Jökull - 01.12.1979, Side 81

discharge of ice down the steep outlet glaciers. The
broad and less active lobes on the northern margin
of Mýrdalsjökull have not advanced yet.
EROSION, DEPOSITION AND LANDSCAPES
Landscapes in Iceland are to a great extent
shaped by glacier erosion during the Pleistocene.
This is evident from the Fjord, cirque and Alpine
landscapes in the Tertiary basalt regions. The
“Palagonite Formation” was built up by subglacial
volcanic activity and later subject to glacier
erosion. The lowlands in Iceland are the result of
glacier erosion as well as marine abrasion. Large
areas in the country are covered by glacial and
fluvioglacial deposits.
The erosion of the present glaciers in Iceland is
still very intense although the glaciers have suffered
deficit for several decades and the velocity slowed
down considerably. Due to the high total budget
the southern glacier outlets from Vatnajökull and
Mýrdalsjökull are able to flow actively right down
to the snouts. Typical valley glacier erosion is found
in the basalt regions in southeastern Vatnajökull.
The glacier outlets are deeply cut into the moun-
tain range. The less active northern lobes terminate
at higher levels. They have not been able to modify
the bedrock by any deep erosion. The rate of glacier
erosion has been estimated for various glacier
basins by measurements of the sediment transport
in glacier meltwater streams. Table I gives a sum-
mary of the results for the main ice caps.
TABLE I. Denudation of land estimated by sediment load
transport (Tómasson 1976)
Region
Vatnajökull
Mýrdalsjökull
Hofsjökull
Langjökull
Other glaciers
Unglaciated areas
Rates of denudation
mm/year
3.2
4.5
0.9
0.4
0.3
0.1
Glacier surges produce extremely high sediment
loads. During the surge of Brúarjökull in 1963—64,
Jökulsá á Brú carried on the average 6470 mg/1 in
suspension. The total transport of sediments during
the year was 25 - 106 tons which corresponds to a
denudation rate of 13.7 mm/year. Two years later,
in 1966, the sediment load had fallen to 1580 mg/1
in suspension and the total annual sediment trans-
port was 6.6 ■ 106 tons.
The high erosional activity of the glacier outlets
which run south from Vatnajökull is evident in the
very large sandur deltas which they have built out
in front of their snouts in Postglacial time (Fig. 1
and Fig. 3). The wide glacier margins terminate on
a broad lowiand area and the heavily laden melt-
water streams spread widely into braided courses
which change continuously and pendulate slowly
across the outwash plain from one side to another.
The lack of vegetation testifies to the migration of
the channels. Jökulhlaups (glacier bursts) caused
by rapid drainage of ice dammed lakes or by sub-
glacial eruptions play an important role in building
up some of the sandur deltas. About 60% of glaciers
in Iceland are situated inside the active volcanic
zone.
Skeidarársandur (1000 km2) is the largest out-
wash plain in Iceland (Fig. 3). It reaches 25 km
inland from the sea and the average slope is 5
m/km. Skeidarársandur is the playground of jök-
ulhlaups from the subglacial lake Grímsvötn which
is situated in a geothermal area in the central
region of Vatnajökull. During the last three to four
decades jökulhlaups from Grímsvötn have occur-
red approximately twice each decade and the total
water volume in each jökulhlaup has been roughly
3—3.5 km3 and the maximum discharge of the
order of 10000 m3/s. Each jökulhlaup has carried
about 30 ■ 106 tons of sediments down to Skeidarár-
sandur. Earlier in this century one jökulhlaup with
a water volume estimated to be the double of that
of recent jökulhlaups occurred each decade with a
maximum discharge of up to 40000 m3/s. Some of
these jökulhlaups were accompanied by volcanic
eruptions in Grímsvötn. During the eruption in
Grímsvötn in 1934 the total sediment transport
down to Skeidarársandur was of the order of 150 ■
106 tons. Further, the western part of Skeidarár-
sandur has been flooded by the lateral ice dammed
lake Grænalón. The maximum discharge of jökul-
hlaups from Grænalón has been of the order of
5000 m3/s. During the recent glacier recession jök-
ulhlaups have become more frequent but smaller in
volume due to the thinning of the damming ice
barriers. Also, the number of dumping glacier lakes
has been greatly reduced.
Mýrdalssandur, Sólheimasandur and Skóga-
sandur have been built to a large extent by jökul-
hlaups caused by volcanic eruptions of the ice cov-
JÖKULL 29. ÁR 79