Jökull - 01.12.1976, Page 50

Fig. 6.
A surface lake in an ice
cauldron which formed
after the jökulhlaup from
the Katla area, Mýrdals-
jökull, 25 June 1955. The
cauldron is about 1 km
across. — Photo Sigurdur
Thorarinsson, 27 June
1955.
Myncl 6.
Lón i sigkatli, sem mynd-
aðist við jökulhlaup frá
Kötlusvœðinu 25. júni
1955. Þvermál sigketils er
um 1 km. Ljósm. Sigurð-
ur Þórarinsson 21. júní
1955.
which was formed during a jökulhlaup from
the Katla area in Mýrdalsjökull in 1955. The
depression is about 100 m deep and the glacier
is 300—400 m thick in the area. A similar water
pond is formed during the jökulhlaups in the
river Skaftá. Fig. 7 shows a cross-section through
the glacier and the subglacial water cupola
(Björnsson 1975). The surface lake is formed
when the glacier surface drops down and a free
water table intersects the glacier surface. A
water table is situated at a certain depth be-
neath the glacier surface (observed at 35 m
depth in drill holes in a neighbouring area). An
ice cauldron is formed during the jökulhlaups
and the water table drops down corresponding-
ly, Fig. 7b. A permeable glacier responds by
increased waterflow towards the cauldron. The
result is a drawdown of the water table in the
surrounding aquifer whereas the water table
rises beneath the ice cauldron, Fig. 7c. If the
water table rises above the glacier surface, a
lake is formed in the ice cauldron. At Mýrdals-
jökull in 1955 a lake was observed two days
after the cauldron was formed, Fig. 6. The
depth of the ice cauldron is reduced gradually.
Ice flows into the cauldron and a water cupola
may rise again and the situation is back to
Fig. 7a. However, if a surface lake remains a
Mynd 1.
Þversnið, sem sýnir mynd-
un lóns í sigkatli.
a) Lón undir jökli
b) Sigketill, sem myndast
við jökulhlaup
c) Lón i sigkatli
Fig. 7. Illustration of the formation of a surface lake in an ice cauldron at a geothermal area.
a) A subglacial water cupola beneath a surface depression
b) An ice cauldron, formed when water drains out of the water cupola
c) A surface lake in the ice cauldron.
48 JÖKULL 26. ÁR