Jökull - 01.12.1979, Page 19

Fig. 11. Profile from Kverkfjöll on the north Vatnajökull margin to Axarfjördur showing the
elevation of subglacially erupted volcanoes and how this may be used to define the surface of the
Pleistocene ice sheet. After Walker, 1965.
covers an area which is essentially identical with
the neovolcanic zone and measures about 30.000
km2. The boundary to the underlying Plio-
Pleistocene series is usually marked by an uncon-
formity and a stratigraphic hiatus of as yet un-
known length. Exceptions are known from the
Snaefellsnes, Skagi, Tjörnes and Slétta peninsulas
where this boundary lies within rock sequences
that extend back into the Plio-Pleistocene and as a
whole unconformably overlie tilted and eroded
Tertiary basement. The unconformity found
elsewhere at the base of the Upper Pleistocene
series is caused by volcanic products of the axial rift
zones extending far beyond the rift axis forming a
transgressive apron of lava flows. Entire volcanic
systems may also develop on the upper Plio-
Pleistocene crust which are then offset en echelon
relative to the main rift axis as for instance the
Hekla and Prestahnjúkur volcanic systems.
The volcanic rocks of the Upper Pleistocene fall
into two types with regard to structure and
morphology. One type comprises extensive sub-
aerial lava flows erupted during interglacial
periods. Glacial erosion has deprived them of their
surface features exposing the more coarse grained
interiors. In older literature they are often referred
to as “grey basalt” or “dolerite” (Icel. grágrýti).
The second type comprises subglacial pillow lavas
and hyaloclastite rocks formerly referred to collec-
tively as “Palagonite Formation” (Icel. móberg).
Volcanic units from the Upper Pleistocene can
often be traced to their respective eruption sites or
craters. Among the basaltic lavas only the lava
shields have been preserved as entities with still
visible craters. Among the subglacial basaltic rocks
products of both fissure eruptions and lava shields
are preserved: Serrated ridges of pillow lava mant-
led by breccias and hyaloclastite tuffs piled up
above erupting fissures. Eruptions from central
craters formed mounds of pillow lava and hyalo-
clastite, sometimes capped by lava and having the
form of table mountains. Some of the younger ones
constitute most impressive morphological features,
e.g. Bláfjall, or Herdubreid in northern Iceland.
They have been used to determine the ice thickness
of the Pleistocene ice sheet (Fig. 11).
The genetic relation between the Pleistocene ice
sheet and the fragmental basaltic rocks became
established during the early 20th century. How-
ever, the correlation with the morphology and
structure of the resulting volcanic mountains
remained obscure much longer. Even though not
known exactly how a subglacial volcanic eruption
proceeds, the main course of events has been
reconstructed (Fig. 12) based on general con-
siderations and observations of such eruptions after
they became visible as well as the structure of the
subglacially formed mountains themselves.
Molten basalt would melt 10 times its volume of
ice if allowed to cool. An average fissure eruption
which typically proceeds at a very high initial ex-
trusion rate should be capable of melting up to 1
km3 of ice within a few days. A subglacial lake is
formed which, if deep enough, provides conditions
for the formation of pillow lava. Continued melting
of the ice causes instability of the growing pile of
pillow lava and slumps become important which
cause breccias to form, among which pillow frag-
ments are a main constituent. The growing mound
would eventually reach shallow water and the
eruption accordingly change in character to an ex-
plosive phreatic eruption. At this stage vast quan-
tities of glassy tuffs erupted which subsequently
envelope the subaquatic products and emerge well
above the water surface. The subglacial fissure
eruptions usually do not proceed beyond these first
2
JÖKULL 29. ÁR 17