Jökull - 01.12.1979, Side 22

Fig. 14. Extent of main
ice sheet in Iceland at the
Pleistocene/Holocene
transition (10.000 y. ago).
Submerged areas are also
shown. Compiled from Th.
Einarsson, 1973. Slightly
modified.
South Central Iceland and decreased towards the
Reykjanes Peninsula in the southwest and the
Slétta Peninsula in the northeast. Within in-
dividual volcanic systems eruptions concentrated
in the central volcanoes and locally on the fissure
swarms. Individual volcanic systems have pro-
duced anywhere from negligible amounts of lava
and tephra (Tindfjallajökull, Hofsjökull) up to
30—40 km3 (Mýrdalsjökull, Grímsvötn and a fiss-
ure swarm north of Torfajökull). Single lava flows
measure by volume most commonly between
0.1 —1.0 km3, but the largest fissure lavas are 12 —
15 km3. They are exceeded still in volume by some
of the lava shields the largest of which are between
15 and 20 km3.
Möst of the lava shields occuring in the axial rift
zones formed in early Postglacial time. They seem
to be a direct continuation of the type of eruption
that prevailed towards the end of the last glaciation
giving rise to the table mountains or tuyas which so
diversify the landscape of the neovolcanic zones.
Besides being common in certain time intervals the
lava shields álso appear to be limited by area. Thus
the Theistareykir volcanic system in northern Ice-
land has produced only lava shields in Postglacial
time and so has also the axial rift zone segment
between Langjökull and Lake Thingvallavatn. The
axial rift zone southwest of Vatnajökull which is
parallel to the latter on the other hand has fissure
volcanism as the sole type of eruption. These two
parallel branches of the axial rift zones thus appear
to complement each other as regards lava type and
mode of eruption.
Lava shields described above are of olivine
tholeiite composition. They occur in highly fissured
areas and are sometimes fissure aligned. It has been
concluded that they started from a fissure, but as
the eruption proceeded, the emission of lava came
to be concentrated at one point. The lava shield
was the result of the piling up of innumerable flow
units which issued from a central vent. The rate of
effusion of this low viscosity lava appears to have
been very low (around 5 m3/s?), but the eruption
may instead have continued for years. Eruption of
alkali olivine basalt in the flank zones occasionally
also produces lava shields of which Surtsey is the
most recent and only historic example.
Fissure eruptions are quite different in their
mode of extrusion. During the first few hours or
days the rate of extrusion may exceed 1000 m3/s
and the lava spreads rapidly and widely as a simple
lava sheet. Lava extrusion then continues at a
greatly reduced rate. The fissure lavas are generally
more evolved petrologically. Perhaps the different
mode of extrusion is a consequence of the depth of
origin; the olivine tholeiite of the lava shields being
erupted directly from the mantle without a delay in
a magma chamber, whereas the olivine free lava of
fissure eruptions is erupted after being delayed in
magma chambers at crustal levels.
20 JÖKULL 29. ÁR