Jökull - 01.12.1960, Blaðsíða 23

duction of the eruption on June 25th 1955 has
been some three million m3.
An eruption of similar intensity as the initial
phase of the Hekla eruption 1947 could, ac-
cording to these assumptions, produce a jökul-
hlaup more than ten times larger than the
hlaup of 1955, or of similar intensitv as a
normal Kötluhlaup (jökulhlaup from Mýrdals-
jökull associated with eruption of the volcano
Katla).
It may be difficult to give a physical ex-
planation of such a rapid melting of the
glacier as is herc suggested (some 104 m3/sec
of ice), if the volcanic material is brought to
the subglacial surface through one vent, but
this problem will not be discussed in details
here.
The behaviour of subglacial volcanic erup-
tion is not known in details. These eruptions
seem usually to be highly explosive and pro-
duce only tephra (volcanic ash). Such eruptions
must be very active in melting the ice, as long
as they have not broken through the glacier.
The tephra will be suspended in the meltwater
and freely give its heat energy to the water.
The melting speed of ice in direct contact
with water depends on several factors, such as
the temperature and turbulence of the water,
the porosity of the ice, the pressure etc. The
importance of each of these factors is not.
known to the writer, but the temperature and
turbulence of the water are probably the most
important. If the contact melting is not suffi-
ciently fast, the tempeature of the water will
rise untill the vapour pressure exceeds the static
pressure of the glacier. This high pressure will
Iift the glacier ice from the earth and allow
the overheated water to flow under it, to in-
crease the melting area sufficiently to keep the
water below its boiling point. Fig. 1 illustrates
the writer’s opinion of the possible evolution
of such an explosive subglacial eruption.
If a subglacial eruption is not explosive, the
process will be different. The lava will be
cooled very fast at the surface by direct contact
with the ice or. meltwater. Its flow will be
very slow, but it will grow in thickness to form
a dome shaped mountain. The surface layer of
this mountain will be characterized by the con-
tact with the ice (móberg), but the interior
will be compact and coarse grained, as it cools
very slowly. An evidence of this structure is
Bedrock
Tuffbreccia and pillowlava
Subaerial lava
Magma-gabbro,
Glacier ice
Fig. 2. Three stages in the evolution of sub-
glacial volcano of non-explosive eruption. The
third stage shows the volcano after having
reached through the glacier. If the glacier melts
away, the mountain will get the form indica-
ted by dashed line (table mountain). — Þrjú
stig flœðigoss undir jökli. Punktalinan á mynd
III sýnir útlínur fjallsins eftir að jökullinn er
bráðnaður.
21