Jökull - 01.01.2005, Side 39

The Late Miocene Tinná Central Volcano, North Iceland
for over 8 km into the central Tinnárdalur valley; on
the west side it disappears into the mountain Hofsfjall
7 km from the assumed crater. The south and north
extensions are not known but the width of the lava
is at least 6 km. The maximal thickness is found to
be 460 m but it was originally greater as the highest
peaks have been eroded away.
The tephra layer is extremely voluminous and in-
dicates a large and explosive plinian eruption. The
thickness of the airborne tephra has been measured in
15 sections in the area (Table 2) and the isolines in
Figure 4 are based on these measurements. It must,
however, be kept in mind that a map based on this
material is not as reliable as maps of Holocene tephra
sectors that are founded on tens or hundreds of soil
sections, such as those of the Icelandic Hekla layers
(Larsen and Thorarinsson 1977). The direction of the
main axis of the tephra sector is towards the east, in-
dicating a westerly wind during the main phase of the
eruption. Farther east (and higher up) the wind direc-
tion might have been more southerly and southwest-
erly, curving the tephra sector towards the north.
The total volume of the Skati rhyolite can be esti-
mated using the figures given above. According to ge-
ological mapping, the area of the dome is over 80 km 2
(Figure 1) and if the mean thickness is 100 m then
the lava volume is 8 km3. Furthermore, gross es-
timates indicate that the volume of the Skati tephra
corresponds to at least 10 km3 of dense rock making
the total volume equivalent to 18 km3 of acid magma.
This is by far the largest monogenetic rhyolite forma-
tion yet reported in Iceland and an order of magni-
tude larger than any Holocene plinian eruption in the
country (Simkin 1994). Compared with well-known
plinian eruptions of historical times, it is larger than
Krakatau 1883 (1.2 km3) but smaller than Tambora
1815 (80 km3) (Pyle 2000).
The Hvítárdalir Dome. In the narrow tributary
valley Hvítárdalir a rhyolite dome, or ridge, is buried
in the lava pile (Figure 5). An excellent cross section
can be seen in the steep slopes on both sides of the val-
ley. It is at least 100 m thick, but the base of the dome
is unexposed below the bottom of the valley. The di-
ameter of the dome along the river is 750 m. Chemi-
cal analyses show close petrological relations with the
Tinná rhyolite (Table 3). A pronounced sedimentary
layer, a fossil scree, is situated between the dome and
the surrounding lavas banking up against it. The dome
has reverse polarity and might be of similar age to the
Skati Dome and a member of that formation.
The Tinná andesite formation. After the great
eruption of the Tinná rhyolite, volcanism continued
with production of intermediate lavas. Several an-
desite lavas, 20-70 m thick, and a few acid lavas
that pile up against the southern slopes of the Skati
Dome have been mapped. Their origins are some-
where south and southeast of the dome; thus, the cen-
tre of volcanic activity seems to have migrated in that
direction immediately after the Skati eruption.
The Tinná tholeiite formation is a pile of thin-
layered lavas with thin interbeds covering the andesite
lavas, banking up against the Skati rhyolite dome and
encircling it. The maximal thickness of the pile is
160 m. The upper limit of these layers is marked by
a shift of the magnetic field from reverse (R) to nor-
mal (N). The production rate of the tholeiites seems to
have been high in the light of the thin interbeds and the
magnetic reversal field measurements show how the
strength of the field declines and increases again after
the shift recording a polarity transition zone. Mag-
netic transition zones are believed to have a duration
of only 1,000–8,000 years, so if they are recorded in a
lava pile, the eruptions must have been frequent (Mer-
rill and McFadden 1999, Coe et al. 2000, Pedersen et
al. 2002).
The Keldudalur dacite formation. Keldudalur is a
tributary valley in upper Austurdalur. Acid rocks are
found in several places near its mouth and in the val-
ley itself, all of which are thought to be exposures of a
single formation. The rock type is dacite (68% SiO2)
(Hjartarson 2003). It forms a ridge or a row of two
or more volcanic domes trending north-south. This
is partly covered by loose scree and till so its base
and contacts with the adjacent rocks have not been
observed. Its highest part reaches 600 m a.s.l. The
visible size is 3 x 2 km and its maximal thickness is
at least 200 m. Assuming an elliptical shape and a
mean thickness of 100 m, the volume of the ridge is
0.5 km3.
JÖKULL No. 55 39