Jökull - 01.12.1977, Síða 44

H;j. The lithics are mostly basalt, the rhyolite
and ignimbrite fragments found in the younger
layers being totally absent. Colour changes are
less conspicuous than in H4 and H3. Maximum
thickness known in Hekla’s vicinity is ca. 40 cm.
VOLUME OF THE ACID HEKLA LAYERS
The great volume of some of the acid Hekla
layers, as shown in Table II, raises the question,
why none of these eruptions have resulted in
the formation of a caldera. This may be partly
explained hy the fact that if a rnagma chamber
exists beneath Hekla it is likely to be of a shape
similar to that shown on Fig. 22, as it is limited
on both the SE and NW side by fissures parallel
to the Hekla ridge at a short distance from the
ridge and reaching down to deep-seated basalt
magma. A roof of the shape shown on Fig. 22
is rather resistant against collapse. It should
also be kept in mind that Hekla is not, like e.g.
Askja and the Krafla caldera, connected with
an active fissure swarm, along which escape of
magma can take place (cf. Björnsson et al.
1977). The activity of Hekla is, however, similar
to the activity of the Icelandic central volcanoes
and, considering also the shape of the volcanic
edifice, it seems natural to regard Hekla as a
central volcano in a youth stage.
SOME REMARKS ON THE CHEMISTRY
OF THE ACII) HEKLA LAYERS
It has been shown (Tliorarinsson 1967a)
that tlie silica content of the initial products
of liistorical (stage 3) Hekla eruptions is a
function of the length of the preceding repose.
The last material to appear in each eruption
is usually lava with silica 53—54%.
Whether this also holds for all prehistorical
(stage 2) Hekla eruptions remains to be seen.
Field evidence indicates that eruptions were
indeed less frequent during stage 2 than in
historical tinies, but applying the function: re-
pose-length versus SÍO2 content, applicable for
stage 3, a repose of ca. 350 years is needed to
produce initial tephra with 74% SÍO2 such as
found in H4, and it is doubtful whether so
long a repose ever occurred during stage 2.
42 JÖKULL 27. ÁR

B A S C M A G M A
Fig. 22. Skelton drawing showing the likely
shape of a presumable magma chamber beneath
Hekla. The section is drawn perpendicular of
the Hekla ridge and the width and height of
the ridge is on the same scale as the distance
to the basalt fissures on both sides.
Mynd 22. Þversnið gegnum Heklu og þá kviku-
þró, sern liklega er undir henni, og gegnum
basalt gossþrungur til beggja hliða.
(From Thorarinsson 1970).
According to Sigvaldason (1974) Hekla has
produced two types of magma, dacite and ba-
saltic andesite, in the past 7000 years. The two
liquids are believed to result frorn successive
partial melting of subsided basalts, which were
originally produced on the western rift zone
and carried by drift over the heat anomaly be-
neatli the eastern volcanic zone.
Basically, only two liquids of different com-
position should form: a low temperature melt-
ing silicic component and an andesitic com-
ponent produced by further melting at higher
temperatures.
The chemical variation found in the Hekla
tephras can not, according to Sigvaldason (1974)