ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

fragments of roman ceramic, which date to the I and II
centuries P.C., had been recollected. Given their erosion
grade, these materials are slightly prior to the sedimentation
of this level and by extension to the remaining superficial
platforms.
The intermediate level of Sant Pol Beach, which had deposited
during a phase of relative rising of the sea level, correlate
chronologically with the warm pulsation of the Roman
Era. The upper level seems to have deposited during
a phase subsequent to a drop of the sea level. Up to now it
couldn't have been possible to precise neither the age of
the sedimentation of the lower level or the cementation of
the upper level.
MATTI J. ROSSI
Morphology of Postglacial lava flow fields in Iceland
Department of Geography, University of Turku,
FIN-20014 Turku, Finland
Volcanism in Iceland is mainly concentrated in the neovolcanic
zone where active rifting episodes take place. There
are strong indications that lava production rate in the neovolcanic
zone was at its highest during the early postglacial
period. In the later part of the postglacial period the lava
production has decreased. The early peak in lavaproduction
after deglaciation can be explained by the rapid isostatic
rebound of the thin Icelandic crust that induced magma
production and extensional tectonics (Gudmundsson,
1986). Three main types of lava flow fields are recognized:
(1) monogenetic shield volcanoes, (2) regional lava flows
from volcanic fissures, and (3) lava flows from central volcanoes.
Lavas from shield volcanoes and the regional lava
flows are dominantly basaltic in composition, whereas lava
flows from central volcanoes often vary from basalt to more
evolved types.
Monogenetic shield volcanoes were formed mainly in the
early Holocene epoch. The cone of a shield volcano forms
from successive lava lake overflows which are of highly vesicular
shelly-type pahoehoe. A widespread apron surrounding
the cone forms from denser, tube-fed pahoehoe. The
apron that surrounds the cone is commonly much more
thinner than the cone. However, it is often areally more extensive
and often more voluminous than the cone. The largest
monogenetic shield volcano, Skjaldbreidur, has a volume
of approximately 15 km', A shield-producing eruption
has alternating episodes of lava lake overflows and tube-fed
delivery to the distal parts of the flow field. In the
late stages of eruption, the cone volume increases in response
to the increased amount of rootless outpouring on
the cone flanks (Rossi, 1996). Lava-inflation structures,
mainly flow-lobe tumuli, are the most prominent features
of the lava flow fields of shield volcanoes. Flow-lobe tumu-
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li gradate into lava rises, which are larger inflation structures
but both structures have a similar mode of emplacement.
Each flow-lobe tumulus is an individual flow-unit
that inflates and forms tension cracks in the lava crust.
Flow-lobe tumuli are generated by inflation of the lava crust
as a result of magmatic overpressure in the associated
molten lava core (Rossi & Gudmundsson, 1996).
Regional lava flows erupt from volcanic fissures. Fissures
commonly occur in swarms which are 5-10 km wide and
40-80 km long. Regional lava flows typically have aa surface
textures and their flow fronts are fed by open lava channels.
Eruptions of long duration may develop complex lava
flow fields with morphologies varying from tube-fed
pahoehoe lavas to aa and blocky lavas with open lava channels.
Lava from the Laki fissure eruption is one of the most
voluminous regional lava flow with a volume of approximately
12 km',
Lava flows at central volcanoes are fed by volcanic vents
that are connected to shallow magma chambers. Differentiation
of the primary magma is common to these magma
chambers. Therefore, many lava flows from central volcanoes
possess an evolved chemistry. Generally, evolved
magmas have higher viscosities than the primary magmas
and the resulting lavas rarely possess pahoehoe morphologies.
At Krafla central volcano, however, lava flows in the
proximal (near-vent) parts of the flow fields have smooth
sheet-flow surfaces. At greater distances the lava flows develop
aa and blocky surfaces, and the lava delivery occurs
through open lava channels.
Lava surface morphologies are controlled not only by variation
in the rheological properties of the lava, but also by
supply rate at the vent and the amount of branching of the
lava flows. There are strong indications that shield volcanoes
were erupted at a relatively low effusion rate. These
lavas are dominantly of pahoehoe type. Regional lava flows
may have erupted at higher rates. The strong shear forces
in lava flows that are supplied at a high rate could be the
main reason for the development of broken aa and blocky
surfaces.
Open lava channels are common features of regional and
central-volcano lava flows. A lava channel supplies lava
from the vent area to the middle and distal parts of the
flow. Lava flows are non-Newtonian substances and their
margins stagnate because lava has a yield strength. In the
middle parts of the flow, however, shear stresses exceed
the yield stress of the lava and the middle flow region remains
active. In theory, the rheological properties of a lava
flow can be calculated from the levee and channel dimensions,
but several factors complicate that approach. The
1984 open-channel lava flow at Krafla demonstrates that
during the lava emplacement several factors, such as
varying supply rate at the vent, may lead to overflow from
the open channel to the margins of the flow. Thus the initial
channel and levee dimensions of a lava flow may change
drastically during eruption. If lava continues to flow through
a channel at a high rate for a longer time (several
days to weeks), a lava channel may become severely eroded
and, as a consequence, become much wider than the originallava
channel.