Jökull - 01.01.2014, Page 37

Volume estimate of nine prehistoric Katla tephra layers
64°N
64°N
18°W20°W
18°W20°W
64°N
18°W20°W
20 km
20 km
Isopachs in cm
Ucp vol: 0.6 km3
Tmax: 100 cm
Isopachs in cm
Ucp vol: 0.8 km3
Tmax: 100 cm
K-VII
Age: ~910 BC
K-IX
Age: ~1150 BC
20 km
Isopachs in cm
Ucp vol: 0.2 km3
Tmax: 50 cm
K-VIII
Age: ~ 980 BC
1
1
1
1 1
2
2
2
2
2
5
5
5
10
1020
0 1
1
1
1 1
1
1
2
2
2
2 2
2
5
5
5
10
10
10
0
2 550
Figure 6. Isopach maps of nine prehistoric Katla
basalt layers based on 36 measured soil sections
(Figure 1 and Table 1). Age given is calculated age
based on soil accumulation rate (see Table 4 and dis-
cussion in text). The K-I layer is the largest of the
eight layers, with a minimum volume of 2.7 km3
uncompacted (UCP) tephra on land and the small-
est is K-VIII with a minimum volume of 0.2 km3.
The outer visible limit and the Tmax is shown with
a blue line and the 10 cm isopach is in red. Grey
lines are roads and tracks. – Þykktarkort níu forsögu-
legra gjóskulaga frá Kötlu (sjá 1. mynd og 1. töflu).
Aldur er reiknaður út frá upphleðsluhraða jarðvegs
(sjá 4. töflu og texta). Lágmarksrúmmál K-I, stærsta
lagsins af þeim níu sem hér eru metin, er 2,7 km3
reiknað sem nýfallin gjóska á landi og minnsta lagið
er K-VIII, 0,2 km3. 10 cm jafnþykktarlínan er rauð
og blá lína sýnir ystu sýnilegu mörk gjóskulags og
metna hámarksþykkt (Tmax). Gráar línur eru vegir
og slóðar.
CONCLUSIONS
Based on the above, it is clear that the estimated Katla
tephra volume is only the minimum estimate of the
total erupted volume. Part of the erupted material gets
transported away by melt water, part of it is deposited
in the sea and part of it is eroded away. However,
comparison of volume estimates from two time peri-
ods with different tephra layer frequencies confirms
that in prehistoric time, when tephra layer frequency
was high (2.8 layers/100 years) during a period of ac-
tive magma chamber, larger Katla eruptions occurred,
with average tephra volume of 0.9 km3 and a range
of 0.2–2.7 km3. In historical time, when tephra layer
frequency was low (1.0 layers/100 years) during a
period of a simple magma transfer system, smaller
Katla eruptions occurred, with average tephra volume
of 0.4 km3 and a range of 0.04–0.8 km3.
Acknowledgements
This study was financed by the Nordic Volcanological
Center Post-doc program and University of Iceland
Research Fund Contribution Grants for Post-docs.
Sigríður Björgvinsdóttir, Dominic Pyanoe, Guðmund-
ur Óli Sigurgeirsson, Jónas Guðnason, Esther Ruth
Guðmundsdóttir and Edda Sóley Þorsteinsdóttir are
genuinely thanked for their assistance in the field. We
thank Jean-Luc Devidal for his help with electron mi-
croprobe analyses in Clermont-Ferrand, France. Þór-
dís Högnadóttir introduced us to the Surfer© program
and is sincerely thanked. We are grateful to Stefan
Wastegård and Magnús Tumi Guðmundsson for con-
structive reviews.
JÖKULL No. 64, 2014 37