Jökull - 01.12.1986, Side 16

Fig 4. Annual precipitation rates on
Reykjanes peninsula. It is extremely
high in the eastern, mountainous
part of the peninsula. Mynd 4. Úr-
koma á Reykjanesskaga. Sérlega
mikil úrkoma er á fjalllendinu á
austanverðum skaganum.
flat coastal lowlands on both sides (fig. 3). The lavas
erupted during the glacial periods have mostly been
turned into pillow lavas or hyaloclastites of various
appearances through rapid chilling in subglacial melt-
water (for a more detailed lithological explanation of
the process, see Sœmundsson 1980). The hyaloclastite
facies range from fine grained stratified tuffs through
breccias with a matrix of glass fragments to pillow
lavas and finally irregularly jointed massive basalts.
The hyaloclastites (Icelandic “móberg”) form steep-
sided mountains and hills, the fissure eruptions result-
ing in mountain ridges and rows of hills, while the
lava shield eruptions may lead to flat topped table-
mountains. Some of the latter are capped by subaeri-
ally erupted basalt lavas, where and if the eruptions
have penetrated through the icecover.
The hyaloclastite mountains form a continuous
highland for the easternmost two-thirds of the penin-
sula, gradually decreasing in height towards west from
600—700 m asl. to 300—400 m asl. These highlands
are composed of tablemountains and composite
mountain ridges with through going depressions
across the highlands, that are near to 200 m asl. The
highlands experience a heavy precipitation from mois-
ture laden marine air masses ascending their southern
slopes (fig. 4).
On the eastem part of the peninsula the precipita-
tion probably exceeds 3000 mm/y while it may reach
1500—2000 mm/y on the western part (fig. 4). The
wet southerly air masses are also relatively warm, so
that a disproportionate part of the precipitation falls
as rain or sleet. During the winter much snow may be
accumulated in the eastern, higher parts of the moun-
tains, especially in the ski-country around Bláfjöll, but
in the western part of the peninsula the snow cover is
very unstable.
The westernmost 20 km of the peninsula are flat
lowlands, covered with postglacial and interglacial
lava fields with some isolated hyaloclastite hills
protruding from the lava cover (fig. 3). The northwest
extension of the peninsula, the Rosmhvalanes penin-
sula, reaches only 60 m asl. and is totally covered by
interglacial basalts, that reach below sea level. In this
western part of the peninsula the precipitation is
about 1000 mm/y. The outflow of groundwater is at
some places hydrogeologically concentrated in narrow
spring areas, some issuing more than 1 m3/s (see fig-
2). The easternmost spring areas lie on the border of
the Neovolcanic zone where the young and permeable
rocks are cut off by older rocks (fig. 3). Farther west
the springs are located directly at the coast, the
springs in Vatnsleysuvík and Vogavík (locations, see
fig. 10) only appearing at low tide.
HYDROGEOLOGICAL ASPECTS
The experience from various areas in Iceland indi-
cates strongly that there is a correlation between
permeability and rock types, at least in a relative
order (Sigurðsson & Sigbjarnarson 1985). Here the
classification of permeability will be restricted to the
types present on the Reykjanes peninsula. It is as
follows, in order of decreasing permeability:
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