Jökull - 01.12.1999, Side 91

Gravity model studies of the volcanic island Surtsey, Iceland
Þröstur Þorsteinsson1’ and Magnús T. Guðmundsson2
‘Department of Physics, University of Iceland, Hjarðarhaga, 107 Reykjavík, Iceland
2Science Institute, University of Iceland, Hofsvallagötu 53, 107 Reykjavík, Iceland
Abstract - Gravity data collected by Cameron et al. (1992) have been used to model the internal
structure ofSurtsey, a volcanic island formed in an eruption ojfthe south coast oflceland 1963-
1967. Several 2 1/2-dimensional forward gravity models were constructed, using existing data on
the volcanic history and stratigraphy as constraints. The models show that the observed gravity
anomalies can be explained by density variations within the volcanic edifice. The units ofthe best
fitting models are: i) a core ofhyaloclastite tuffs formed during the explosive phase ofthe eruption
in 1963-1964; ii) a submarine delta ofpillow lava breccias formed during the effusive eruptions
after April 4, 1964; iii) subaerial lava; iv) subaerial tephra; and v) relatively dense sediments
making up the northern peninsula. The volume of intrusions within the volcanic pile making up
the island, is apparently too small to register in the gravity field. The models suggest that no
appreciable volume ofpillow lava exists under Surtsey.
INTRODUCTION
The island of Surtsey was formed in an eruption off the
south coast of Iceland 1963-1967 (Fig. 1). The island is
the southernmost part of the Vestmannaeyjar island
chain, a central volcano complex that forms the south-
em end of the southward propagating eastem volcanic
rift zone in Iceland. Before the emption, the sea bed at
the eruption site was at 130 m depth (Jakobsson and
Moore, 1982). When the eruption ended, an island had
formed, capped with subaerial lava but primarily made
of tephra.
The formation of Surtsey offered an unique oppor-
tunity to study shallow water volcanism: The emer-
gence of a volcanic island, the progression of subaeri-
al volcanism from explosive to effusive activity lead-
ing to the formation of a subaerial lava shield. Surtsey
can be classified as a marine table mountain (Kjart-
ansson, 1966), similar to the sub-glacially erupted
table mountains that formed during the Pleistocene in
Iceland and elsewhere (Kjartansson, 1943; Mathews,
1947; Bemmelen and Rutten, 1955). Table mountains
are usually composed of a basal unit of pillow lavas,
overlain by hyaloclastites and capped with subaerially
empted lavas. Surtsey is the only marine table moun-
tain that has been observed to form in a single emption.
An unresolved question in submarine and sub-glacial
volcanism is at what depth or under what conditions
pillow lava forms, i.e. to what extent are table moun-
tains formed of pillow lava and to what extent are they
composed of pyroclastic glass. Pillows are generally
formed at higher confining pressures than pyroclastic
glass which is formed by fragmentation of magma at
low to moderate pressure (e.g. Jones, 1969). Moreover,
where degassed magma flows into water, pillow lava
forms (Moore, 1975).
Several models have been put forward for the
stracture of Surtsey, some assuming a core of pillow
lava formed during the submarine phase of the emp-
tion (Kjartansson, 1966; Einarsson, 1968) while a
more recent model based on drilling on the island in
1979 suggests the absence of such a core (Jakobsson
and Moore, 1980, 1982). A different model was pro-
posed by Cameron et al. (1992) on the basis of gravi-
ty modeling. A drawback in the modeling of Cameron
et al. (1992) was that density differences between
units in the volcanic construct that lie above sea level
*now at University of Washington, Seattle, Geophysics Program, Seattle WA 98195 USA
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