of two mem-
bers, Skati lava and Skati tephra. This is an excep-
tionally thick and extensivemonogenetic eruptive unit
that has built up a mountain, the Skati Dome, rising
at least 500 m above its base with the highest peaks
in the present Skatastaðafjall area (Figure 3). The
eruption seems to have started with a powerful plinian
phase. Viscous rhyolite lava was extruded along with
the tephra emission. It covered the pyroclasts from
the earliest phase of the eruption but the pyroclastic
fall from the later phases continued on top of it. The
crater site is not known but the conduit is most likely
buried below the main body of Skatastaðafjall moun-
tain. From there the lava flowed out in all directions
from the crater on a sole of black basal breccia of ob-
sidian aggregate. Towards the east it can be traced
JÖKULL No. 55 37