Jökull - 01.12.1977, Page 39

SOME CHARACTERISTICS OF H4
H4 is and unwelded and poorly stratiíied
Elinian airfall deposit. The tepltra consists of
pumice and ash with silica ranging from 74 to
57%, various lithics, and ntinor amount of
crystals. Within the deposit there are horizons
tvliere lithic fragments are exceedingly abundant
(~ 50%). Distinct colour changes occur as the
pumice becomes more basic, going from bright
white at the base to brownish-black at the top.
H4 is divided into four units according to
the colours of the pumice: white, greyish-yellow,
greyish-brown, and brownish-black. Within each
unit a small gradation in colour occurs. In the
vicinity of Hekla (< 30 km) all four units are
found, but never more than three in each
locality. Tliis is due to changes in wind direc-
tion during the eruption and since it is known
that the pumice becomes clarker as the eruption
proceeds, it has been possible to íollow these
changes.
The changes in colour are not accompanied
by changes in grain size except where one of
the units is partly or totally missing due to
different wind direction at that stage of the
eruption.
Tlie lithics include material exposed in out-
crops on the flanks of the volcano such as basalt
and hyaloclastites, but also rhyolite and ignim-
brites wliich are not exposed. At the base of
the tephra layer the lithics are mostly basalts
but rhyolite and liyaloclastites become increas-
ingly abundant in the upper part.
It shoulcl be noted that H4 is heavily erodecl
in most localities in the Elekla area, and some-
times only the white part remains. Where mea-
surcd, thickness of H4 does not exceed 3 meters.
This should not, however, be taken as maximum
thickness.
The following description of H4 refers to the
proximity of Hekla where the tephra deposit is
thick enough to be studied in some detail. No
undisturbed section was obtained closer to the
Hekla fissure (Summit Crater) than 7.5 km.
White unit.
The lowermost unit, consisting of white
pumice, lithics, and plagioclase crystals, makes
up ahnost half of the total volume of H4. A
drastic change in grain size occurs within the
unit. It can be divided into 3—4 sub-units (Fig.
15) on the basis of grain size characteristics ancl
the proportion of lithic fragments in the tephra.
This part of the tephra was mainly carried to-
warcls N and NE.
Fig. 15. Main direction
of the spreading of the
white sub-units of H4.
The different sizes of the
arrows indicate difference
in area and the dots and
rings indicate difference
in coarseness.
Mynd 15. Örvar sýna i
hvaða átt undirdeildir
hins Ijósa hluta H4 dreifð-
ust, en stcerð örvanna gef-
ur vísbendingu um mis-
munandi útbreiðslu.
JÖKULL 27. ÁR 37