Náttúrufræðingurinn - 1966, Blaðsíða 40

34
NÁTTÚRUFRÆÐINGURINN
More recently similar views have been advanced on the origin of mountains
of the stapi type, first by W. H. Mathews (1947) on the Tuyas in British
Columbia, and then by R. V. van Bemmelen and M. G. Rutten (1955) on the
tablemountains of Northern Iceland, whereupon this theory of „intraglacial
accumulation" was accepted by most students of the móberg formation (in-
cluding the present writer) as the most probable interpretation of the origin
of stapis. However, the validity of the tlieory had not been proved fully in the
case of any Icelandic mountain. It was opposed by Trausti Einarsson (1958
and 1962), and facts menlioned in the present article are not indicative of such
an origin for all mountains of the stapi shape. Further, the theory lacked
actualistic support, as repeated subglacial eruptions in Iceland in historic time
do not seem to liave created any stapis.
It was not until the present writer’s investigation of the volcano Leggjabrjótur
in Central Iceland that evidence was brought forth of at least this mountain
liaving been formed in accordance with his hypothesis of 1943 (Kjartansson
1964 and 1966).
Finally, the eruption of Surtur, beginning in November, 1963, and still
slightly active in January, 1966, lias now piled up a kind of stapi before our
eyes. But this new stapi, Surtsey, was not like its l’leistocene counterparts formed
in an ice-sheet, but in the sea. Consequently it is somewhat anomalous, especial-
ly with regard to its sliape. The lack of surrounding ice-walls may be responsible
for less steepness of its submarine slopes and the lieavy erosion of oceanic
waves has already severely deformed its flanks about the sea level.
In the formation of Surtsey as well as in tliat of the older stapis, mutatis
mutandis, four different stages and four corresponding rock facies are dis-
linguishable (Fig. 7): — (1) Basaltic magma extruding on the bottom of deep
water (or thick ice) forms a subaqueous pile of pillow-lava. High pressure pro-
hibits explosive activity. — (2) As the top of the pile reaches a level of sufficiently
decreased water pressure explosions set in, probably at a depth not exceeding
20—30 metres. Explosive activity, conditioned by easy access of water into the
vent, continues with production of pyroclastic material. Crater walls of this
material rise above the water level. — (3) Wlien these walls become continuous
around the crater and sufficiently watertight the activity turns effusive. Lava
of the shieldvolcano type covers the emerging top of the pile. — (4) Lava
streams flowing into the water (or onto the ice) are solidified rapidly by its
chilling effect and pile up to form high and steep subaqueous fronts consisting
of a mixture of brecciated and pillowy lava.