Jökull - 01.06.2000, Síða 5

Guðrún Larsen
A sustained eruption column of vapour, gases and
tephra usually develops during the first hours of sub-
aerial activity at Katla. On the first day of the 1918
event the eruption cloud reached an elevation of 14 km
above sea level as measured from Reykjavík, about
160 km west of the volcano (Eggertsson, 1919). At
closer range it was seen as a fast rising cloud with
a convoluted upwind margin, and the basal part was
described as black, becoming whitish in the upper
reaches (Sveinsson, 1919; Jóhannsson, 1919). Light-
ning flashed in and above the cloud at intervals of a
few seconds and tephra fall commenced to the east
of the volcano within an hour of the first cloud sight-
ing. On the second day of eruption, tephra fall was
reported from Reykjavík and from Höfn 200 km to
the east of the volcano.
Tephra fall can occur at any time during Katla
eruptions but the explosive activity is usually most in-
tense, and tephra production greatest, during the first
days (S.t.s. Ísl. IV, 1907-15; G. Sveinsson, 1919). The
tephra is deposited both as lobate fans and as thin veils
around the volcano, manifesting a distribution pat-
tern which is significantly different from that of short-
lived plinian eruptions of similar magnitude (Figure
2). The reported tephra fall area in the 1918 eruption
exceeded 50.000 km  on land. Occasionally, tephra
from the first days of Katla eruptions has reached the
Faroe and Shetland Islands and during the 1625 erup-
tion tephra fall reached the European mainland (Thor-
arinsson, 1981). The direction of the main tephra fall
has been to the east, northeast or southeast in eight
out of 17 eruptions for which the tephra distribution
is known (Figure 3). The maximum thickness of air-
borne Katla tephra is not known as the thickest part
is not preserved due to deposition on the glacier, but
thicknesses in excess of 0.5 m have been reported (G.
Sveinsson, 1919). The maximum measured thickness
of compacted tephra at distances of 25 km from the
source is  30 cm.
The volume of airborne Katla tephra varies be-
tween eruptions by at least three orders of magnitude.
The most voluminous tephra layer is thought to be the
K-1755 layer with an estimated volume of 1.5 km

of freshly fallen tephra (Thorarinsson, 1975). The
smallest tephra layers mapped so far have volumes of
SKAFTÁ
TU
NG
NA
Á
Vatnajökull
1
3
10
20
0.5
19° 00' 18° 00'
63°
40'
64°
00'
1
0.5
0.1
3
VíK
Katla
0.1
0 10 20 KM
Katla tephra layer
1625 AD
3 Isopachs in cm
5
Mýrdals-
jökull
KBK
Figure 2a. Dispersal of the 1625 Katla tephra.
isopachs of proximal and distal deposits on land. –
Dreifing gjósku í Kötlugosinu 1625. Jafnþykktarlínur
gjóskunnar á landi.
less than 0.02 km

. The volume of water-transported
volcanic debris is unknown but two estimates indicate
volumes of between 0.7 and 1.6 km

during the K-
1918 eruption (Larsen and Ásbjörnsson, 1995; Tóm-
asson, 1996).
The duration of documented Katla eruptions since
1625 has varied from 2 weeks to over 5 months (Table
1) but some eruptions probably fall outside this range.
The last three Katla eruptions to melt through the ice
cover lasted 20-28 days. The average repose period
since 1500 AD is 47 years with maximum deviations
of 33 and 34 years. The shortest repose period known
so far is 13 years, between the eruptions in 1612 and
1625, while the longest one is about 80 years.
4 JÖKULL No. 49