Jökull - 01.06.2000, Blaðsíða 10

Holocene eruptions within the Katla volcanic system,
tween cloud and ground also occur. People and live-
stock have occasionally been killed by lightning, even
at distances of 30 km from the volcano (S.t.s. Ísl. IV,
1907-1915). In the 1918 eruption the telephone could
not be used nor electricity maintained for extended pe-
riods of time (G. Sveinsson, 1919). Other electrical
phenomena, such as St. Elmo’s fire, are also reported.
Lightning may be the most serious - and the most un-
derestimated - hazard of future Katla eruptions.
Explosive silicic Katla (Sil-K) eruptions
Explosive silicic Katla eruptions have only recently
been recognized as a distinct phase in the Holocene
activity of the Katla volcanic system (Larsen, 1994;
Larsen et al., in press). Such eruptions were not ob-
served or described during historical time. A few of
the tephra layers produced in these eruptions were
previously known as significant key layers in the re-
gional tephrochronology of S-Iceland (Larsen, 1984)
due to their distinct grain characteristics and easy
identification and correlation in the field. Others have
only recently been mapped.
Some 12 Holocene silicic tephra layers originat-
ing below the Mýrdalsjökull ice cap have been identi-
fied so far (Larsen et al., in press). All were erupted
between ca. 1700 yrs BP and ca. 6600 yrs BP (Ta-
ble 3 and Figure 5). Older layers are found but their
origin has not been verified. The location of the vent
area below the ice cap can be inferred by plotting the
axes of some of the silicic tephra layers (Figure 6).
The axes of two bilobate tephra layers meet within the
caldera, indicating a vent area near its centre. Vents at
the caldera fracture cannot be excluded in other cases.
Silicic domes are found at the caldera rim and some
of them, e.g. at E-Kötlukollur, are thought to be of
late-glacial age as they have chemical characteristics
similar to pre-Holocene tephra from the Katla system
(Lacasse et al., 1995). Some of the Holocene sili-
cic tephra layers may be part of dome-forming erup-
tions. The caldera is unlikely to have been icefree
during prolonged periods of the Holocene and the
silicic magma was most likely erupted under similar
conditions as the basaltic magma, in the presence of
ice/meltwater in hydromagmatic explosive eruptions.
Table 3. Radiocarbon dates and estimated age of
silicic tephra layers from the Katla system. (From
Larsen et al., in press). – Geislakolsaldur og áætl-
aður aldur súru gjóskulaganna frá Kötlu (samkvæmt
Guðrúnu Larsen o.fl., í prentun).
SILK tephra Age B.P. Lab. no
Layer YN 1676  12 GU-7091
Layer UN 2660  50 SSR-2805
Layer MN 2975  12 GU-7021
Layer LN 3139  40 GU-7019
Layer N4 c. 3600
Layer N2 c. 4200
Layer N1 c. 4900
Layer A1 c. 5000
Layer A7 c. 6200
6400  80 U-4604
Layer A8 c. 6400
Layer A9 c. 6600
Most of the silicic tephra layers are lobate, with
two or three well defined main lobes, the largest layer
being a noteable exception (Figure 7). Many of the
layers are thin and consist of fine ash, while the largest
tephra layers have grains in the lapilli range as well.
Three layers contain distinct needle shaped grains and
have accordingly been named the upper (UN), middle
(MN) and lower (LN) needle layers.
The silicic Katla tephras have a distinct glass
colour and grain characteristics. The tephra has an
olive-green to greyish-green hue when seen in soil-
sections. The coarser grain sizes consist of three dis-
tinct grain types: rods of fibrous glass, up to sev-
eral cm long and a few cm in diameter, with elon-
gated vesicles and very thin walls, breaking easily into
small "needles"; equant or slightly elongated grains of
highly vesiculated glass with irregular vesicles; and
poorly vesiculated, black scoriaceous grains. The
maximum observed length of rodlike grains is over
8 cm with a diameter of 1-2 cm at a distance of 30
km. Lithics have not been found so far. The needles
are unique among the Holocene Icelandic tephras but
have some resemblance to the platy fines of the 1362
tephra from the ice-covered Öræfajökull volcano.
JÖKULL No. 49 9