Jökull - 01.12.1967, Qupperneq 12

Accorcling to rough measúrements tlie total
volume of the glacial burst water was only
about 3.5 G1 ± 25%.
DISCUSSION
Due to the great danger to human lives and
property caused by the larger jökulhlaups from
Mýrdalsjökull it would be feasible to be able
to 1) predict jökulhlaups, 2) give a warning at.
the moment when the hlaup sets off, and 3)
see immediately of what kind the jökulhlaup is.
As the unusual jökulhlaup of June 25 broke
out just when thickness measurements were
being carried out on Mýrdalsjökull by seismic
sounding methods one may ask, if the dyna-
mite explosions hacl set off this jökulhlaup.
This is not considered likely and it should be
pointed out that the hlaup broke out from
Höfdabrekkujökull at 20.00 hours and between
14.30 and 19.00 hours 8 earthquakes (M =
2—4) that originated in the Katla area were
recorded in Reykjavík (Tryggvason 1960). It
cannot be said at tliis stage in which way these
earthquakes are related to the jökulhlaup but
it seems likely that they were caused by sub-
glacial volcanic activity. The expedition stav-
ing on Mýrdalsjökull at the time of the jökul-
hlaup felt one earthquake ancl frequently the
seismic sounding work had to be postponed
because of disturbances on the geophones.
One might also ask if the jökulhlaup in
1955 had drained all the subglacially accumu-
latecl water in the Katla depression so large
lilaups would not take place in the near fu-
ture. This seems unlikely, liowever, as the
cauldrons have already disappeared.
The jökulhlaup in 1955 showed clearly that
water can accumulate under the ice cover.
Most likely only part of this accumulated water
was drainecl by the lilaup. Such subglacial ac-
cumulation of water throws some light on, or
even explains, tlie large jökulhlaups which may
have reached a flow rate of one hundred to
several hundred thousands m3/sec as deduced
from photographs and other reliable sources.
In order to follow such subglacial accumula-
tion of water it would be important to mea-
sure changes in the altitude of the surface of
the ice cover in the Katla depression at least
every 5 years. It w'oulcl also be of great value,
if a hole could be drilled through the ice
cover into the water basin.
REFERENCES
Sveinsson, G. 1919: Kötlugosið 1918 og alleið-
ingar þess. (The eruption of Katla in 1918
and its consequences) (Icelandic). Gefið út
að tilhlutan Stjórnarráðs Islands. Reykja-
vík 1919, 59 bls.
Thorarinsson, S. and Rist, S. 1955: An investiga-
tion of Katla ancl the glacial burst from
Katla in the summer 1955. A preliminary
report (Icelandic). Jökull 5, 43—46.
Thorarinsson, S. 1957: The jökulhlaup from
the Katla area in 1955 compared with other
jökulhlaups in Iceland. Jökull 7, 21—25.
Tryggvason, E. 1960: Earthquakes, jökulhlaups
and subglacial eruptions. Jökull 10, 18—22.
JÖKULHLAUP ÚR MÝRDALSJÖKLI
25. júni 1955 og 20. janúar 1956
Sigurjón Rist,
Vatnamælingadeild Orkustofnunar.
ÁGRIP
1) Hinn 25. júní 1955 brauzt út jökulhlaup
undan Mýrdalsjökli og féll í árnar Múlakvísl
og Skálm. Samtímis mynduðust tvö ketilsig i
jökulinn, en ekkert eldgos kom upp. Rúmmál
ketilsiganna var samtals 28 Gl ± 15% (1 Gl =
10e ms). Magn hlaupvatnsins var árnóta.
2) Hlaup kom á ný í Múlakvisl 20. janúar
1956. Það var af öðrum toga spunnið, komið
úr jökullónum i randfjöllum jökulsins. Magn
þessa hlaups var um 5.5 Gl.
JÖKULHLAUPIÐ 1955
I greininni hér á unclan um þykktarmælingu
á Mýrdalsjökli í júní 1955 er drepið á það,
að hlaup kom einmitt úr jöklinum, meðan
starfað var að mælingum. Einnig var gerð
nokkur grein fyrir Jressu lilaupi af Sigurði Þór-
arinssyni og Sigurjóni Rist (1955) og Sigurði
Þórarinssyni (1957).
Af frásögn sjónarvotta má ráða, að hlaup-
vatn kom kl. 20.00 (GMT) hinn 25. júní 1955
í Rjúpnagili undan vesturhorni Höfðabrekku-
246 JÖKULL 17. ÁR