Jökull - 01.01.2005, Blaðsíða 69

Holocene tephra stratigraphy and eruption frequency of Katla, S-Iceland
Figure 8. The Holocene tephra
layer frequency calculated for
five different running cycles (i.e.
periods of 200, 400, 500, 700
and 1000 years). Regardless of
the cycle length, the resulting fre-
quency curve is always character-
ized by two peaks, although the
younger peak is broader and pos-
sibly double. – Fjöldi gjóskulaga
á tímaeiningu (reiknaður fyrir
200, 400, 500, 700 og 1000 ára
tímabil) í samsetta jarðvegssnið-
inu. Tveir til þrír toppar koma
fram óháð því hvaða tímaeining
er valin.
(or possibly three) peaks, regardless of the periodicity
used. This is exemplified here by the 500 year period-
icity, which reveals a significant increase in the basalt
eruption frequency during the periods 2.5–4.5 ka and
7.0–8.5 ka (Figures 8 and 9). A similar trend is ob-
served in a soil section west of the volcano (Larsen,
unpubl. results). The eruptions that produced the
dacitic SILK layers have a repose time of 100-1000
years (Larsen et al., 2001) and at first glance they
appear to be fairly evenly distributed through time.
However, a closer examination indicates two peaks in
the frequency distribution for the SILK layers, one at
ca. 7.1–7.4 ka and another at 3–4 ka (6200–6600 14C
yr BP and 2700–3600 14C yr BP; Larsen et al., 2001).
These peaks correspond to those of the frequency dis-
tribution for the basalt eruptions. Although the SILK
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