Jökull - 01.12.1979, Page 24

Fig. 15. Tectonics of the Krafla fissure swarm in
northern Iceland. Different parts of the swarm
rifted during different deflation events of the Kra-
The faults and fissures with their vertical hade
and horizontal opening are intimately associated
with dyke injection as is best seen in the dissected
older lava piles, where the majority of faults and
dykes stand nearly normal to the stratification im-
plying a subvertical attitude at the time of for-
mation. Normal faults with a hade of up to 60“ also
occur. Their relation to the vertical fractures and
dykes has not yet been analyzed. However, the
normal faulting is probably contemporaneous with
the latter which is in accordance with the
prominent graben structures of the fissure swarms.
The throw of individual faults is anywhere from a
few meters up to several hundred m. The width of
dykes varies from less than a metre up to more than
20 m. Most commonly the thickness is in the range
1—3 m.
Most of the fissure swarms are focussed on a
central volcano, these together constituting a vol-
canic system commonly between 50 and 100 km
long and 10—30 km wide (Fig. 15). The central
volcanoes have many characteristics that make
them stand apart from the rest of the fissure
swarms. They are the locus of most frequent erup-
tions and maximum lava production on the swarm
making them topographically distinct. Acidic rocks
are virtually confined to them and many of them
have besides an elongated fissure swarm a more
local fissure system of circular outline developed
about a caldera. The majority of the eroded central
volcanoes studied sofar have calderas varying in
diameter from about 5—10 km. Most of them also
developed cone sheet swarms extending well
beyond the caldera margins. Of some 13—15 cen-
tral volcanoes in the axial rift zones at least 7 have
reached the caldera stage. Caldera collapse has
been related to either explosive acidic eruptions
producing sheets of ash flow or air fall tuffs (Krafla,
Tindfjallajökull, Askja) or to withdrawal of magma
from magma chambers underneath them (Askja,
Grímsvötn). The central volcanoes of the axial rift
zones have low relief relative to their surroundings
(Reykjanes Peninsula, northern volcanic zone) ex-
cept where they grew under glaciers. In that case
most of the eruptive products piled up around the
core area of the volcano as hyaloclastite and pillow
flacentral volcano. The space between the 31. Oct.
1976 and 6. Jan. 1978 events rifted during a deflation
event of 13. May 1979. Areas of rifting from Björns-
son et al., 1979. Neighbouring fissure swarms are
shown also.
22 JÖKULL 29. ÁR