Jökull - 01.12.1982, Blaðsíða 29

hydrological efFects of the Pleistocene glaciation and
the rapid deglaciation about 104 years ago appear
significant enough to have been able to provide for
the initiation of many of the LT hydrothermal sys-
tems that are now active.
At this end, it is of interest to reflect on whether
any field techniques or experiments are available to
obtain further data or information on the LT hydro-
thermal processes discussed above. The present
writer wishes to point out that elastomechanical
techniques appear to offer some possibilities in this
respect.
Any transient type of heat removal from the sub-
surface is associated with a thermoelastic contrac-
tion that is easily estimated at about 10"11 m3/J
removed (Bodvarsson 1976). Hence the total rate of
contraction associated with the Reykholtsdalur
system is of the order of 2 x 108 X 1R" = 2 x
10-3m3/s or about 6 X 104 m3/year. This is a very
considerable reduction in volume ofthe source rock
that may lead toobservablesurface strain or tilts, in
particular, in the case of concentrated sources such
as the CDM model. If the volume reduction is
restricted to an area ofsay 60 km2 only, the associat-
ed central surface subsidence could amount to one
millimeter/year and the rate of tilting could be of
the order of 100 nanoradians/year. Elastomechani-
cal methods may thus offer some possibilities of
monitoring CDM processes.
In this context, it is of interest to note that Einars-
son, Klein and Bj'ómsson (1977) have concluded that
the high degree of seismicity in the Borgarfjordur
area may be caused by fracturing by tensile stress.
acknowledgement
This work was supported by the U.S. National Science
Foundation under Grant EAR 8023850.
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