Jökull - 01.01.2014, Side 32

B. A. Óladóttir et al.,
from source) of e.g. the K-1755 tephra layer in Kriki,
where it measures 120 cm. These Tmax values are
probably underestimated as it is known that up to half
of erupted material can be deposited very close to
source, as in the Grímsvötn eruption of 2004 (Odd-
sson et al., 2012), and can result in up to 30 m thick
tephra by the craters, as in the Eyjafjallajökull erup-
tion of 2010 (Guðmundsson et al., 2012). This part
of the erupted material, i.e. crater rims around vents
and tephra accumulated in ice cauldrons formed in
eruptions within the Katla caldera, as well as material
transported away by associated jökulhlaups (glacial
outbursts), is excluded in volume estimation in this
study resulting in absolute minimum volume estima-
tions of the erupted material.
Volume estimations and error
Volume estimations are based on measurements of
tephra thickness in different locations around the vol-
cano. Several parameters create errors in volume esti-
mation:
Firstly, a field based error derived from three main
points: 1) The choice of location. An ideal location is
where tephra thickness represents primary fall and has
neither been eroded nor undergone secondary deposi-
tion in order to get the optimal thickness (e.g. not in
or under steep slopes, Figure 2c-d). Proximal deposits
have shown to be more variable than distal ones (En-
gwell et al., 2013). 2) The measured thickness. In the
absence of bedding or grading a choice has to be made
on what thickness of a layer represents its correct
thickness in each outcrop, the observed maximum,
minimum or average thickness (Figure 4a). Minimum
and maximum thickness is often measured and the av-
erage thickness used for volume estimation. Possibly
only part of the tephra layer is primary, its top may
have been eroded away and a choice has to be made
on what the measured number actually reports. A pos-
sible way around this problem could be to measure
mass per unit area but as it cannot account for material
loss due to erosion it is no better for old tephra layers
but would work well for new layers where it is possi-
ble to collect directly from collective trays (Engwell
et al., 2013). 3) Measuring error. There is an error de-
rived from the measurement itself and the uncertainty
increases as the tephra thickness decreases and there
is a known tendency to fit measurements in convenient
size bins (often running on 5 mm, e.g. Engwell et al.,
2013). The total measurement uncertainty has been
estimated∼30%, based on both natural variation (e.g.
erosion, secondary addition of material) and observa-
tional uncertainty (measurement error; Engwell et al.,
2013) and that value is used here when calculating the
error of the thickness (Table 5).
Secondly, the drawing of isopach maps from point
data includes error. Volume calculations are based
on the produced maps and incorporate the error. The
drawing of these maps varies between individuals and
cannot be reproduced in the exactly same way. Ac-
cording to Klawonn et al. (2014) the different smooth-
ing of the contour lines on isopach maps does not
affect the volume estimation but a large uncertainty
is assigned to the thinnest isopachs that most often
leads to underestimations of the actual trend of deposit
thinning. In any case, errors associated with draw-
ing contours on the isopach maps are negligible com-
pared with incertitude arising from different methods
for volume calculation (Klawonn et al., 2014).
RESULTS
Soil sections
In total 40 soil sections have been measured (Figure
1, Table 2). The outcrops were principally measured
between the marker tephra layers V-871 and HS, but
included the Eldgjá layer (∼70 years younger than the
V-871) and a silicic Katla layer (N4; ∼70 years older
than HS) for improved correlation (Tables 1 and 2).
The total thickness of tephra sections ranges be-
tween 1.7 m in Óbrinnishólmi, one of the most distal
sections ∼70 km northeast of the Katla caldera, and
4.7 m in Rjúpnafell, one of the most proximal sections
only∼20 km east of the caldera (Figure 1). The num-
ber of measured tephra layers in individual sections
ranges from 31 in Norður-Hvammur, south of the vol-
cano to 63 in Rjúpnafell (Figure 1). A total of nine
Katla tephra layers were mapped, named K-I to K-IX.
The tephra layer K-V is found in 33 outcrops, K-I in
32, K-IX in 31, K-VI is found in 30 outcrops, K-III,
K-IV and K-VII are in 28, K-VIII in 20 and K-II is
only present in 18 outcrops (Table 2).
32 JÖKULL No. 64, 2014