Náttúrufræðingurinn - 1992, Síða 49

tral volcanoes form a caldera. Nearly all
Icelandic central volcanoes liave devel-
oped such phenomena. One of the excep-
tions has been the Hekla volcanic system.
Recently, a theory has been presented
which suggests a caldera formation in the
Hekla volcanic system during the Holo-
cene. The theory is based on the following
arguments:
1. The subglacial hyaloclastite forma-
tions surround the Hekla volcano but are
nearly absent from the mountain itself.
Volcanic activity is always most intense in
the centre of the volcanic systems, and
there the accumulation of volcanics is
greatest. In all comparable volcanoes in
Iceland, hyaloclastites form the highest
peaks - Hekla volcano is the only excep-
tion.
2. The hyaloclaste ridges southwest and
northeast of the Hekla volcano become
higher as approaching the volcano, but
are abruptly curtailed a short distance
away from it.
3. In two places close to Hekla are
blocks tilting towards the volcano and
these seem to be bounded by arched frac-
tures.
4. Where the Hekla fissure intersects
the assumed caldera rim, there are pecu-
liar xenolith occurrences. This may be in-
terpreted as anomalous volcanic activity.
5. Outside the assumed caldera rim in
the vicinity of Hekla, no lava flows have
been found dating from 4500 years ago
until about 500-600 AD. Thus there seems
to be a time gap of 3000 years. Tephro-
chronology, however, does not indicate
any change in the volcanic activity during
this period. The most obvious explanation
of this time gap is that in the course of the
great eruption which produced the H-4
tephra, a caldera collapse took place. If
this theory is valid, it took about 3000
years to fill the caldera to the rim with la-
va flows.
6. In the area between Hekla and the
Vatnafjöll are some eruptive fissures.
They are discontinuous and an 8-km gap
occurs where they cross the assumed
Hekla caldera.
7. Within the assumed caldera, only in-
termediate magma (andesite) is erupted,
but outside it mostly basaltic magma.
It is likely that the curtailed hyaloclas-
tite ridges, southwest and northeast of
Hekla, have originally continued into the
Hekla mountain itself. In the course of a
caldera collapse, the parts of the ridges
closest to the volcano have subsided. The
fact that basaltic magma is not erupted
within the caldera, may be explained by a
magma chamber at depth beneath the vol-
cano, containing intermediate and acidic
magma acting as a density trap to the bas-
altic magma.
The assumed caldera is about 10 km in
diameter (Fig. 4) but may consist of more
than one smaller calderas, in similar man-
ner as the Askja volcano in Northern Ice-
land.
In historical times, there have been 17
eruptions in the Hekla proper and 5 erup-
tions, mainly basaltic, elswhere in the vol-
canic system (Table 2).
191