Jökull - 31.12.2001, Blaðsíða 11

Geomorphology of Odáðahraun
Lava cover category
Exposed lava cover types are concentrated in the
central portion of the study area, between Askja in
the south and road no. 1 in the north. The largest
classes are a4: Rough lava with partial sand cover
without vegetation and a5: Rough lava with partial
sand cover, respectively. Class a4 is most abun-
, dant between Trölladyngja and Dyngjufjöll ytri in the
south (Figure 3; site A), at Utbruni, and along the
Jökulsá á Fjöllum. Portions of the Upper Pliocene
barren highlands to the east of Jökulsá also fall within
this class. Class a5 is scattered throughout our study
area, but is concentrated in Suðurárhraun and Fram-
bruni in the west (Figure 3; site B), and areas to the
north of Askja (Figure 4). Class a3: Smooth lava, par-
tial clinker cover with sparse vegetation, forms con-
tinuous patches, the largest of which (ca. 105 km2) is
located approximately lOkm north of Herðubreið, as
an apron on the eastern flank of Eggert-Herðubreið-
arfjöll (Figure 3; site C).
Another large, continuous area of a5 occurs ca.
15 km SE of Búrfell. Class a6: Degraded lava, mainly
aa, with vegetation cover is most abundant between
the vegetated area in the north and the barren region,
and portions of both Búrfellshraun (Figure 3; site D)
and Dimmuborgir belong to this class. Two small
classes, al and a2, represent fresh, recently erupted
lavas. Class al: Fresh and glassy lava, mainly aa is
found primarily on rough portions of Krafla lava
flow (Figure 3; site E). Class a2: Fresh and glassy
lava, mainly pahoehoe or aeolian sediment is not a
satisfactory class, as, in addition to fresh lavas such
as Krafla and Askja 1961 (Figure 3; site F), it also in-
cludes substantial areas of dark fine (mainly aeolian)
sand e.g., in the vicinity of Dyngjujökull sandur. With
the given data, however, it was not possible to separate
the two land-cover types.
Barren sediment cover category
The barren sediment category is composed of nine
classes. Class bl: Lag surfaces, is abundant in many
parts of the study region. Lag surfaces form where
fine sediments are blown away, leaving a surface of
pebbles or cobbles that are polished by abrasion. The
largest continuous lag area of bl surrounds an apron
of rough lava (a5; see above) on the eastern flank of
Eggert-Herðubreiðarfjöll, and totals almost 60 km2 in
size (Figure 3; site G); another substantial lag area is
found to the north of Sellandafjall near Mývatn. Ad-
ditionally, class bl is abundant on Tertiary, eroded till
surfaces on both sides of the Rift Zone. Class b2:
Reclaimed erosion area with dry grass is very limited
in area, and abundant only in the northern eroded ar-
eas. It occurs on sites where reclamation measures
have been taken by sowing grass seeds on eroding sur-
faces, i.e. on the western edge of the Nýjahraun near
road no. 1 (Figure 3; site H). Due to the partial grass
cover, some of these areas may actually experience
deposition at present.
Classes b3: Fine sediment, b4: Medium sediment.
b5: Coarse sediment, and b6: Alluvial sediment cover
large areas (in total ca. 25%) in the southern and cen-
tral parts of the study region. Alluvial sediments (b6)
mixed with braided channels dominate the immedi-
ate vicinity of the ice margin (Figure 3; site J); these
channels often fall into classes a4 and b5, most likely
due to high moisture content and exposed lava blocks.
Outside the alluvial fringe, fine and medium sedi-
ments (b3 and b4) dominate the sandur area between
Dyngjujökull and Askja (Figure 3; site K; Figure 5).
This region experiences vigorous aeolian activity,
and may act as a source for the aeolian sediments
found further north on the lava fields. The most strik-
ing depositional sedimentary feature on the classi-
fied image is a 3-7 km wide lobe of aeolian sediment
that extends NE from Suðurárbotnar for almost 50 km
(Figure 3; site L; Figure 6A). This lobe terminates at
Nýjahraun, where sand trapping by lyme grass has re-
sulted in substantial dune fields. The formation is not
as distinctive on the ground as it may appear on the
image, however, due to the discontinuty of the veneer
of fine and medium sediments (b3 and b4) mixed with
underlying lava.
An offshoot sand stretch, possibly originating
from this formation, extends between Sellandafjall
and Bláfjall all the way to Dimmuborgir (Figure 3;
site M). Three other distinctive sand lobes extend
south on the rough Frambruni lava flow (a4), to the
west of Dyngjufjöll ytri. The northernmost stretch is
JÖKULL No. 51 9