Jökull - 01.12.1965, Page 5

Fig. 2. Open water in
the Grímsvötn caldera at
the foot of Vatnshamar.
— Opið vatn við rœtur
Vatnshamars.
Photo S. Thorarinsson,
July 1953.
published in Jökull (Rist 1961). The two maps
are in good agreement, but on Þorbergsson’s
majr the absolute heiglit figures of the móberg
hillocks Depill and Stóri Mósi on the west side
of the caldera (cf. the map Fig. 7) differ 1.7
and 1.0 m respectively from the figures on
Jóhannessons map. Although the accuracy of
both maps is probably about the same I have,
in order to avoid confusion, used Þorbergsson’s
height figures on Stóri Mósi and Depill when
calculating the height figures in my tables and
diagrams as Þorbergsson’s map has already been
in use for sonre years.
DETERMINATIONS OF THE HEIGHT
OF THE WATER LEVEL
AT DEPILL — STÓRI MÓSI.
The expeditions visiting Grímsvötn have
nearly always found open water along the
western wall of the Grímsvötn caldera at the
foot of Vatnshamar (Fig. 2), Depill and Stóri
Mósi. Since 1955 we have nearly every spring,
besides four times in September, measured the
absolute height of this water level bv measur-
ing the height difference between it and the
cairns erected on the summits of Depill, Stóri
Mósi and the still lower Litli Mósi (1416 m).
These measurements are listed in Table I and
the height changes of the water level shown
on the diagram Fig. 3. The figures for 1957
and 1965 are based on nreasurements with a
Kollsman sensitive altimeter (height difference
between Depill and the water level), and the
error should be within ± 1 m in 1957 when
the measurements were repeated nrany times,
ancl within ± 2 m in 1966. The rest of tlie
figures is based on levelling exact enough to
keep the error within ± 0.3 m. Tlie absolute
height figures are based on the lieight of Stóri
Mósi beirig 1429 m according to Þorbergsson’s
map (Rist 1961).
Three jökullilaups have occurred in Gríms-
vötn since the annual expeditions started in
1953. The first one was in July 1954, culmina-
ting July 18th (Thorarinsson 1954, Rist 1955),
the second in January 1960, culminating Janu-
ary 24 ancl the most recent one in August—Sep-
tenrber 1965, culminating Sept. 8. Frequent re-
connoitring flights over the Grímsvötn area
during the jökulhlaup in 1954 enabled us to
observe that the sinking of the firn-ice floor
of the Grímsvötn caldera had just begun on
July 14, four days before the culmination of
the jökulhlaup, ancl that this subsidence was
practically finished in July 1954. Thus the
main subsiding took place within a week.
Consequently the subsiding during a jökul-
lilaup is shown with vertical lines on the dia-
gram Fig. 3. The whole-drawn vertical lines
show the amount of sinking as measured dur-
ing the first visit to Grímsvötn after each hlaup,
but obviously the figures so obtained are mini-
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