Náttúrufræðingurinn - 1987, Blaðsíða 59

evolves into a magma chamber. If the sill is
to expand and evolve into a chamber,
magma must continue to flow into it
(through dikes) from the reservoir below
at sufficiently high volume rate. Ideal sites
for high horizontal compressive stresses
within dike swarms, hence for magma
chambers, are rock formations where indi-
vidual layers have different elastic prop-
erties. The rocks formed during the Pleis-
tocene have notably different elastic prop-
erties and, when buried in the volcanic
zones, they form more promising sites for
magma chambers than the Tertiary rocks.
This may explain why the number of cen-
tral volcanoes (most of which are fed by
shallow chambers) during the late Pleis-
tocene is proportionally greater than dur-
ing Tertiary time.
5) It is estimated that during individual
volcanic events the volume of dike and
eruptive material is, on average, 0.02% of
the total volume of the corresponding
magma reservoir but 0.05% of the total
volume of a magma chamber. Provided a
reservoir does not supply magma to a
chamber during an eruption, or at a
negligible volume rate, the volume of dike
and eruptive material from a chamber in
individual volcanic events is less than 0.25
km3 and usually less than 0.1 km3.
6) It is argued that many historical vol-
canic events in Iceland, considered to be
single eruptions, were in fact composed of
many smaller eruptions. In such volcanic
sequences the chamber is reloaded
between events (eruption and/or intru-
sion). The estimated volumes of historical
eruptions in Iceland thus cannot be used to
calculate the volumes of the corresponding
chambers or reservoirs. It is concluded
that, from the point of view of volume, a
typical historical eruption could have been
fed by a shallow magma chamber.
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