Náttúrufræðingurinn - 1984, Blaðsíða 54

SUMMARY
Pele’s hair II.
Mode of formation,
composition and structure
by
Magnús Ólafsson*, Púll Imsland and
Guðrún Larsen,
Nordic Volcanological Institute,
University of Iceland, Reykjavík
*Now at National Energy Authority, Reykjavík
The eruption mechanism leading to Pele’s
hair formation is restricted to lava fountaining
activity. A narrow vent forms a nozzle, which
strongly accelerates the out-rush of magma. The
magma is then drawn out into threads at the
same time as the containing gases are exsolved
and expanded. The magma must be relatively
fluid and gas rich.
The article describes, primarily through
photomicrographs, common internal features of
Pele’s hair from three different volcanic erup-
tions in íceland, the Krafla eruption of Novem-
ber 1981, the Surtsey eruption (Augiist 1966)
and the Laki eruption, 1783, which all produced
basalts. The primary features are pipe-like gas
vesicles and crystals. The vesicles are frequently
observed to contain liquid bubbles, which in
turn may contain small dust-like crystalline part-
icles. The crystals observed are euhedral olivine
phenocrysts and skcletal plagioclase microlites
of quench origin. The glass of the threads has
been analysed with a microprobe (Table I), but
the gas, the liquid of the bubbles and the dust-
like particles are of unknown composition. Pyc-
hnometer-bottle determination of the specific
weight of the finely crushed glass of the Krafla
eruption gives 2.79g/cm3, which corresponds to
that of the degassed magma at the craters.
The study of the Pele’s hair reveals that the
threads generally range in diameter from 0.001
- 0.01 mm. Occasionally, the diameter mea-
sures between 0.1 and 0.2 mm. Around the
olivine phenocrysts the threads are thicker than
elsewhere. Pele’s tears and irregular glass lumps
with spherical gas vesicles appear in small amo-
unts. The original length of the threads is uncer-
tain, but threads up to nearly 10 cm have been
measured.
A dacitic tephra from the prehistoric „upper
needle Iayer“ (Guörún Larsen 1978) was analys-
ed, as an example of an icelandic silicious
magma, which did produce features that re-
semble the Pele’s hair of the basalts. The grains
of this tephra have a characteristic needle-shap-
ed form. The short needles remind of the Pele’s
hair in many respects, they have the same
grooved outer surfaces and contain elongated
gas vesicles, which may contain liquid bubbles.
Related features have been observed in other
silicic tephras in Iceland.
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