Jökull - 01.12.1979, Page 11

1500
Hor-izontal Scale Vertical Scale
Fig. 2. Sections from east to west through the Tertiary series of eastern Iceland. The AA’section is
the northernmost and the CC’section farthest to the south. Note thickness variations up dip and
along strike. After Walker in G. Bödvarsson and Walker, 1964.

^SUMMIT GROUP _ PARASITIC
Thin basic and intermediate lavas
Rhyolite lavas
I Rhyolitic welded tuffs
Acid tuffs and agglomerates
Intrusions, mainly acid
Breiðdatur Volcano
Flood-basalts forming envelope of voicano
í...\ Extent of propylitization
Length of section approx. 22 miles (35 km)
Vertical scale approx. 2 x horizontdl
Fig. 3. Schematic section through the Breiddalur Tertiary central volcano in eastern Iceland.
Underlying and enveloping lavas are indicated. From Walker, 1963.
sometimes form discrete cone sheet swarms of up to
15 km in diameter. Individual sheets dip at an
angle of 30—40° towards an apex beneath the core
area of the central volcano. The proportion of in-
trusive rocks may locally exceed 50% in the more
deeply eroded ones whereas the dyke intensity out-
side them rarely exceeds 10%. The great proportion
of intrusive rocks, at shallow depth gave rise to a
temporary hot water convection cell which caused
hydrothermal alteration of the core of the central
volcanoes to propylitized rock far above the zeolite
facies metamorphism of the volcanic pile elsewhere
(see chapter 8).
The dyke or fissure swarms of so called axial rift
zones are anywhere from 10 km to over 100 km in
length. They are characterized by extensional tec-
tonic features such as open fissures, graben struc-
tures and crater rows at the surface, and dykes and
normal faults at deeper levels.
The active period of the volcanic systems has
been found to vary from 300.000 years to over 1
m.y. They are preserved as entities in the volcanic
pile, indicating that they grew, drifted off towards
the margin of the current volcanic zone and then
became extinct. New ones replaced them over the
more or less stationary deep-seated zone of magma
generation.
Superpositioning and the present configuration
of axial rift zones predict that the oldest exposed
rocks in Iceland should occur in the farthest north-
west, north and east. Radiometric dating of the
lowest exposed levels in the east indicate that the
oldest rocks in that area are just over 13 m.y. As yet
ages from the northwest and north are fragmen-
tary, but cluster around 16 m.y. for a deep strati-
graphical level in the northwest. Ages from
northern Iceland indicate that the oldest part of the
lava pile there may be around 12 m.y. These ages
JÖKULL 29. ÁR 9