Jökull - 01.12.1976, Side 55

HITI í BORHOLU I VESTMANNAEYJUM
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stops. The temperature drops further to 9° C
in December 1973 and is 8.6 in late March
1974 and 8.7 in December 1974. A chart re-
corder was connected to an uncalibrated therm-
istor in the discharge water during most of
February and March 1973; no sudden (<2 hrs)
changes of temperature exceeding 1° C were
seen.
Discharge measurements (Fig. 2) were made
by taking the time for a 22-liter bucket to fill
from a 1" hose connected to the valve, at zero
gauge pressure. Water levels after July 2 were
measured with a plumb line. Fig. 2 shows a
very similar pattern to that of Fig. 1, i. e.
maxima near the beginning of the eruption
and in late May, a minimum in March and a
continuously falling water level after July 2.
In December 1973 the water level was at 12.6 m
depth, in late March 1974 13.2 m and in Dec-
ember 1974 14.3 m. Some of the day-to-day
fluctuations seen in this data are due to errors
of measurements, and some to changes of mete-
orological origin. Tidal effects appear to be
less than 10 cm in amplitude at spring tide,
when the tidal amplitude in the harbour is
about 65 cm.
The hydrostatic pressure at the top of the
hole was measured a number of times with a
spiral manometer. After closing of the valve,
tliis pressure took 1—2 hours to equalize. The
measurements are not very accurate, but indi-
cate that 0.1 atmosphere of pressure head by
closed valve corresponds roughly to 0.1 liters/
sec discharge at zero pressure (inset in Fig. 2).
This has been employed in Figs. 2 and 4 by
making a water level change of 1 m equivalent
in scale to 0.1 1/sec discharge.
Upon close inspection of Fig. 1 and 2 we see
that there is a time lag of some 5—10 days be-
tween temperature and pressure. This is shown
well by the loop of Fig. 4, which also accen-
tuates the relatively rapid pressure drop near
the end of the eruption.
Temperature profiles were measured in the
hole a few times with a recording thermometer
(Amerada-type) with absolute accuracy of about
1° C. Its sensitivity is sufficient to detect a
change of a small part of a degree. Results are
shown in Fig. 3. The temperature increases in
the first days of the eruption but after the end
of volcanic activity it decreases to a value simi-
lar to that measured in 1967. On extrapolating
JÖKULL 26. ÁR 53