Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 62

58
This island is genetically a stapi (tablemountain) admit-
tedly somewhat anomalous, especially in regard to its shape
as it was formed in the sea, not in an ice-sheet. Most of the
peripheral vents remained submerged throughout their period
of activity. The existence and position of three of them is
indicated only by submarine hills (seamounts) with depths
88, 67 and 74 metres respectively, revealed by the echo sound-
ing of the Icelandic Hydrographic Survey in the summer of
1964 (Fig. 3). From the top of the fourth submarine mound,
called Surtla, depth 25 metres, small explosion clouds rose
into the air in the last days of December 1963. Finally two
of the peripheral vents, Syrtlingur, active during the summer
1965, and Jólnir, active in the first half of 1966, managed
to form crater islands. But both were abraded by the ocean
shortly after they had ceased tlieir activity. The respective
shoals or seamounts are not shown by the depth contours in
Fig. 3 as they had not come into existence by the time of the
sounding. But breakers still rise on the crest of Jólnir by low
tide and heavy sea.
The seamounts marked 88, 67 and 74 in Fig. 3 are formed
by entirely submarine activity without visible explosions.
This is also true of Surtla (25 m) except for the slight ex-
plosions reaching above sea level in the last stage of its pil-
ing up.
For three reasons previously pointed out (Kjartansson
1966), the submarine piles formed by the Surtsey eruption
including the oldest part of the submarine pedestal of this
island are most probably made up of pillow-lava of the type
forming the base of most of the Pleistocene móberg mountains:
(1.) The basal position of this type of rock in the móberg
mountains indicates that its formation was conditioned by a
pressure in sufficiently deep water (under floating ice) to
prevent explosion of the extruding magma. The quiet extru-
sion, without visible explosions of the magma that formed
the submarine piles around Surtsey, indicates just the same
conditions during their formation.