Jökull - 01.01.2015, Síða 63

The Eldgjá lava flow on Mýrdalssandur
19º00’W
63
º2
8’
N
63
º3
6’
N
18º40’W 18º20’W
Vík
Ha
Kötlujökull
1.5
2
2.5
3
4
5
6
1300
1918
1900
1600
2
1.5
2
4
Jökulhlaup advance
Coastline changes
Major roads
Impact area 0 12km
R
Hj
Figure 2. Advancement of jökulhlaups
in hours from the onset of a large Katla
eruption. Based on simulations from
Vatnaskil Engineers (Gudmundsson et
al., 2008). Approximate coastline loca-
tion between 1300 and 1918 is shown
with dashed lines (Nummedal et al.,
1987). Ha: Hafursey, Hj: Hjörleifshöfði,
R: Rjúpnafell. – Áhrifasvæði jökul-
hlaupa undan Kötlujökli. Bláar lín-
ur sýna reiknaða framrás hlaupsins í
klukkustundum frá upphafi goss í Kötlu-
öskjunni samkvæmt hermun Verkfræði-
stofunnar Vatnaskila. Áætluð lega
strandarinnar á mismunandi tímum er
sýnd með brotalínum.
Larsen, 2000). The largest jökulhlaups carry with
them huge amounts of sediments and ice blocks from
the glacier. Estimates of the total sediment volume in
the 1918 jökulhlaup range from 0.7 to 1.6 km3 (Tóm-
asson, 1996; Larsen and Ásbjörnsson, 1995).
The environment at the Mýrdalssandur outwash
plain has significantly changed since the Eldgjá erup-
tion. The lava fields changed the topography, hy-
drology, utilization potential of the area east of
Mýrdalsjökull and the runoff of rivers and jökulhlaups
(Larsen, 2000; 2010). Since the Eldgjá eruption large
jökulhlaups accompanying eruptions at Katla volcano
have only emerged from beneath the Kötlujökull out-
let glacier (Thorarinsson, 1975). The high topogra-
phy of the Álftaver lava created a barrier prevent-
ing jökulhlaups from flowing to the east and direct-
ing them to the south. The outwash plain got thicker
with every jökulhlaup until it reached the elevation of
the lava field. Jökulhlaups were then able to flow to
the east over the lava, burying it in alluvium (Larsen,
2000; 2010). Today, the Álftaver lava field is partially
buried beneath the Mýrdalssandur outwash plain and
the location of its edge has been uncertain. With time
and continued volcanic activity and jökulhlaups from
Katla, progressively larger parts of the lava flow will
be buried beneath Mýrdalssandur.
Previous studies on sand thicknesses at the Mýr-
dalssandur outwash plain
In 1978 seismic refraction and reflection profiling and
vertical electrical soundings with a Schlumberger ar-
ray were conducted at the Mýrdalssandur outwash
plain by the National Energy Authority (Figure 3) to
map the thickness of the pumice-rich sediments. The
seismic soundings only cover the area around Hjör-
leifshöfði, while the resistivity survey included the
area east of Hafursey. The total sediment thickness
above the bedrock was revealed to be up to 60 m in the
area around Hjörleifshöfði increasing to 120 m around
Hafursey (Thorarinsson and Guðmundsson, 1979).
A total of 29 shallow boreholes were drilled into
the Eldgjá lava flow at Mýrdalssandur in 2009–2012.
In areas A and C (Figure 3) the drill went through
the lava flow showing a thickness ranging from 18 to
25 m. The uppermost 6–10 m is highly porous while
the lower 15–18 m is made up of dense basalt, which
lies mostly in 2–3 zones consistent with the lava being
emplaced as a series of lobes (Vegagerðin, 2013).
The lava edge has been defined on the basis of vis-
ible lava on the surface, which is done on the geolog-
ical map by Jóhannesson et al. (1990). Larsen (2000)
suggested that the edge lay further to the west than
previously thought (Figure 1). The suggested edge
JÖKULL No. 65, 2015 63