Jökull - 01.12.1977, Page 40

H-4, white tephra. Lithics.
Sub-units (b) (c) <d)
w%
Fig. 16. Histograms of
lithic fragments in the
white unit of H4 on
Saudafell, 8 km N of
Hekla. The “shots” are
stippled.
Mynd 16. Kornastœrðar-
dreifing bergbrota úr
Ijósa hluta á Sauða-
felli.
The two sub-units terrned (a) and (b) have
not been found together in any of the localities
and it is therefore not known which one is
erupted first.
The sub-unit (a) consists of an extremely
fine-grained white ash. It is not found in the
immediate vicinity of Hekla and seerns to reach
maximum thickness (ca. 10 cm) 30—40 km N
of the volcano. There tlie mean grain size is
5.5 <J> (0.02 mm). In some places it forrns a hard-
ened crust. It was carried almost directly to-
wards N and can be traced all over Northern
Iceland.
The profile referred to for sub-units (b)—(d)
is at Saudafell, 10 km N of Hekla’s Summit
Crater, where the white unit is more than two
meters thick.
Sub-unit (b) is a relatively thin layer found
at the bottom of the deposit in the area N of
the volcano and seems to be of local extension
only. The distribution indicates that it erupted
from the extreme NE encl of the Hekla fissure.
The pumice is coarse-grained, the niean grain
size in the Saudafell profile is in excess of —3 $
(8 mm) and the largest pumice clasts measure
up to 15 cm (maximum diameter). The lithics,
mostly basalt, make up to 19 weight % of the
tephra. Their histograms are bimodal (Fig. 16).
The largest lithic fragments are “shots” and
indicate violent explosions cluring the forrna-
tion of the sub-unit, which may represent the
opening stage of the eruption.
Sub-unit (c) is rclatively coarse-grained and
forms the main part of the white tephra at
Saudafell. The mean grain size of the pumice
is —2 <J> (4 mm) and lithic fragments, mainly
38 JÖKULL 27. ÁR
basalt, constitute 21 weight % of the tephra.
Stratification is lacking and the lithics are even-
ly distributed in the tephra. The mode of
eruption was probably continuous uprush with-
out any distinct explosions.
Together (a) and (c) form the main part of
the unit. Both were carried chiefly towards N.
Fielcl evidence in Northern Iceland suggests
that (c) was carried sligthly more westwards
than (a), this is thought to be due to advection,
the change in wind direction with elevation,
indicating that the extremely fine-grained ash
of (a) was transported at higher levels than the
nrore coarse-grained (c) (Fig. 18).
Brownish Greyish Greyish
black brown yellow
Fig. 17. The distribution of the three upper
FH-units near Hekla.
Mynd 17. Útbreiðsla dökkbrúna, grábrúna og
grágula hluta H± nœrri Heklu.