Rit (Vísindafélag Íslendinga) - 01.06.1967, Qupperneq 66

62
ed in the sea by explosive activity. Then, in August 1966,
effusive activity was renewed hy a crater opening on the flank
of the shield volcano where it is still going on (May 1967).
In this last (effusive or shield volcano) stage of activity
two different facies are formed: (1) Lava streams that so-
lidify on land become sheets with the structural character-
istics of shield volcanoes (3 in Fig. 4), and (2) lava streams
that advance into water (an intraglacial lake or the sea),
are chilled and solidify rapidly. They heap up to form high
fronts with steep subaqueous slopes.
The echo soundings of 1964 indicated occasional steepness
of about 30° down from the south shore of Surtsey. This
steepness is comparable with that of the sides of the Icelandic
stapis, though in the latter it may exceed 50° (Herdubreid
53°, Hlödufell 50°), but these steepest slopes are probably
to some extent due to restraining walls of ice during their
formation and to subsequent glacial erosion.
As mentioned above, rock structures in the upper parts of
the stapis are often chaotic; coarse breccias containing pil-
lows and angular fragments of lava. Occasional rough strati-
fication shows an outward dip of some 25 °—35° correspond-
ing to the angle of repose of this material when uncemented
and under water. Similar rock structures are to be expected
on the submarine slope off the lava shore of Surtsey (4 in
Fig. 4).
In the two past field seasons J. G. Jones (1966) has studi-
ed what he calls the “Laugarvatn volcanoes” for comparison
with what is known about submarine volcanoes (guyots).
The undefined term “Laugarvatn volcanoes” no doubt refers
to the móberg mountains north of Laugardalur in Árnessýsla
and possibly includes the stapis Hlödufell and Skridan the
study of which in 1941 lead me to put forth a hypothesis of
intraglacial accumulation for the origin of this type of moun-
tains. Jones found in the Laugarvatn volcanoes all the same
four rocks facies in the same stratigraphic sequence as I have
assumed for Surtsey (outlined above from Kjartansson 1966).