Jökull - 01.12.1979, Page 25

Fig. 16. Changes in land elevation at the center of the Krafla caldera during the current rifting
episode that has been continuing since 1975. Upper diagram shows rapid deflation events alternating
with periods of slow inflation. Lower diagram shows comparison with an earlier rifting episode of the
same volcanic system. The bars show historically recorded events of increased volcanic and tectonic
activity very likely accompanied by rapid deflation.
lava and very little was added to the flanks.
Examples are the volcanoes under the Vatnajökull
ice sheet. Many central volcanoes in the axial rift
zone can be classified as composite shield volca-
noes, on the basis of their shape and the rock types
erupted. Others do not conform to this definition
because the prevalent subglacial eruptions have
modified their shape.
Since 1975 the Krafla volcanic system of the
axial rift zone in northern Iceland has been active,
and much has been learned there about the
mechanism of rifting and the interplay between
magmatic processes in the central volcano and
rifting in the associated fissure swarm. Historical
accounts indicate that such rifting is episodic, and
occurs in northern Iceland every 100—150 years
affecting one particular swarm or a part of it at a
time. During the interval between two successive
episodes, tensional stress accumulates which is
released during the rifting episodes. The central
volcanoes appear to play a very active role during
such episodes by allowing the ascent of magma that
collects at shallow depth in their roots. From the
resulting inflation of the volcano the rate of inflow
of magma has been found to be constant at Krafla
since 1975, amounting to about 5 m3/s. From time
to time during the rifting episode, which may last
for several years, magma is injected from the shal-
low reservoir into the fissure swarm, the controlling
factor being the magma pressure that must reach a
certain level to initiate sudden rifting. As a result
the central volcano deflates (Fig. 16). Different
segments of the fissure swarm have rifted during
individual deflation/rifting events. The extension
across the Krafla fissure swarm along most of its
length already amounts to about 4 m correspond-
ing to about 200 years of tensional stress build up
assuming a mean half-rate of spreading of 1 cm/y.
Geodetic measurements across the Krafla fissure
swarm are consistent with crustal separation
manifest slightly as continuous creep but mainly as
episodic events with up to several m of extension
and subsidence occurring within a few years period.
This kind of stress release appears to hold for other
parts of the axial rift zone as well.
The central volcanoes appear to operate in-
dependantly of the transecting fissure swarms;
from about 20 volcanic eruptions occurring on
average per century only a few extend to the fissure
swarms, the rest is confined to the central volcanoes
themselves. The reason for this must lie in different
“loading time” for the central volcanoes on the one
hand and the fissure swarms on the other. The
central volcanoes possess a magma trap in their
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