Jökull - 01.12.1977, Side 30

and Other Acid Hekla Tephra Layers
GUDRÚN LARSEN,
NORDIC VOLCANOLOGICAL INSTITUTE, UNIVERSITY OF ICELAND
SIGURDUR THORARINSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ICELAND
ABSTRACT
The postglacial activity of Hekla and its vici-
nity is divided into three stages according to
tlre type of volcanic activity and the chemical
composition of the eruption products.
The second stage is characterized hy large
Plinian eruptions. Isopach maps of the five acid
tephra layers produced during this stage are
presented with calculations or estimates of their
areal coverage, volume and age.
The second oldest of the acid tephra layers,
H4, is described in some detail. Ils age is about
4000 C14 years. It covers ca. 78.000 km- on land
and its total volume as freshly fallen on land
and sea is ca. 9 km3. The tephra consists of
pumice ancl ash with SiO2 ranging from 74%
at the bottom of the layer to 57% in the top-
most part, minor amount of crystals and various
lithics. H± is divided into four units according
to the colour changes occurring as the pumice
becomes more basic: white, greyish-yelloiu, grey-
ish-broiun and brownish-black. The lowermost
tuhite unit makes up about 50% of the volume
of the layer. Grain size reaches maximum in
the greyish-yellow unit.
Apparent changes in wind direction during
the fall of the tephra indicate that the bulk of
the layer was probably produced in less than
11/2 day.
The chemical data available on H± and the
larger, but almost identical, H3 are briefly dis-
cussed. Detailed sampling of the H3 tephra and
microprobe analyses of the glass reveal a fairly
gradual change in Si03 from the bottom to the
top of the layer. The occurrence of white veins
in darker pimiice clasts from some units of H±
and H3, most prominent at the boundaries be-
tween the units, indicates a mixing process
between two end-members. So far, this is also
supported by the chemical data.
Hekla is a central volcano in a youth stage.
A deep position and limited widtli of its magma
chamber may explain why none of its big ex-
plosive and acid eruptions have resulted in the
formation of a caldera.
INTRODUCTION
Tephrochronological stndies indicate that
the postglacial activity of Hekla and its vicinity
can be divided into three stages:
1) Fissure eruptions producing basalt lava.
The basalts were erupted from fissures
slightly S and SE of the present Hekla ridge
and flowed mainly towards SW where they
banked against the Búdaröd moraines. This
stage ended 6—7000 years ago.
2) Plinian eruptions proclucing tephra with
silica content of 70—74°/o in the initial
phase of the eruptions and decreasing more
or less gradually to ca. 57% towards their
end.
A distinct maximum in production of acid
and intermediate tephra occurs in the peri-
od between ~ 4400 and ~ 2000 B.P. A few
basaltic lavas were erupted from small fis-
sures NE of the present Hekla ridge during
this period.
3) Eruptions beginning xvith a short Plinian
phase, followecl by mixed activity producing
voluminous lavas of basaltic andesitic (ice-
landite) composition.
This is contrary to stage 2 where the bulk
28 JÖKULL 27. ÁR