Jökull - 01.12.1979, Side 9

2 Outline of the geology of Iceland
KRISTJÁN SAEMUNDSSON
National Energy Authority, Reykjavík
GENERAL ASPECTS
Vertical sections of the volcanic sequence in Ice-
land expose up to 1500 m of rocks below which lie
at least another 2—5 km of extrusives. At this depth
seismic layer 3 (Vp = 6.5 km/s) is reached which
possibly constitutes the base of extrusive rocks.
Deep drilling in Reykjavík, near Akureyri in
northern Iceland, and in Reydarfjördur in eastern
Iceland has confirmed a minimum 4 km thickness
of the lava pile.
The exposed volcanic pile is built predominantly
of basalt (80—85%) and acidic including inter-
mediate rocks constitute about 10%. The amount of
sediment of volcanic origin is in the order of 5—
10% in a typical Tertiary lava pile but much higher
in Quaternary rocks.
Among the basalts three main lava types have
been recognized in the field. 1) compound flows of
olivine tholeiite, 2) simple flows of tholeiite with
little or no olivine and 3) flows porphyritic in
plagioclase and/or pyroxene. Gradations exist bet-
ween the three but as a rule they are distinctive
enough in the field to provide mappable strati-
graphic units. Certain lava types are characteristic
for distinct types of lava morphology. Thus the
olivine tholeiite often produces lava shields which
are seen in the lava pile as thick pahoehoe flows
consisting of numerous thin flow units. The olivine
poor tholeiite is characteristic of fissure erupted aa
lavas. Central volcanoes erupt both types but the
latter is more abundant forming unusually thin
flows, free of phenocrysts, possibly erupted in a
superheated state. The porphyritic lavas are more
often produced from fissures and range among the
most voluminous flows erupted in single eruptions.
In outcrop they are usually distinctive in their
thickness and massive appearance.
Of the acidic rocks some 60—70% are lavas and
intrusions. 30—40% consist of pyroclastic material
deposited as agglomerate in vent regions, as ash
flow tuff sheets or as airfall tuff beds carried
downwind from the source for long distances. Some
of the ash flow tuff sheets constitute easily mapped
marker horizons in the volcanic pile. They are
commonly welded and individual sheets seldom
exceed 5 km3 by volume.
Alkalic rocks are limited to branches of the neo-
volcanic zones that are termed flank zones, and are
superimposed on the tholeiite rocks that build up
the lava pile.
The lavas of the pile dip gently on a regional
scale, generally towards the central part of Iceland
(Fig. 1). The dips increase gradually from near zero
at the highest exposed levels of the pile to about
5—10° at sea level. The increase in dip is matched
by individual lava groups thickening down the
direction of dip (Fig. 2). The regional tilt thus must
have been imparted to the pile during its growth,
which takes place within axial rift zones that are
stationary for long periods of time. Extension and
subsidence in these zones was matched by dyke
injections and lava pouring out at the surface.
Lateral transport due to crustal spreading would
gradually remove the growing pile away from the
zone of accretion and no more lavas would add to
it. At this point erosion begins to modify the topo-
graphy by carving out valleys and exposing the
internal structures of the pile.
Structural relationships indicate that the lava
pile grew as lenticular units from elongate volcanic
systems which included swarms of dykes and
fissures usually localized about central volcanoes.
These units attain their greatest thickness in the
area of the central volcanoes which are also marked
by the occurrence of acid and intermediate rocks
among the copious basalt outpourings (Fig. 3).
They commonly develop calderas. Intrusive bodies,
up to 10 km2 in area, of dolerite, gabbro and/or
granophyre are exposed in most eroded central
volcanoes studied to date. The roots of the latter are
also invaded by intrusive sheet complexes which
JÖKULL 29. ÁR 7