Jökull - 01.12.1978, Blaðsíða 24

Fig. 3. Large boulders
and stones on the sur-
face of Raudhóla-
sandur. View towards
north.
Mynd. 3. Stórgrýtisdreif á
Rauðhólasandi. Horft til
norðurs.
sandur-plain is the large number of stones and
boulders of basalt, hyaloclastite and tillite lying
on its surface (Fig. 3). The biggest boulders
measure 2.4 m in diameter. The concentration
of the boulders decreases towards north and
west (Fig. 4), and so does the hyaloclastite and
tillite content (Fig. 5). These rocks are less
resistant to water-abrasion than lava blocks.
Besides the stones and boulders the sandur-
plain is built mostly of sand and gravel (Fig.
e).
About one kilometre to the southeast of
Raudhólasandur a small hill rises about 30 m
above the surroundings (Fig. 2). This hill
consists mostly of pillow lava and breccias and
is cut through by a 100—150 m wide and 35 m
deep gorge. Rounded basaltic lava boulders
(0.5— 1 m in diameter) were found widely
scattered on the slopes of the gorge leaving no
doubt about its fluvioglacial origin. The gorge
must have been cut as a superposed gorge,
when the hill was buried beneath a cover of
glacier ice. The direction of the gorge is
towards the southeastern corner of Raud-
hólasandur, parallel to the ice movement.
Thus it is most likely that the cutting of the
gorge and the formation of the sandur-plain
were synchronous, formed at a time when the
ice margin lay at the southern terrace of
Raudhólasandur.
The large number of boulders on the top of
the sandur-plain and the cutting of the gorge
indicate that an immense volume of water
flowed here during a short period, i.e. a catas-
trophic glacier outburst or a jökulhlaup took
place. Catastrophic floods from glaciers are
common in Iceland. They may occur when
water is released suddenly from ice-dammed
lakes or from meltwater storages at subglacial
geothermal areas or the sites of volcanic erup-
tions. The first possibility is very unlikely in
this area. Such ice dammed lakes are usually
blocked up in tributary valleys. The only pos-
sible valleys for damming of this kind in the
area are situated between the Illvidrahnjúkar
mountains, but they run parallel to the ice
movement. It is more likely that the jökul-
hlaup occurred in connection with a sub-
glacial eruption. Several eruptions took place
in this area during the Lateglacial and Post-
glacial time (Kjartansson 1965).
The table mountain Miklafell is situated at
the northeastern corner of Hofsjökull, about
20 km to the southeast of Raudhólasandur.
Table mountains are formed in subglacial
eruptions and consist of hyaloclastite with
basaltic lavas on the top. If the eruption lasts
long enough, the crater is built up over the
massive ice and basaltic lavas begin to flow
(Kjartansson 1943). Thus table mountains can
be used to estimate the thickness of the ice at
the time of their formation. In the case of
22 JÖKULL 28. ÁR