Jökull - 01.12.1979, Síða 71

Fig. 11. Map of the northern part of the Surtsey volcanic island off the south coast of Iceland. The
area of primary tepra is shown stippled. The extent of the hydrothermal area (horizontal lines) and
surface exposures of the palagonitized area (crosshatching) as in Sept. 1976. From Jakobsson 1978.
studies show that the xenoliths are most probably
pieces of shallow acid instrusions.
However, low-temperature alteration and
leaching of glassy rocks is extensive, and the
incipient marks of burial metamorphsim are seen
regionally in the Tertiary lava piles of eastern,
northern and western Iceland.
Hyaloclastites (Icel. móberg) i.e. basaltic or basal-
tic — andesitic rocks, which have been quenched
and granulated by contact of the magma with
water are widespread in Iceland. They form due to
subglacial or subaquatic eruptions and constitute
for example the main part of the Upper Pleistocene
“Móberg Formation”. Recent examples of this
formation are also known, e.g. the products of the
phreatic phases of the Surtsey eruptions 1963—
1967. As a result of quenching, the hyaloclastite is
mainly made up of translucent brownish glass,
called sideromelane.
Palagonitization
The sideromelane is unstable and leaching and
alteration starts relatively quickly depending
mainly on the access of water and ambient
temperatures. The overall process of alteration of
the sideromelane is called palagonitization, and the
term palagonite is used for the vitreous,
TABLE 3. Typical electron microprobe analyses
of a sideromelane grain and its palagonite rim, in
weight per cent. Fe203* is total Fe as Fe203, and
H20 is assumed to add up to nearly 100% wt. The
Saefell tephra, Vestmannaeyjar.
wt.% Sidero- Pala-
melane gonite
Si02 45.1 36.0
tío2 2.5 3.9
ai2o3 16.0 10.4
Fc203* 12.2 18.1
MgO 5.0 4.6
CaO 11.9 9.9
Na^O 2.8 0.2
KjO 0.5 0.7
Sum 96.0 83.8
JÖKULL 29.ÁR 69