Jökull - 01.12.1999, Page 92

were not considered, making their results unreliable.
Here we present new gravity models of Surtsey, using
the data of Cameron et al. (1992).
THE GEOLOGY OF SURTSEY
The Surtsey eruption was detected on 14 October 1963,
when an eruption column was observed rising from the
sea. The following day a crater rim became visible
(Thorarinsson etai, 1964; Einarsson, 1965). A more or
less continuous explosive eruption lasted for 5 months,
building up an island made of pyroclastic glass and
tephra. The island rose to almost 200 m above sea level
as horseshoe-shaped tuff cones were formed around the
two craters. On 4 April 1964 the island was large
enough to seal off the active crater from the sea. The
style of eruption changed from explosive to effusive
with subaerial lava forming on the southem part of the
Fig. 1. The location of Surtsey. The inset shows the region of
the map relative to the main island of Iceland. The region
shown on Figure 2 is indicated with a box. - Staðsetn-
ingarkort. Lega 2. myndar er sýnd með rammanum utan um
Surtsey.
island. The island was progressively enlarged towards
the south where a flow-foot breccia of pillow lava (pil-
low breccia) was built up below sea level, forming a
base on which the subaerial lava advanced. When the
eruption ended in June 1967, the island had an area of
2.8 km2 and the total volume of erupted material was
estimated 1.1-1.2 km3, of which 60-70% was tephra
(Jakobsson and Moore, 1982).
Activity during the emption was not confined to the
island of Surtsey (Fig. 2). A submarine ridge was
formed 2 km ENE of the island between 28 December
1963 and 6 January 1964. Two smaller islands were
formed in 1965 (Syrtlingur) and 1966 (Jólnir). Both is-
lands disappeared within weeks after activity ceased.
No subaerial lava had been formed, making the uncon-
solidated piles of hylaoclastites easy prey for marine
erosion (Þórarinsson, 1968).
Since the end of the emption, erosion has reduced
the size of Surtsey significantly (Jakobsson and Moore,
1982). Deposition has also created a peninsula on the
north side, mainly made up of fragments of the lava
forming the southem part of the island (S. Jakobsson,
pers. com. 1996).
Shortly after the volcanic activity ended, alteration
of the tephra into palagonite was detected (Jakobsson,
1978). The commencement of alteration has been linked
to the formation of a geothermal area with shallow intru-
sions providing the heat (Axelsson et al, 1982).
Little is known about the nature of the bedrock in
the area. In a drill hole on Heimaey, 20 km NE of Surts-
ey (Fig. 1), about 180 m of basaltic tuffs and lavas were
underlain by 650 m of sedimentary rocks (Tómasson,
1967). Similar conditions may exist under Surtsey.
During the Surtsey eruption, Kjartansson (1966)
proposed a model for the internal stmcture of the is-
land. It was based on observations during the emp-
tion, but to large extent, on analogy with table moun-
tains on mainland Iceland (Kjartansson, 1943). The
model proposed that pillow lava makes up the bulk of
the volume beneath sea level, with the tephra as a rel-
atively thin coating. Einarsson (1968) proposed a
slightly different model having a higher proportion of
tephra, with pillow lava mainly confined to the areas
under the craters. On the basis of results from the core
extracted in 1979, Jakobsson and Moore (1982) put
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JÖKULL, No. 47, 1999