Jökull - 01.01.2016, Síða 73

Grain characteristics of silicic Katla tephra layers
of Volcanic Ash (1974) and Volcanic Ash (1985) who
kindly inspected a sample of the SILK-LN tephra in
2014, did not know of any other tephra having this
characteristics (Heiken, personal comment, 2014).
Hydromagmatic/Phreatomagmatic eruptions
As many Icelandic volcanic systems are ice-covered,
„wet eruptions“ are the most common type. Phreato-
magmatic/hydromagmatic eruptions therefore charac-
terize the volcanic activity in Iceland (e.g. Thordar-
son and Larsen, 2007; Thordarson and Höskuldsson,
2008), with basaltic eruptions the most frequent.
Hydromagmatic eruptions are divided into three
categories: Phreatic eruptions where magma is
not directly involved but provides the heat source,
phreatomagmatic eruptions and phreatoplinian erup-
tions where the magma fragments as a result of inter-
action with water and becomes extremely fine grained
tephra. Fragmentation processes in hydromagmatic
eruptions are much more complex than in magmatic
eruptions. The presence of external water can affect
both fragmentation and cooling at various stages of
an eruption.
Wohletz (1983) did a research on eruptive mate-
rial formed by the interaction of magma and water.
He designed a graph that shows maximum and min-
imum fragmentation caused by external water (Fig-
ure 4) which demonstrates the mass ratio of water
and magma against efficiency and approximate me-
dian grain size. If the mass ratio of water/magma is
less than 0.3, the efficiency decreases and the grains
get bigger. In that kind of situation there are two
things that maintain the explosive activity of the erup-
tion; on the one hand volatiles and on the other hand
steam. Once the mass ratio of water/magma is about
0.3, or even slightly higher, the magma-water interac-
tions maintain the explosions in the eruption. At this
stage, the efficiency is at its maximum, the fragmenta-
tion of magma is at its greatest and the smallest grains
are produced (Cas and Wright, 1987; Wohletz, 1983).
Grains formed in phreatomagmatic basaltic erup-
tions have been classified into five main types and
their formation has been clarified by experiments
(e.g. Morrisey et al., 2000; Wohletz, 1983). Blocky
grains form by brittle fracture, when deformation
rates exceed the tensile strength of the melt and also
by thermal contraction in quenched portions of the
melt. Fusiform grains with fluidal surfaces form from
portions of melt that fragment prior to quenching.
Moss-like grains form by viscous deformation under
tensional stress conditions. Drop-like grains develop
from the effect of surface tension from fluid melt.
Plate-like grains are thought to be pieces stripped
off quenched crust. One of the characteristics of
phreatomagmatic basaltic eruptions is also the pres-
ence of lithics from substrata of the vents and from
conduit walls.
Figure 4. Mass ratio of water/magma versus efficiency
and grain size (Wohletz, 1983). – Massahlutfall
vatns/kviku á móti afkastagetu og kornastærð.
Phreatoplinian eruptions are less common com-
pared to the other two types and not as well un-
derstood (White and Houghton, 2000; Morrisey et
al., 2000; Francis, 2001; Francis and Oppenheimer,
2004). This category was first recognized and named
by Self and Sparks (1978), partly based on obser-
vations of the widespread unit C in the Askja 1875
eruption. The characteristics described by them were
abundant fine ash, even near the source, well-bedded
deposits and accretionary lapilli, all indicating the
presence of water. Self and Sparks (1978) con-
cluded that the extreme fragmentation was due to
magma-water interaction superimposed on fragmen-
tation caused by vesiculation and expansion of gases
in the magma itself. The wide dispersal indicated de-
position from a high eruption column. About 90% of
the total grains size in Askja unit C was smaller than
1 mm (Sparks et al., 1981).
JÖKULL No. 66, 2016 73