Jökull - 01.12.1986, Page 6

Fig. 2. Distribution of geothermal
manifestations in Borgarfjördur and
the Southem Lowlands. Locations
of the geothermal systems in Reyk-
holtsdalur and Upper-Árnessýsla are
shown. Also some Upper-Quatern-
ary volcanics outside the TL-zone. —
Jarðhitastaðir í Borgarfirði og á
Suðurlandsundirlendi. Sýnd lega
jarðhitasvœðanna í Reykholtsdal og í
ofanverðri Árnessýslu. Einnig eru
sýndar ýmsar síðkvarterar gosmynd-
anir, sem eru utan TL-gosbeltisins
og liggja mislœgt ofan á mun eldra
gosbergi.
by generation of new crust in the western zone over
the last 6—7 My and the relatively recent develop-
ment of the eastern zone about 1 Mya.
A trend of decreasing heat flow with distance from
the Reykjanes-Langjökull zone is clearly observed but
it is less than predicted by the kinematic volcanic rift
zone model of Pálmason (1973). However, Pálmason
(1973) considers that the discrepancy was not greater
than expected in view of the assumptions made for the
model calculations. Rapid erosion in Quaternary
times has been considered to contribute significantly
to an increase in the present near surface thermal
gradient in Iceland (Bödvarsson 1982) and this may
explain the above mentioned discrepancy (Pálmason
1973). Deep convection of ground water through the
flood basalt pile drifting out of the volcanic zone
would cause cooling to be faster than anticipated by
the kinematic model of Pálmason (1973). Intrusion of
magma from the volcanic zone and into the old crust
would have the opposite effect. It is tempting to think
that such intrusive activity has occurred because vol-
canic eruptions through old crust have occurred in
quite a few places outside the volcanic zone as already
mentioned.
High-temperature geothermal activity is widespread
in the zones of active volcanism and rifting in Iceland.
The longest stretches devoid of such activity include
the TL-zone and the area between Torfajökull and
Köldukvíslarbotnar in the eastern volcanic zone. Only
two very small high-temperature fields, Hveravellir
and Prestahnúkur, occur within the TL-zone, and one
field, the Geysir field, at its eastern margin (fig, 1).
The manifestations at Prestahnúkur consist of a few
weak fumaroles. At Hveravellir and Geysir natural
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