Náttúrufræðingurinn - 1987, Blaðsíða 101

SUMMARY
Rates of some
geomorphological
processes in
Iceland
by
Sigurður Steinþórsson
Science Institute, University of Iceland
Geoscience Building
Reykjavík
Iceland owes its very existence to an
underlying mantle plume which ís respons-
ible for high volcanic productivity, and at
the same time serves to buoy up the relati-
vely thick oceanic crust (10 km). As a
consequence Iceland is the longest ridge
segment (400 km) and largest landmass
(103,000 km2) exposed anywhere along the
mid-ocean rift system. Second, owing to its
position in the North Atlantic Ocean, vig-
orous wave action is an active eroding
force along the coasts, whereas the
Pleistocene glaciation resulted simultaneo-
usly in impressive subglacial volcanic form-
ations of pillow lavas and hyaloclastite
along the volcanic zones (Fig. 3), and m
extensive glacial erosion towards the co-
asts. In general, Iceland is the scene of
rapid geological processes, both construct-
ive and destructive. In the article an at-
tempt is made at a first-order estimate
the rates of some of the major ones.
The volume of volcanics erupted in Ice-
land during the Holocene is estimated abo-
ut 4 2 km3 per century. For comparison,
the volume of the Laki 1783 lava flow is
some 12 km3, similar to that of the largest
lava shields in Iceland. The volume of the
country above sea level is about 51,500
km3, equivalent to the volcanics produced
in 1.2 Ma (million years). Therefore, over
90% of the rock ”created“ at the rift zones
during the geological history of Iceland
(14.5 Ma) has disappeared below sea level
in one way or another.
First, recycling of volcanics in the rift zone
is considered. Comparing the rate of crust-
al formation (200 km2/Ma per unit length;
10 km thick crust, 20 km/Ma spreading
rate), the proportion of dykes in the crust
(40%), and the rate of volcanic produc-
tion, about 11% of rock must be recycled,
i.e. remelted and brought to the surface
again. Spreading, and volcanic activity, is
confined to discrete fissure swarms in
which the recycling should be considerably
more rapid than the above average numb-
er. In addition, the productivity varies
along the rift zone (Fig. 2), with enhanced
recycling in C Iceland compared to the
distal ends of the rift zone, in keeping with
the variation in “degree of evolution” of the
basalts along the rift zone (Fig. 3, lower
part).
Second, the effects of thermal contraction
in the crust are examined in terms of eqns.
(1) to (3), shown in Fig. 5. Two-thirds of
the crust produced sink below sea level
due to cooling in the crust.
Third, erosion rate estimated from sedi-
ment transport of rivers is estimated
25,000 km3/Ma.
Fourth, topographic analysis in two areas
in Iceland indicated local erosion rates of 1
mm per 3 Ma and 15 Ma, respectively. The
difference is partly due to selection of
domain, one analysis taking only to mo-
untain valleys, and neither analysis to the
continuation of the valleys below sea level
(fjords).
Finally, the effects of isostacy are consi-
dered, both sagging by sediment in S Ice-
land, and isostatic rebound as a result of
deglaciation and erosion (Fig. 6). The re-
sults are summarized in Table 1, the bal-
ance of which can be improved (6,000 vs.
40,000 km3) by assuming lower erosion
rates in the Tertiary than indicated by the
sediment load of present day rivers.
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