Jökull - 01.12.1979, Side 26

roots into which magma is fed directly from the
mantle. When the magma pressure overcomes
lithostatic pressure a volcanic eruption is due to
begin. “Loading” of the fissure swarms is of a dif-
ferent kind, being related to the accumulation of
tensional stress as a result of crustal spreading.
The mean half-rate of spreading averaged over
the last few m.y. is 1 cm/y. Upwards of 100 years of
tensional stress build up appear to be necessary
before a fissure swarm is ready to rift which is often
triggered by a magmatic event in the associated
central volcano. A central volcano may erupt
several times before the transecting fissure swarm
splits up, provided an ample magma supply. If the
magma supply is on the other hand limited, the
fissure swarm may split up several times before an
eruption of the central volcano occasionally goes
off.
Tectonics of the flank zones
Two branches of the neovolcanic zones have
poorly developed extensional features, and are
grouped together as flank zones. Those are the
Snaefellsnes volcanic zone and the southern part of
the South-Iceland volcanic zone. Their volcanism
extends back ioto thé Plio-Pleistocene only, and no
equivalents are known from the Tertiary. The vol-
canic products lie unconformably upon older piles
of volcanics that at least partially suffered erosion
before the younger volcanism started. The flank
zones are petrologically different from the axial rift
zones in that they erupt mostly transitional to
alkalic lava types (see chapter 8). Most of the cen-
tral volcanoes of the flank zones form large volcanic
edifices, that are cone shaped strato-volcanoes
(Snaefellsjökull) or ridge shaped ones which are
enlongated parallel to the fissure trend (Hekla,
Eyjafjallajökull). Many of them have a large sum-
mit crater and a caldera has formed in three of
them. The main reason for the marked topographic
expression of the strato-volcanoes of the flank zones
is tectonic: Large scale extensional faulting and
Fig. 17. Tectonics of the Snaefellsnes volcanic
zone and its connecting segment to the Langjökull
axial rift zone. Eruption sites shown are mainly of
Postglacial and Last glacial age. Before that time
activity was concentrated in the middle part of
Snaefellsnes. Some of the larger and morphologi-
cally distinct faults are shown. Note inconspicuous
N-S trending faults and crater rows.
24 JÖKULL 29. ÁR