Jökull - 01.06.2000, Page 19

Guðrún Larsen
Table 5. Chemical composition of basaltic tephra from the Eldgjá fires, Katla system. Bulk sample from the
bedded tephra layer where it is 4 cm thick. Slight differences between gla and glb may indicate different source
areas on the >75 km long fissure. The third type, glx, may be scavenged from bedrock. – Efnasamsetning
basísks glers úr Eldgjárgjósku. Heildarsýni af 4 cm þykku lagskiptu gjóskulagi. Mismun á efnasamsetningu gla
og glb má líklega rekja til uppruna á mismunandi hlutum gossprungunnar. Glerkorn með lægra TiO , glx, gætu
verið aðskotakorn úr berggrunni.
Eldgjá tephra n SiO  TiO  Al  O  FeO MnO MgO CaO Na  O K  O P  O  Total
E 934/938 gla 6 46,07 4,65 12,10 15,09 0,18 5,07 10,64 2,72 0,71 97,22
0,36 0,25 0,36 0,49 0,03 0,28 0,25 0,09 0,04 0,55
E 934/938 glb 5 47,75 4,65 12,34 13,93 0,18 4,61 9,68 2,99 0,88 97,01
0,35 0,09 0,16 0,08 0,03 0,14 0,25 0,10 0,07 0,50
E 934/938 glx 3 49,45 2,52 12,38 13,40 0,19 5,41 10,77 2,50 0,39 97,01
0,29 0,11 0,38 0,09 0,02 0,07 0,05 0,08 0,02 0,58
lying lava had solidified before they were emplaced.
The volcanic signal from Eldgjá in the Greenland ice
cores extends over 3-6 years (Hammer, 1984; Zielin-
ski et al., 1995) and may be a realistic indicator of
the length of the eruption. The effects, however, have
lasted to this day.
ENVIRONMENTAL CHANGES IN THE
PAST 11 CENTURIES
The most extensive environmental changes in inhabi-
tated areas in Iceland, caused by volcanic activity dur-
ing the last 1100 years, began by the Eldgjá erup-
tion. These changes can be assessed by geologi-
cal/tephrochronological studies and with the help of
written sources, both the early chronicles that briefly
describe the conditions and natural phenomena met
by the Norse settlers, and later contemporary sources
(Íslensk Fornrit I; S.t.s.Ísl. IV, 1907-15). Only the
more "local" effects of the historical eruptions will be
treated here, i.e. those of lava emplacement, tephra
falls and jökulhlaups. The regional effects, in particu-
lar those of the Eldgjá event, are discussed elsewhere
(e.g. Zielinski et al., 1995; Thordarson et al., in press).
Prehistoric changes are not addressed here.
The pre-Eldgjá environment
A partial reconstruction of the pre-Eldgjá environ-
ment is possible on the basis of current geological and
historical knowledge.
Álftaversafréttur north of Atlaey was extensively
vegetated prior to the Eldgjá eruption. Thick soil
had formed on the Hólmsáreldar lavas since their em-
placement ca 6800 14C yrs ago (cf. Figure 5). On the
surrounding hyaloclastite hills even thicker soil with
up to 170 individual tephra layers had accumulated to
more than 8 m thickness in places. This soil cover is
still preserved on some of the hills, e.g. in Atlaey. The
major river of the area, Hólmsá, flowed in places on
the Hólmsá lava between banks of soil several metres
thick, similar to its present course at Hrífuneshólmi,
just before it joins Kúðafljót.
In front of of the gap occupied by Kötlujökull a
low and wide alluvial cone is implied by the geome-
try of the encircling Eldgjá lava (Figure 8). Before the
emplacement of the lava, meltwater and jökulhlaups
probably had free passage in easterly directions, while
a gently curving terminal moraine in the northern part
of the Álftaver district (Figure 8) protected the vege-
tated region to the south.
In the Meðalland district the Eldgjá lava (Figure
8) is at least partly underlain by sandur deposits or al-
luvial flats, presumably by the rivers Tungufljót and
Skaftá. The area may also have been overrun by pre-
Eldgjá jökulhlaups from below Kötlujökull. Primary
bedded Eldgjá tephra can be seen immediately below
the lava, resting directly on top of the sandur, about 1
km east of the farm Botnar. In the Landbrot district
the Eldgjá lava flowed across large wet areas, as ev-
idenced by extensive fields of rootless cones. Thick,
broken-up soil along its northern edge, in places thrust
into heaps or small ridges, indicates that part of this
area was vegetated when overrun by the lava.
18 JÖKULL No. 49