Jökull - 01.06.2000, Síða 12

Holocene eruptions within the Katla volcanic system,
0 10 20 km
N4
YN
LN
N4
N2
MN
YN
Mýrdalsjökull Ice Cap
19° 30' 19° 00' 18° 00
63°
40'
63°
40'
VíK
Figure 6. Axes of thickness for some silicic Katla
tephra layers extended into the caldera, indicating po-
tential source areas within the caldera. – Þykktarásar
nokkurra súrra Kötlulaga eru framlengdir þannig að
þeir ná inn í öskjuna. Tvö gjóskulaganna eru tvíása
og ásarnir skerast innan öskjunnar, sem bendir til
upptaka inni í henni. Ásar hinna gjóskulaganna lenda
þar á milli. Þegar um einn ás er að ræða er þó ekki
hægt að útiloka að upptökin gætu verið í öskjubrotinu.
Some of the silicic tephras have been dated by ra-
diocarbon analyses of organic material immediately
above or below the tephras. The youngest layer, YN,
is dated at 1676  12 14C yrs and the second youngest
and largest, UN, is dated at 2660  50 14C yrs (Table
3). The oldest layer verified to be silicic Katla tephra
was erupted about 6600 14C yrs BP.
The 12 silicic eruptions identified so far are not
evenly spaced during the period in question. The
eruption frequency was highest between ca 6200 and
6600 14C yrs ago when three eruptions occurred, and
between ca 2700 to 3600 14C yrs ago when four of
the twelve eruptions took place. Thus, the interval be-
tween eruptions has varied from about 100 to about
1000 14C yrs.
The glass composition of the 12 tephra layers
analysed so far is similar for all the tephras. The
SiO  content lies in the range of 63-67% (Table 4 and
Larsen et al., in press) and the overall composition
has remained remarkably stable over five millennia.
Grains of basaltic and rhyolitic glass, possibly scav-
enged from the vent or conduit, occur in at least one of
the layers. The composition of the silicic magma dif-
fers significantly from that of the Pre-Holocene silicic
tephra deposits on the southern slopes of the volcano
(Lacasse et al., 1995).
The duration of the silicic eruptions is not known.
The geometry of the tephra layers indicates that the
tephra was erupted in separate bursts forming dis-
tinct well defined fans or lobes. Some of the lobes
are narrow, indicating short-lived events (minutes or
hours). Some of the tephra layers are bi- and trilo-
bate, and changes in wind-direction between deposi-
tion of individual lobes indicate relatively long quiet
periods (hours, days, weeks). This implies that many
of the eruptions consisted of several relatively short-
lived explosive events at intervals of unknown length.
Intermittent activity may even have continued for a
few years, similar to the 1821-23 activity at the neigh-
bouring Eyjafjallajökull volcano. Another possibil-
ity is that the activity was continuous but only tephra
from the largest events was deposited outside the ice
cap. The volume of airborne silicic tephra indicates
relatively small eruptions, of similar or smaller mag-
nitude than the typical Katla eruptions. The distribu-
tion of the tephra suggests that the explosive activity
was of low intensity, not capable of supporting high
sustained eruption columns.
Jökulhlaups accompanying eruptions in the area
defined by the axes of thickness within the caldera
(Figure 6) could, under present conditions, escape
through any of the three gaps occupied by the glaciers
Entujökull, Kötlujökull and Sólheimajökull. Jökul-
hlaups accompanying eruptions at the caldera fracture
could also escape along other routes, depending on the
location of the vents. No water-transported material
with the chemical characteristics of the Holocene sili-
cic tephras has been found on the flood plains around
Mýrdalsjökull, but glass chemistry has revealed that
several of these eruptions contributed to ocean-rafted
pumice, which has been found on coasts around the
North Atlantic (Newton, 1999; Larsen et al., in press).
The wide distribution is more likely the result of the
properties of the pumice, which allowed it to stay
afloat for a long time, than an indication that the
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