Jökull - 01.12.1982, Page 24

THE E FFECTS AND TIME SCALE OFTHE
PLEISTOCENE GLACIATION IN ICELAND
The long periods of heavy glaciation of Iceland
during the past 2 to 3 million years must have
interfered greatly with ground water hydrology on
the island. Of particular importance in the present
context is the last period ofglaciation that extended
over about 10 5 years (Dansgaard et al. 1969) and
ended only 104 years ago. Ground water recharge
must have been quite limited during this period and
hydrothermal circulation may have been severely
curtailed.
Another important hydrological effect may have
resulted from the relatively rapid rate of reduction
of the surface l<fad at the end of the glaciation peri-
od. The deglaciation generated a differential up-
warping of the island such that the central parts
rose about 100 m relative to the coastal regions.
This process can have taken place during a few 103
years only and is likely to have resulted in major
scale fracturing of crustal formations. Subsurface
permeability to water may have been enhanced
quite substantially.
Considering this sequence of events, it is by no
means improbable that the rapid deglaciation was
an important milestone in the development of geo-
thermal systems in Iceland. Many, and possibly
most, of the presently active LT systems in Iceland
may thus have been activated only 104 years ago
(Bodvarsson, 1980).
Compared to the above processes, the relaxation
times for thermal conduction anomalies are quite
long. For example, to reach a quasiequilibrium on
the spatial scale of one kilometer, times of a few 104
years are r equired, and the figure for 10 kilometers
is a few 106 years. In view of the rapid dynamics of
the surface processes, the length of the thermal
relaxation times indicate quite clearly that local
steady state crustal conduction temperature fields
are quite improbable in the Icelandic environment.
Moreover, it is of interest to note that heavy
glacial erosion has carved out long deep valleys and
fjords. The rate of erosion is uncertain, but the time
scale involved has no doubt been of the order of 105
years rather than 10b years. An important combin-
ed effect of the rapid erosion and the long thermal
relaxtion times is the resulting anomalous subsur-
face temperature field in such regions as the Eyja-
fjordur in the North. The magnitude of the purely
conductive temperature anomaly can be estimated
(Bodvarsson 1950) but there are uncertainties result-
ing from advective/convective effects due to the flow
of subsurface waters.
Finally, it is of some interest to mention that the
deglaciadon 104 years ago has affected the subsur-
face crustal stress field, in particular, below the
deep glacial valleys. Since ffacture spaces ofonly 1
millimeter aperture are important in the geo-
thermal scenario, it is not inconceivable that the
stress reduction has affected the subsurface thermal
hydrology.
THE ENERGY SUPPLY OF THE LOVV
T EM P ERATURE ACTIVITY
The total integrated mass output of the LT
springs in Iceland is now estimated at 1800 kg/s
(iSaemundsson and Fridleifsson 1980). Assuming an
average temperature of 80°C the total rate of energy
dissipation by the LT springs is about 0.6 GVV (one
GVV = 109VV). It is evident that this figuredoes not
include dissipation due to subsurface flow losses
and locally elevated conduction surface heat flow in
the thermal areas. VVe can only surmise that the
actual total dissipation figure is well above 0.6 GVV
and probably no less than 1 to 2 GVV. Moreover,
since the LT activity is restricted to parts of the
island, only a fraction of the total estimated con-
duction heat current of 15 GVV (Bodvarsson 1982a)
would be available for heat supply. In fact, Bjornsson
(1980) assumes that the total conduction current
available within the recharge region of the LT activ-
ity is only 5 GVV. Since the rate of actual heat
recovery depends very critically on the subsurface
pattern of flow, only some fraction of the available
current can actually be recovered. Finally, on the
steady state model, one would have to expect that
the water issued by the springs is only a leak from a
larger flow of water from the highlands out to the
ocean. Hence, it is likejy that more heat is being
taken up than indicated by the output of the
springs.
At this juncture, we havenomeans ofquantifying
the conjectured energy currents, but it would
appear that the steady state model for the entire LT
activity is energetically somewhat marginal and
hence a less likely proposition.
ROCK/VVATER HEAT TRANSFER IN THE
ICELANDIC ENVIRONMENT
Iceland is built up almost entirely of a series of
flood basalts with interbedded layers of sediments,
22 JÖKULL 32. ÁR