Náttúrufræðingurinn - 1998, Blaðsíða 115

Straumsvík, or the area in general, has been car-
ried out. The article deals with these matters by
listing the processes, whether meteorological or
geological, and describing their general nature,
causes and effects.
The meteorological processes are primarily
severe storms and rain associated with very
deep and fast, hurricane-like depressions com-
ing up the Atlantic from south-west. These are
relatively comraon during the winter months,
accompanied by strong winds, which are the
main cause of damage. Most common is the
blowing of corrugated iron sheets off roofs and
walls. Once loose, the sheets may cause damage
wherever they come down and hit things or liv-
ing creatures. Hence the damage is commonly
twofold, first the damage to structures where
ihc sheets were originally and secondly the
damage to things or people hit by the sheets.
Coastal flooding is a complicated process.
It’s initiated by meteorological phenomena, low
pressure, heavy storm and high sea, but modi-
fied by marine conditions, such as water tem-
perature, tide and fetch, celestial conditions,
such as the relative position of the sun and
moon, and by the geological nature of the coast
exposed to the flooding, its materials, structure
and general landscape. Coastal Hooding is com-
mon in Iceland, especially in thc south-western
part. For three reasons, the Straumsvfk area is
favourably located in relation to this type oi
flooding: a) It’s sheltered by the Reykjanes Pe-
ninsula from the flood surges approaching from
south and soulh-west, which are the common-
est and generally the severest ones. b) The west-
erly winds that could pose a threat to
Straumsvík are relatively infrequent. c) Because
of the closeness to Greenland the fetch is too
short for severe floods to build up.
Earlhquakes and ground físsuring are com-
mon phenomena in Iceland and both are relevant
in the Straumsvík area. At a short distance,
south and south-east of the area, lies the bound-
ary between the two crustal plates of the area,
the North-American Plate and the Eurasian
Plate. At this plate boundary the crust ol the
North-Atlantic is generated and from there it
drifts in opposite directions. Al the boundary it
generally drifts in defined events that start
when pressure, which has built up at the bound-
ary, is released in earthquakes. Earthquakes
here can reach up to 6 on the Richtcr scale at a
distance of only about 10 km from Straumsvík
and even up to more than 7 within 50 km from
Straumsvík. During earthquakes, fissures form
in the crust and the edges of the fissures are off-
set to form faults. It is common that such faults
reach the surface of the ground which is then
torn open and offset. At the surface these faults
and fissures tend to form swarms with parallel
arrangement of the features. Here such swarms,
with a north-east trend, are close and in fact cut
the area in question. The crust here is thus bro-
ken by past events, and faulting and fissuring of
the ground can be expecled in the fulure.
Basaltic volcanic eruptions of effusive nature
occurring in the volcanic rift zone, one of the
surface expressions of the plate boundary, could
result in relatively thin fluid lava flows towards
Straumsvík. The Straumsvík area is made of such
lavas and the aluminium plant itself is localed on
the youngest of these lavas, the Kapelluhraun,
which formed in an eruption below the hills of
Undirhlíðar, merely 8-9 km south-east of the
plant. This eruption took place only about 850
years ago. A lava flow of this type coming from
craters at Undirhlíðar would reach Straumsvík in
a relatively short time; hence the time for pre-
cautionary actions is very short after such an
eruption has started.
Land subsidence and a relatively fast coastal
erosion are characteristic features of the region.
In places the land subsides with an average
speed of around 2 mm per year. This is the result
of a) the movement of the crustal plate away
from the plate boundary, towards deeper ocean
and cooler environment, and b) the pressure on
the rigid plate itself, which is caused by new
volcanic material building up on top of the plate
within the volcanic zone. The crust seeks to
maintain isostatic equilibrium. The strata on the
shore are made of vesicular and heavily cracked
friable rock, the fresh basaltic lava. It breaks
down easily in the relatively harsh marine and
weather conditions, where temperatures com-
monly lluctuate around 0°C during the winter-
time, with rapid shifting of frost and thaw, an
effective agent of breaking down rock.
PÓSTFANG HÖFUNDAR/ÁUTHOR's ÁDDRESS
Páll Imsland
Skipasundi 46
IS-104Reykjavík
273