Jökull - 01.01.2005, Blaðsíða 68

Bergrún Arna Óladóttir et al.
Table 4: Periods of compositional patterns (I-VIII in column 1). Column 2 lists the tephra layers limiting each
period; column 3 shows the length of each period, column 4 shows the calculated tephra layer frequency; and
the average time between formation of successive tephra layers is given in column 5. Approximately half of his-
torical Katla eruptions are recorded as tephra layers in the study area. Consequently, the true eruption frequency
is most likely double that of the tephra layer frequency and the true mean repose time is close to the half of the
value listed for the average time between successive tephra layers (see text for further discussion). – Tímaskeið
sem afmarkast af snöggum breytingum á aðalefnasamsetningu Kötlukviku (I-VIII í fyrsta dálki). Í öðrum dálki
eru yngstu og elstu gjóskulög á hverju tímaskeiði, í þriðja dálki er lengd hvers tímaskeiðs, í fjórða dálki er
fjöldi gjóskulaga sem féllu á rannsóknasvæðinu á hverjum 100 árum, og í fimmta dálki er meðaltímalengd milli
gjóskulaga reiknuð. Um helmingur þeirra gjóskulaga sem féllu á sögulegum tíma finnst á rannsóknasvæðinu.
Samkvæmt því má ætla að raunveruleg tíðni Kötlugosa á forsögulegum tíma geti verið tvöfalt hærri en gildin í
fjórða dálki og að meðalgoshléin geti verið helmingi styttri en sýnt er í fimmta dálki.
Time Bounding Length Tephra layer frequency Average interval
period layers (years) (per 100 years) (years)
I 1918AD–Eldgjá 980 1.00 98
II Eldgjá–AT19 800 2.15 47
III AT21–AT77 1700 2.80 35
IV AT79–HA4 630 3.15 32
V HA5–HA34 1750 1.35 73
VI HA35–HA43 510 1.55 64
VII HA46–RF20 700 2.60 39
VIII RF21–RF43 650 3.15 32
there are 20 known eruptions from the Katla volcano
and at least 10 of those tephra layers are present in
the area east of the volcano. About half of the erup-
tions are therefore preserved in soil profiles east of the
volcano, so the number of tephra layers there can be
doubled to estimate the total number of eruptions from
Katla. An implicit assumption here is that the weather
pattern (wind directions, strength etc.) has remained
similar during the last 8400 years.
The overall tephra layer frequency in the compos-
ite soil profile measured for this work (Figure 4a and
b), is 2.6 tephra layers per 100 years in prehistoric
times (190 eruptions in 7300 years). Considering only
tephra layers from the Katla system, the prehistoric
frequency is 2.2 layers per 100 years. A minimum
value of 1.4 layers per 100 years is obtained if we only
consider layers that have their Katla origin confirmed
by microprobe. The value 2.2 layers per 100 years
is a factor of 2 higher than the frequency obtained
for the historical period (∼1 event per 100 years for
historical tephra layers in the eastern sector; Larsen,
2000). Accepting our argument as presented above,
that the eastern sector only preserves one half of the
tephra layers produced by explosive Katla eruptions,
the long-term average eruption frequency in prehis-
toric times could be as high as 4 events per 100 years.
This frequency corresponds to a total of ∼300 explo-
sive basaltic Katla eruptions over 7300 years (i.e. the
period 1100 to ∼8400 years ago). This estimate can
be verified (or disproved) by a systematic study of the
Holocene tephra stratigraphy around Katla volcano,
focusing on measuring soil profiles in the region to
the west and south of Mýrdalsjökull.
Temporal changes in the Holocene eruption fre-
quency
The Holocene eruption frequency is evaluated using
periodicities of 200, 400, 500, 700 and 1000 years
and the data are plotted on Figure 8. The results show
that the frequency distribution is characterized by two
68 JÖKULL No. 55