Jökull - 01.12.1979, Side 23

In Postglacial time eruptions of intermediate
and acid magma have occurred in several volcanic
systems, most profusely in Hekla which alone
accounts for more than 1 /2 of the volume produced
during this time interval. There have been erup-
tions of both lava and tephra, the rhyolitic magma
being highly explosive. Rhyolite flows are known
only from the Torfajökull central volcano. There
rhyolitic magma although viscous has not formed
domes but only thick flows of small horizontal ex-
tent. Four volcanoes, Hekla, Askja, Oraefajökull
and Snaefellsjökull are responsible for most of the
acid explosive eruptions. The tephra produced in
some of these eruptions has dispersed over vast
areas and provides a basis for time stratigraphic
studies of Postglacial soils. Patient work by S.
Thorarinsson has established the sequence of
tephra layers in the soil over most of Iceland, mak-
ing tephrochronology a powerful method of dating
Postglacial events (see chapter 4).
Towards the end of the last glaciation the sea
level rose eustatically. The lowlands were inun-
dated by the sea and marine clay, silt and gravel
were deposited, often forming prominent terraces.
The highest shorelines are about 100 m above the
present sea level farthest inland in southwestern
Iceland, elsewhere they are commonly around
40—60 m. The age of the maximum transgression
is around 11.000 years corresponding to the
Alleröd-interstadial. Isostatic land rise caused the
sea to retreat and equilibrium was established
around 9000 years ago near the present sea level.
Fluvioglacial outwash covers about 5000 km2.
The outwash plains are most extensive south of
Vatnajökull and Mýrdalsjökull where catastrophic
floods (Icel. jökulhlaup = glacier bursts) frequently
occur, the major ones caused by eruptions of the
subglacial volcanoes.
The soil cover in Iceland is discontinuous and
delicate. The interior is more or less barren. Es-
pecially in the areas of young volcanism desert-like
conditions prevail as the water soakes into the
porous ground. The young volcanic terrain of the
interior supplies immense quantities of wind blown
dust and volcanic ash making the soils of the
fringing areas sandy and dry. In the Tertiary and
Plio-Pleistocene areas the soils are more boggy and
peat is extensively developed. Since settlement time
Iceland has been deprived of much of its soil and
vegetation cover, and the birch woods have been
reduced so thoroughly that today only insignificant
remnants are left.
TECTONICS
The tectonics of Iceland is controlled by its
position on the Mid-Atlantic Ridge, with exten-
sional features predominating. In detail the tec-
tonic pattern is, however, complicated and many
aspects of it are as yet inadequately studied. Active
tectonism occurs in the neovolcanic zones and
along oblique or transverse tectonic zones that
connect offset segments of the neovolcanic
branches either internally or to the submarine
mid-ocean ridge. The Tertiary and Plio-Pleistocene
areas appear to be tectonically inactive except
where they are cut by the transverse tectonic zones.
In situ stress measurements in the Tertiary and
Plio-Pleistocene areas have begun just recently. So
far they yield maximum compressive stress per-
pendicular to the axial rift zones, which is charac-
teristic of intraplate conditions in general.
Tectonics of the axial rift zones
Among the neovolcanic zones the axial rift zones
(Fig. 1), mark the trace of the plate boundary where
active plate growth is taking place. They erupt
tholeiitic rocks and have very pronounced linear
extensional tectonic features and are flanked by
volcanic piles which dip and become progressively
younger towards them. These piles comprise the
Plio-Pleistocene and most of the Tertiary series.
From the distribution of dyke and fault swarms in
the flanking piles it is clear that they formed under
the same tectonic regime as prevails in the axial rift
zones of today.
Fracturing of the axial rift zones is concentrated
into fissure swarms up to 20 km broad and
sometimes over 100 km long. Their trend is variable
but rather uniform within each branch of the axial
rift zones. They typically form en echelon arrays
which may be dextral or sinistral depending on the
trend of the individual branches relative to the
direction of spreading, which is east-westerly (near
N 100° E). The dominant structures of the fissure
swarms are the volcanic fissures, noneruptive gap-
ing cracks and faults or fault bundles with vertical
hades and spaced apart only a few tens of m up to a
few km. When traced along their trend the faults
and fissures tend to be sinuous and branching and
they often consist of short en echelon segments.
Their trend clusters around a maximum which lies
in the direction of the swarm. Sometimes cross
faults and volcanic fissures striking normal to the
main trend are seen but they are rare.
JÖKULL 29. AR 21