Jökull - 01.12.1969, Síða 129

Fig. 6.
A great ice reef.
Extreme height
2 m.
through the cracks and transforms the dry snow
slush. Slush snow as thick as 40 centimeters has
been observed on the ice cover. In this way the
ice cover gains 40 centimeters thickness of com-
pact snow ice.
When there is little or no snow cover on
the ice the temperature fluctuations affect the
ice easily. When the ice expands shoreward it
forms somewhere, socalled "bárugardar”, that
is a waveformation from loose ice fragments
(Sigfinnsson 1957).
Against steep spits and islands where the ice
cannot move shoreward, cracks leap right across
the lake. When the ice is under great pressure
major ice reefs and ice keels are formed (Figs.
4 to 6). The ice keels result in a rapid melt-
ing of the ice floes, leaving a rather broad
opening, which usually gets again an ice cover
of blue-ice. As soon as the formation of ice
wrinkles begins, the shoreward pressure ceases.
When the ice cover is shrinking narrow fissures
form across the lake against the same steep
spits. During relatively warm winters water
openings in Mývatn are wide; water clearings
are named “eyda” by local people.
At a glance the statement warm winter —
cold Mývatn, cold winter — warm Mývatn
sounds contradictory, but such is the result of
systematic temperature measurements at the
same localities extending over period of many
years by Sigfinnsson. This justifies a closer ob-
servation. In warm winters there are large
pools of open water and melt water of 0° C
is intermingled with the lake water. In cold
winters on the other hand, Mývatn is protected
and isolated from the cooling effect of the
atmosphere by a cover of ice and snow. Mea-
sured bottorn temperature under these condi-
tions is 2.4° C against 1.4° C in warm winters.
As Fig. 2 illustrates the outlet of the lake
is on its west side. This figure shows a stream
lead at socalled Alar. Shoals on lava ridges
protruding froni the exceedingly smooth plain
of diatomaceous earth direct the slow warm
bottom current to the surface. The figure also
demonstrates the variable size of the stream
lead, during prolonged frost periods it is just
like a borehole.
Closer to the outlet of the lake a still larger
wateropening ensues, a stream lead called Ála-
brot, also variable in size. Should there be any
questioning as to the causes of ice jams at the
Mývatn outlet? The Álabrot stream lead
contributes the cooling surface eddies at the
JÖKULL 19. ÁR 1 25