Jökull - 01.01.2016, Síða 74

Þorsteinsdóttir et al.
Fragmentation in most of the phreatoplinian erup-
tions described by Houghton et al. (2000) was brought
about by vesiculation and bubble expansion, as well
as by quenching by external water. The pumice clasts
were vesicular enough to imply that the magma had
already formed foam and perhaps begun to fragment
or disintegrate before it first encountered external wa-
ter. The fragmentation resulting from the magma-
water interaction could be the result of thermal con-
traction upon quenching, brittle failure caused by high
strain rates resulting from expansion of steam or both
these mechanisms. Lithics appear to be less com-
mon in phreatoplinian eruptions than in the basaltic
ones. This could suggest that the fragmentation
caused by water takes place at the interface between
the magma and ice/water, rather than deeper in the
conduit (Dellino et al., 2012).
Knowledge about silicic phreatoplinian eruption
columns or plumes is limited. It has been suggested
that limited amount of water will have little effect on
the height and dispersive power but excessive amount
will cause column collapse and pyroclastic flows or
surges (Houghton et al., 2000).
Only two examples of recent subglacial explosive
silicic (>63% SiO2) eruptions were found in litera-
ture (Guðmundsson et al., 2012; Kratzmann et al.,
2009). It is suggested that some phases of the 1991
Hudson eruption were phreatoplinian because of the
fine ash produced. However, no phase of the Eyja-
fjallajökull eruption was classified as phreatoplinian,
although about 94% of the tephra from the first phase
(14-16 April) was smaller than 1 mm and up to 50%
smaller than 0.063 mm (Gudmundsson et al., 2012).
METHODS
Tephra samples were collected from soil sections at
Framgil on Álftaversafréttur and Einhyrningur west
of Markarfljót (Figure 2), 22 and 20 km from cen-
tre of caldera, respectively. At each location the
tephra layers were cleaned, photographed, measured
and macroscopic features described such as bedding,
grading, colour, texture, grain size and grain types (for
details see Thorsteinsdóttir 2015).
Grain analyses
Sieving by hand (to avoid breaking/abrading) was
used for size fractions larger than 4Φ (0.063 mm)
and settling velocity less than 4Φ. A Sedigraph (Mi-
cromeritics, 2010) was used for grains smaller than
4Φ. The results from both methods were combined
and plotted on a graph, showing the complete grain
size distribution of each tephra sample. The elongated
grain shape of the SILK tephra may affect the size
results because particle size techniques assume that
grains are spherical. A comparison between the SILK
tephra layers is however considered justified.
Grain morphology analysis was carried out on se-
lected samples from four eruptions. The parameters
ruggedness (ratio of convex perimeter to total perime-
ter, CPERIM/PERIM), elongation (ratio of minimum
to maximum diameter, DMIN/DMAX) and circularity
(4πAREA)/(PERIM)2 were measured using an image
analysis program (Eiríksson et al., 1994).
Scanning electron microscope (SEM) images on
selected tephra samples were obtained using a Hitachi
TM3000 electron microscope, in order to demonstrate
potential differences between tephra layers which
might reflect different eruptive environments.
RESULTS
Samples from 10 SILK tephra layers were analysed
for grain size and 4 samples for grain shape, including
the previously analysed SILK-LN layer (Thorsteins-
dóttir et al., 2015). The main focus was on possible
changes during the period from 2800 to 8100 years
ago. The tephra samples were collected at similar dis-
tance, 20–22 km, from the center of Katla caldera.
The thickness axes are, however, not known for all
the layers. Tephra layers SILK-YN, SILK-MN and
SILK-A9 were not included in this study. The remain-
ing layers (except SILK-A11) were grain size anal-
ysed, and SILK-N1, SILK-A8, SILK-A11 and A12
were analysed for grain shape (Table 3).
Changes in grain size
The younger part of the silicic tephra sequence ap-
pears coarse grained while the older part is fine
grained (Table 3). The largest grains in the coarser
section belong to the grain size categories from -3Φ to
74 JÖKULL No. 66, 2016