Jökull - 01.12.1979, Síða 20

Fig. 12. Growth of a subglacial, monogenetic
volcano. Stage A: A pile of pillow lava forms in
deep meltwater lake. Stage B: Slumping on the
flanks of the pillow lava pile produces pillow brec-
cia. Stage C: Hyaloclastite tuffs are erupted under
shallow water conditions. Stage D: A lava cap
progrades across its own delta of foreset bedded
breccias. Basically from Jones, 1968, with slight
modifications.
stages. Eruptions from a central crater that would
normally produce a lava shield commonly reach a
late stage where fluid lava again starts to flow high
on the volcanic mound above the influence of ice or
meltwater. A lava capping is formed which, how-
ever, upon flowing into the surrounding lake
crumbles into breccia and forms a delta over which
the subaerial lava progrades. The recent Surtsey
eruption was similar in character to the latter two
stages.
Many examples are known where intermediate
and acid lava erupted beneath Pleistocene ice
sheets. In the case of the more acid members the
ensuing volcanic structure formed a steep-sided
dome of rhyolite consisting of distinctive rock
varieties. Among these are lobes of glassy rhyolite
encrusted with a thick layer of pitchstone resemb-
ling huge pillows. The lobes are embedded in a
granulated glass matrix evidently derived from the
crust of the lobes themselves.
Rocks belonging to the Upper Pleistocene series
can be related to a large extent to volcanic systems
that are still active or dormant; the ones regarded
as extinct are still recognizable from their topo-
graphy and incipient dissection. Altogether some
30 volcanic systems have been defined within the
neovolcanic zones of Iceland of which only 6 can be
considered extinct (Fjallgardar, Snaefell, Breida-
bunga? Esjufjöll, Geysir, Grafningshálsar) since no
volcanic activity has occured in them during Post-
glacial time.
The sequence of glacials and interglacials during
the Upper Pleistocene is incompletely known. Very
few absolute age data are available and age
relationships have to be inferred from indirect
evidence. Within the volcanic complexes groups of
lava flows alternate with tillite beds and hyalo-
clastite rocks giving information about several in-
terglacial/glacial cycles.
On the Tjörnes Peninsula the lower part of the
Upper Pleistocene is represented by a 200 m thick
section (Fig. 8) involving two interglacial/glacial
cycles. A tillite bed at the base of this section indi-
cates that the Matuyama/Brunhes magnetic
reversal occured during a cold climate interval. In
late Upper Pleistocene time the Tjörnes Peninsula
suffered erosion; only occasionally the fringes were
covered by volcanic products, among which it is
difficult to establish a stratigraphic order. The
Snaefellsnes Peninsula may have a longer con-
tinuous record of the Upper Pleistocene series but
as yet the sections have not been worked out in
detail. They include thick piles of subglacial brec-
cias, a feature even more characteristic of the Up-
per Pleistocene series in southern Iceland. The
breccia piles which are commonly several hundred
m thick, and only of a local extent, upset to a large
degree the stratoid nature which so characterizes
the older parts of the volcanic pile in Iceland. As an
example a composite stratigraphic column from
the Hengill-Grafningshálsar volcanic systems in
southwestern Iceland is shown (Fig. 13). It is more
18 JÖKULL 29. ÁR