Jökull - 31.12.2001, Page 14

Jukka Kciyhkö et al.
eral metres above the normal river level (Figure 3;
site P; Figure 6B). An additional outwash plain (also
referred to as Grjót; ca. 5x lOkm) has formed to the
south of Grímsstaðir, on the western bank between the
current channel and the road F88 to Áskja (Figure 3;
site R).
Siliceous tephra classes (b7 and b8) dominate the
tephra fan that formed during the 1875 eruption on
the ENE side of Askja (Figure 3; site S). The contin-
uous fan deposit covers ca. 63 km (class b7), and is
surrounded by class b8, a mixture of light-coloured
tephra and darker sand/lava. The eastern edge of
the fan has been eroded (and perhaps, partly buried)
by aeolian (and fluvial?) activity. Class b8 can also
be found interleaved with hyaloclastite formations or
loose palagonitised sediments (b9), the latter being
rather light-coloured (i.e. with reasonably high re-
flectance in visible wavelengths). Dyngjufjöll ytri
(Figure 3; site T) is the largest hyaloclastite forma-
tion in this area, followed by the N-S oriented ridges
(Fjallgarðar) between Möðrudalur and Grímsstaðir in
the east. In addition to these subglacially formed sed-
imentary formations. light-coloured sediments occur
in places as loose scree and exposed indurated strata,
for example at the Sydra formation to the north of
Grímsstaðir (Figure 3; site U). These supposedly (cf.
Van Vliet-Lanoe et al., 2001) interglacial fluvial and
lacustrine deposits are now being eroded by wind and
water.
Miscellaneous category
The miscellaneous category is dominated by the class
c2: Vegetated soil (ca. 20% of the total area). Vege-
tated areas are abundant in the north-west of the study
region, around Mývatn (Figure 3; site V). These vary
from well-drained dwarf-shrub heaths to wetlands and
patches of dense mountain birch scrub, as described
in the 1:40 000 Vegetation map of Iceland (1982).
The Vegetation Map of Iceland 1:500 000 (Guðjóns-
son and Gíslason, 1998) depicts three vegetation types
for the study region: heath, birch scrub and wetland.
Of the 30 classes generated by our unsupervised TM
clustering, six were unambiguously identified as veg-
etation - these were classes in which “greenness” ob-
scured the underlying surface characteristics. Four
classes represented well or moderately-drained soils,
and two classes represented wetlands. A typical er-
ror in remote sensing studies of the subarctic biome
is the partial misclassification of Betula nana domi-
nated wetlands as Betula pubescens scrub due to their
similar reflectance patterns (cf. Káyhkö and Pellikka,
1994). The vegetated area defined in our TM classi-
fication agrees well with the area shown on the map
by Guðjónsson and Gíslason (1998). The current
project, however, does not concern vegetated areas
per se, which is why vegetation is not considered here
in more detail.
Other classes within the miscellaneous category
are cl: Hydrothermal alteration or solfatara, c4: Ice
and snow, and c3: Water bodies. The largest occur-
rence of class cl is the Námafjall solfatara area near
Mývatn (Figure 3; site X). During the image acquisi-
tion in July, some of the mountain tops, e.g. Herðu-
breið (Figure 3; site Y) and Trölladyngja (Figure 3;
site Z), and the rim of Askja caldera in the west had
ice and/or snow cover.
SYNTHESIS AND DISCUSSION
Distribution of land cover types
Unsupervised clustering of the Landsat TM image,
followed by manual merging of the classes, revealed
the principal characteristics of the land cover types of
Ódáðahraun. This region is dominated by unvegetated
lava fields, often covered with loose surficial sedi-
ments consisting of mobile, redeposited aeolian sand.
As primary soil cover is almost completely absent
from most of the area, this region cannot be consid-
ered to be undergoing erosion, sensu stricto, as there
is no soil cover to be eroded. Instead, this region acts
as a transport pathway for the sediment released by
glaciofluvial processes at the ice cap margin. Continu-
ous expanses of vegetation, threatened by the advanc-
ing semi-desert, occupy the north and north- west of
the region. Arnalds (1992a, b) has demonstrated that
the margins of Ódáðahraun erode by the gradual en-
largement by wind action of small erosion scars. This
process proceeds until the majority of the landscape
has become barren with only few scattered remnants
of soil cover (rofabarðs).
12 JÖKULLNo. 51