Jökull - 01.12.1972, Blaðsíða 41

area occupied by the Flveradalur and ice-
cauldron thermal area exceeds two or possibly
3 km2, as much as some previous estimates of
the entire Kverkfjöll thermal area.
The magnitude of the volcanic event which
formed the ice cauldron can be estimated from
tlte volume of the cauldron proper. By photo-
grammetrically computing the parallax factor
for stereoscopic pairs of the 28 August 1968
Plus X aerial photographs, using a parallax
bar, and assuming an elevation of 1620 meters
for the surface of the ice-dammed Jökullón,
the elevation of the floor of the ice cauldron
is approximately 1635 meters. The volume of
the cauldron proper is thus estimated to be
0.02 km3. The volume of the total subsidence
is at least double this quantity. Some gas ex-
plosions undoubtedly took place near the sur-
face of the cauldron floor in 1959, spreading
sand and mud over the firn surface in the
vicinity of the cauldron. Yet it is nearly certain
that the cauldron that formed in 1959 resulted
almost exclusively from ice melt. The energy
required to melt .04 km3 of ice would be
1.2 X 1023 ergs or 1.2 X 1016 joules.
Kverkfjöll eystri
Points of thermal emission along a several
kilometer-long segment of the precipitous
Skarphédinsjökull escarpment of Kverkfjiill
eystri appear on the 1966 direct-record imag-
ery but are near the margin (Fig. 4) of the
70 mm film strip and are not sharply deline-
ated. Thorarinsson (1953, p. 20) in 1953 found
warm exhalations and ice perforations here at
an altitude of 1920 meters, the highest altitude
thermal activity in Iceland; thermal activity
was not observed during expeditions in 1933
and 1946, but it was noticed again in 1956
('Thorarinsson 1956, p. 42). The 1966 infrared
data indicate continued emission from a geo-
thermal source at this locality. Several possible
smaller geothermal manifestations along the
west-facing bedrock escarpment and to the
northeast below the Skarphédinsjökull escarp-
ment of Kverkíjöll eystri (Fig. 5) also appear-
ed on botli 1966 and 1968 images.
Kverkjökull outlet tongue
Between Kverkfjöll vestri and Kverkfjöll
eystri, a graben-like couloir, which is mainly
an erosional feature, channels and constricts
the movement of Kverkjökull outlet tongue
(Figs. 2 and 5) as it curves slightly to the north-
west in its descent from the Kverkfjöll massif.
The bedrock escarpments defining the couloir,
swept clear of talus accumulations of slide-
rock by the aclvancing glacier, appear as warm
features on the imagery (Fig. 4), taken 22
August 1966 (2357 IMT). Comparison of
diurnal cooling curves of basalt bedrock and
glacier ice would undoubtedly indicate higher
surface temperatures for the basalt at that
time of the day in August at latitude 64° N;
hence the light tones representing the bedrock
surfaces on the imagery do not necessarily
indicate geothermal sources. Ice fractures
presumably convecting warmer air to the sur-
face are abundant on the neck of Kverkjökull,
particularly near the eastern bedrock wall
(Figs. 4 and 5). But because even shallow
crevasses (less than 100 m) usually appear warm
on predawn thermal imagery, these indications
of convection of warm air do not necessarily
imply geothermal sources.
The generally lighter tones of Kverkjökull
outlet tongue compared to ice surfaces of ad-
jacent upland glacier surfaces, are, perhaps,
accounted for by the generally northward slope
of Kverkjökull; accordingly, the white surface
may reflect a small amount of twilight solar
radiation at wavelengths shorther than 3p.
A dark, channel-like pattern (Fig. 4) possibly
indicating a subsided trough or low area in
the glacier surface may mark the position of
a subglacial tunnel system upstream from a
glacier portal. The low temperatures indicated
by the image dark tones could result from
downslope movement of cold air along the
surface of the trough, a phenomenon noted
elsewhere on sloping fluted glacier surfaces
(Mark Meier, oral communication, January
1971). A linear depression in the glacier sur-
face of Kverkfjöll upstream from the glacier
portal is also noticeable on aerial photographs
(above point c, Fig. 3) and particularly on
stereoscopic pairs, but can not be traced as far
upstream on the aerial photographs as on the
infrared image (Fig. 4).
Perhaps the most unusual and significant
feature recorded in both 1966 and 1968 is the
warm melt-water stream (Figs. 4, 5, and 6) em-
JÖKULL 22. ÁR 39