Jökull - 01.01.2005, Síða 70

Bergrún Arna Óladóttir et al.
Figure 9. Number of Katla tephra layers per five hundred years of the composite soil profile, covering ∼8500
years (0–1 ka is historical time). The SILK layers (YN, N3, A9, A12) that were not present in the composite
soil profile are included. Solid line only includes basaltic Katla tephra layers that have been analysed for major
element composition. The mean value is 1.4 events per 100 years. Using the total number (= 172) of Katla
tephra layers based on field identification, the mean value is 2.2 layers per 100 years in prehistoric times and
the true mean frequency is likely to be close to 4 events per century. In historical time the tephra layer frequency
on the east side of the volcano is ∼1 event per 100 years, whereas the true eruption frequency is ∼2 event per
100 years (e.g. Larsen, 2000; see text for further details). – Fjöldi Kötlugjóskulaga á hverjum 500 árum í sam-
setta jarðvegssniðinu (0–1 ka er sögulegur tími). Öll þekkt súr Kötlulög eru höfð með í talningunni þótt sum
þeirra fyndust ekki í samsetta sniðinu (YN, N3, A9, A12 bætast við) til að kanna tengls súru gjóskulaganna við
toppa í gostíðninni. Heildregna línan sýnir fjölda efnagreindra gjóskulaga. Meðaltalið er 1.4 gos/100 ár. Ef
gjóskulög greind sem Kötlulög á útlitseinkennum eru tekin með (samtals 172) er meðaltalið 2.2. gos/100 ár á
forsögulegum tíma. Í ljósi þess að helmingur Kötlulaga frá sögulegum tíma finnst austan við Kötlueldstöðina
(þ.e. 10 af 20 Kötlulögum, sbr. Guðrún Larsen 2000) er líklegt að gostíðnin á forsögulegum tíma hafi verið
nálægt 4 Kötlugosum á öld.
layers are produced during periods characterized by
both high and low basalt eruption frequency, there is
an obvious correlation between the increase in dacite
eruption frequency and basalt eruption frequency.
Periods I-VIII and eruption frequency
Significant variations are also observed between the
periods defined by the compositional patterns (Table
4). The lowest frequency of explosive basalt erup-
tions is found in period I (historical time), whereas
the highest frequency is in period IV. The latter cor-
responds to the younger frequency peak in volcanic
activity at the Katla system (Figures 7 and 9). The av-
erage time interval between the formation of basaltic
tephra layers is shortest in periods IV and VIII, 32
years in both instances, but longest in periods I (98
years) and V (73 years). Note that the time inter-
vals quoted here are two times longer than the true
repose time between eruptions, assuming again that
the tephra sequence east of Katla volcano represent
only half of the total number of its eruptions. The av-
erage repose times as calculated from the composite
soil sections for each period (Table 4) can be com-
pared between different time periods since the risk of
missing out tephra layers is expected to be the same
throughout the time studied. Consequently, the aver-
70 JÖKULL No. 55