Jökull - 01.01.2014, Side 33

Volume estimate of nine prehistoric Katla tephra layers
HS
H4
1
2 3
Ph
o
to
: S
ig
u
rð
u
r Þ
ó
ra
ri
n
ss
o
n
Hrífunes
Historical time
Hemra
Prehistoric time
a
b c
Figure 4. a) Soil section between the tephra
marker layers HS and H4, showing vari-
able thicknesses for a given tephra layer. b)
Historical soil section (photo S. Thorarins-
son) vs. c) prehistoric one. Field obser-
vations indicate more voluminous eruptions
from the Katla volcano during the last ∼3000
years of prehistoric time than during histor-
ical time (see text for further discussion).
– a) Gjóskulög á milli HS og H4, sýnir
hversu breytileg þykkt gjóskulags getur verið.
b) Jarðvegssnið frá sögulegum tíma (ljósm.
Sigurður Þórarinsson). c) Jarðvegssnið frá
forsögulegum tíma. Athuganir í mörkinni
benda til þess að stærri gos hafi átt sér stað
í Kötlu undir lok forsögulegs tíma heldur en á
sögulegum tíma (sjá frekari umræðu í texta).
Tephra marker layers
The soil profiles were correlated using 19 tephra
marker layers, of both local and regional extent (Ta-
ble 1). Nine are from the Katla volcanic system, five
silicic (YN, UN, MN, LN, N4) and four basaltic (El-
dgjá, Hrafnkatla, K-E, K-N). Six marker layers are
from the Hekla volcano, five silicic (Hx, Hy, Hm, Hn,
HS) and one basaltic, referred to as the Blue layer.
The remaining four are from Veiðivötn-Bárðarbunga
(V-871, basaltic part), Torfajökull (Grákolla, silicic),
Eyjafjallajökull (Ey-H, silicic) and an intermediate
tephra suite of an unknown origin, here referred to as
the coarse suite (Table 1).
The coarse tephra suite consists of three layers of
a very similar age (∼420 BC), most often seen as 1–2
coarse grained layers. Where only one layer is found
it is difficult to tell which of the three layers of the
marker suite is present. However, as they are so close
in time it is possible to use them as a marker layer
suite, regardless which one is present in a given out-
crop. The so-called Blue layer was erupted ∼610 BC
and is ∼200 years older than the coarse tephra suite.
It is found in 27 of the measured outcrops and the
isopach map hints at an origin from Hekla (see Tables
1 and 2).
The silicic Katla layer, UN, is the only layer found
in all outcrops, the V-871, Hn, and HS are present in
30 outcrops or more, Eldgjá, Hrafnkatla, the coarse
suite, the Blue layer, Hm and LN are found in 20–
29 outcrops, Hy and N4 are found in 10–19 outcrops
and YN, Ey-H, Grákolla, Hx, K-E, MN and K-N are
observed in less than 10 outcrops (Table 2).
JÖKULL No. 64, 2014 33