Jökull - 01.01.2015, Blaðsíða 69

The Eldgjá lava flow on Mýrdalssandur
DISCUSSION
The results presented here clearly show a sharp in-
crease in depth to magnetic sources to the west of
what is assumed to be the edge of the Álftaver branch
of the Eldgjá lava flow (Figure 7). Although, eight
Holocene lava flows are known to have originated in
the Katla volcanic system and emplaced around the
Mýrdalsjökull glacier (Jóhannesson and Sæmunds-
son, 1990; Óladóttir et al., 2008), only the 7700 years
old Hólmsá lava flow and the Eldgjá lava (934 AD) are
known in the study area. Therefore, only the Hólmsá
lava (7700 years old) is expected to be present in the
study area in addition to the Álftaver branch of the
Eldgjá lava. The area and volume of the Hólmsá lava
flow is poorly constrained since it is mostly covered
by the younger Eldgjá lava flow (Larsen, 2010). Thus,
here it is assumed that the Eldgjá lava flow is the only
contributing factor of the observed anomalies how-
ever not excluding that the Hólmsá lava flow might
have some effect on the profiles.
Sedimentation rates at the Mýrdalssandur out-
wash plain
Based on the magnetic profiles, we estimated sedi-
ment thickness and sedimentation rates at Mýrdals-
sandur since the Eldgjá lava flow was emplaced.
Three profiles suggest that the top of the lava flow at
its edges is at ∼10 m depth. The thickness of the lava
flow according to borehole data is 18–25 m (Vega-
gerðin, 2013). However, lava flows are often thin-
ner near the edge and the indications of buried lava
ledges suggests that this is also the case with the Eld-
gjá lava beneath Mýrdalssandur. However, it is also
possible that the porous upper part of the lava has been
removed by jökulhlaups as lava block fragments are
seen on the surface in some parts of Mýrdalssandur
(Vegagerðin, 2013).
Using 10 m as the thickness of sediments on top
of the lava flow and 10 m as lava flow thickness by the
edge, a rough estimate of the sediment accumulation
since the Eldgjá eruption is ∼20 m. The area of Mýr-
dalssandur west of the lava edge is about 220 km2.
Assuming 15–25 m thickening since the eruption in-
dicates a post-Eldgjá accumulation of 3–5 km3. This
value is probably an underestimate, since the outwash
plain has grown into the ocean (Figure 2) and the
filling of the Kerlingafjörður fjord that was located
west of Hjörleifshöfði (Larsen, 2000) is only partly
included.
Area and volume of the Eldgjá lava flow
Assuming that the Eldgjá lava flow is composed of
three branches, the Álftaver-, Meðalland- and Land-
brot lava fields (Larsen, 1979; 2000; 2010) and adding
64 km2 to the area published by Larsen (2000) we
obtain a total area of 844 km2 (Figure 8). Adding
the 1.4±0.5 km3 obtained here to the Eldgjá lava vol-
ume published by Thordarson et al. (2001) results
in ∼19.7 km3 or, given the expected 10–15% uncer-
tainty, 20 km3 lava volume. The volume of the Eld-
gjá lava flow thus amounts to 80% of the ∼25 km3,
8600 BP Þjórsá lava flow within the Veiðivötn fissure
system, the largest known fissure eruption on Earth
during the Holocene (Hjartarson, 1988; Halldórsson
et al., 2008). This comparison clearly highlights the
Eldgjá eruption as another major lava extrusion event
during the Holocene. The volume of the buried lava
flow may be an underestimate, as the edge could lie
farther to the west in the southern study area, where
the lava flow could not be mapped out (Figure 8). The
south coast lay farther to the north prior to the Eldgjá
eruption and the lava flowed into the ocean. Further
observations along the former coastline may increase
the flow volume.
CONCLUSIONS
1. The extent of the buried part of the 934 AD Eld-
gjá lava flow on the Mýrdalssandur outwash plain was
mapped with magnetic surveys.
2. The lava edge is found to lie 1–4 km further to the
west than suggested by previous surface mapping.
3. The area of the lava flow buried below Mýrdals-
sandur is 64 km2 making the total area of the Eld-
gjá lava flow 844 km2. The volume of the Eldgjá
lava flow beneath Mýrdalssandur is estimated to be
1.4±0.5 km3. This brings the best estimate of the to-
tal volume of the Eldgjá lava flow to ∼20 km3.
4. The sediment accumulation on central and western
Mýrdalssandur since the Eldgjá lava flow was em-
placed is at least 3–5 km3.
JÖKULL No. 65, 2015 69