Density analysis of magmatic and phreatomagmatic phases of the 934 AD
Eldgjá eruption, southern Iceland
William M. Moreland* and Þorvaldur Þórðarson
Institute of Earth Sciences, University of Iceland
*presenting author, email: wmm2@hi.is
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1. Introduction to Eldgjá
Eldgjá is a ~75 km long fissure in southern Iceland (Fig. 1) which erupted in the
10th century and may have lasted as long as 8 years (Thordarson et al. 2001).
It is the largest fissure eruption in Iceland in historic times.
It is of the mixed-cone row type and produced two large lava fields (18.3 km3)
and a widespread basaltic tephra deposit (1.3 km3 DRE). The total erupted
volume of magma was 19.6 km3 (cf. 1783 AD Laki, 15.1 km3)
Proximal tephra deposits are up to 5.5 m thick at ~5 km distance from source
and contain many sub-units which vary in thickness parallel to the fissure,
indicating origin at multiple source vents.
As well as producing a large volume of products, the Eldgjá explosive phases
were very intense, even the purely magmatic ones (Fig. 2).
In order to examine the processes leading to fragmentation, occurring in the
shallow conduit, the density (i.e. vesicularity) of the eruptive products will be
examined.
Figure 1. Map showing location of the
Eldgjá (red) and Laki (yellow) lava fields.
The Eldgjá fissure is composed of several
segments: the Western, W; Central, C;
Eldgjá, E; and Northern, F. The dashed line
marks the extent of which the lava field
margins are covered by sediment
(Sigurðardóttir 2014). After Thordarson et
al. (2001).
3. Results
• All units sampled at Skælingar are magmatic. They produce unimodal, normal
to moderately log-normal density distributions with a mean density of 790-750
kg m-3 (60-70% vesicularity) and a range of around 860 kg m-3 (40%
vesicularity). Layer 8 and lower layer 9 produce broad density distributions
distinct from the other layers.
• The Stóragil section contains both magmatic and phreatomagmatic products.
The phreatomagmatic samples have characteristic plateau-like distributions
with a mean density of 720-750 kg m-3 (70-80% vesicularity) and a range of
around 842 kg m-3 (45% vesicularity). Layer 5b to 5c shows a change from
phreatomagmatic- to magmatic-type density distributions.
Figure 3. Stratigraphic log of
the key-sections: Skælingar and
Stóragil. Magmatic sub-units
are depicted by white with
black pyroclasts, corresponding
to clast size – from medium ash
to bombs, or black layers
representing fine, well-sorted
sub-units.
Phreatomagmatic
sub-units are depicted by the
brown, dashed layers. A thin
silicic layer is present within
the Stóragil section.
4. Discussion
• The change in width of distribution between lower and upper Skælingar layer
5b (yellow star) is due to an increase in the fraction of outgassed clasts. This is
an expected result as the gas from each parcel of magma producing a sub-unit
will rise to the upper part of that parcel leaving the lower part both degassed
and outgassed.
• Although broad density distributions are characteristic of phreatomagmatic
deposits, the distribution shown by Skælingar layer 8 through 9 (red star) may
be due to several separate parcels of magma, at varying stages of degassing,
fragmenting at the same time.
• The change in density distribution shown by layer 5a to 5b (green star) in
Stóragil seems to record the transition from phreatomagmatic to magmatic.
Given that L5a is the last phreatomagmatic sub-unit in the section, this may be
due to activity on the fissure moving out from under the glacier.
5. Future Work
• Thin-sections of representative clasts from each sample have been made.
These will be imaged, both by desktop scanner and scanning electron
microscope, and the images analysed (Fig. 4) as outlined by Shea (2010).
• The results of this will give variables such as vesicle number densities, vesicle
size distributions, and vesicle volume distributions.
• These variables will then be used to investigate the fragmentation mechanism
in the two eruption style end-members: magmatic and phreatomagmatic.
25x
1000
Thickness (cm)
Eldgja
Laki
100
10
1
0.0
5.0
10.0
15.0
20.0
25.0
Distance, km
Figure 2. Plot showing change in deposit
thickness with distance from source for
Eldgjá and Laki.
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2. Methods
Two key-sections, representing magmatic and phreatomagmatic tephra, were
logged (Fig. 3)
100 pyroclasts 8-32 mm in diameter were collected from <5 cm layers within
the sub-units following the method outlined by Houghton and Wilson (1989)
The clasts were then weighed, sealed in parafilm and weighed again to
calculate volume.
Using 2.85 g.cm-3 as melt density, the density and vesicularity of the clasts (Fig.
3, histograms).
100x
Figure 4. SEM images of bubbles. Three types of bubbles discernable: (1) largest, spherical ones; (2) smaller
spherical ones, with thick walls; and (3) small mature, polygonal ones. Bubble nucleation can be very
complex.
6. References
Figure 3 (cont.) The sub-units found in Stóragil are found at the base of Skælingar (L1-4). Activity began in
the west and progressed to the north-east, this is why these layers thin in this direction.
The histograms plot number of clasts within bins of density. Each sample set contains 100 clasts. Samples
were collected so as to best represent a sub-unit. If a sub-unit changed character with thickness that unit
was sampled at least twice to record that change.
Houghton B, Wilson C (1989) A vesicularity index for pyroclastic deposits. Bulletin of volcanology 51:451–462.
Shea T, Houghton BF, Gurioli L, et al. (2010) Textural studies of vesicles in volcanic rocks: An integrated methodology. Journal of
Volcanology and Geothermal Research 190:271–289. doi: 10.1016/j.jvolgeores.2009.12.003
Sigurdardottir, S (2014) The Eldgjá lava flow beneath Mýrdalssandur, S-Iceland. Mapping with magnetic measurements.
Unpublished MSci. Thesis, University of Iceland.
Thordarson T, Miller D, Larsen G, et al. (2001) New estimates of sulfur degassing and atmospheric mass-loading by the 934 AD
Eldgjá eruption, Iceland. Journal of Volcanology and Geothermal Research 108:33–54.