Jökull - 01.01.2005, Page 59

Holocene tephra stratigraphy and eruption frequency of Katla, S-Iceland
Table 2b: Criteria used during field measurements. b) Key parameters used for describing tephra layers in a
soil profile. – Einkenni notuð við greiningu gjóskulaga á vettvangi. b) Önnur útlitseinkenni (þykkt gjóskulags,
snertifletir gjósku og jarðvegs, litur gjósku, kornastærð, byggingareinkenni gjóskulags, bergbrot, annað).
Parameters Description
Thickness Three to five thickness measurements were done on each layer to obtain an average value
and information on minimum and maximum thickness in any one section. Tephra thickness
values are a function of eruption intensity and/or distance from the main dispersal axis.
Contacts Pristine sub-aerial tephra fall deposits have sharp and non-erosional contacts. Deviations
may suggest post-depositional reworking.
Colour Colour of tephra layers reflects their composition. Basaltic tephra layers are typically black,
brownish-black, or greyish black, whereas tephra layers of intermediate compositions are
brown to greyish brown. Felsic layers are typically light coloured, with colours changing
from olive-grey to light grey to yellowish-white to white in accordance with increasing SiO2
contents.
Grain size Estimates of mean grain size and sorting for each layer, classification of grain types and
measurements of five largest grains were used to obtain qualitative information on the erup-
tion type and intensity.
Depositional structures Internal bedding (stratification), bed-forms, size grading and fabric provided information
on modes of transport and deposition (i.e. the part representing the primary fall versus that
produced by local reworking of deposits). In case of primary tephra fall deposits internal
stratification may reflect changes in eruption style or continuity of the activity, whereas size
grading may reflect changes in eruption intensity.
Rock components The relative abundance of pyroclasts and other accessory components, such as wall-rock
lithics and crystal fragments, was documented and can be useful indicator of eruption type
and style.
Other Observations that provide further information about the impact on the local environment
(i.e. presence of tree trunks/molds or material of archaeological significance).
lated thickness of ∼980 cm and provides a continu-
ous record spanning ∼7300 years from ∼1100–8400
years ago (Figures 4a, 4b). The uppermost part of the
prehistoric tephra succession is preserved in a contin-
uous profile at Atley, which for sake of convenience
was divided into 4 sections (Figures 3a-3b).
The∼520 cm-thick soil profile contains 99 tephra
layers. The topmost layer measured and sampled was
the one immediately above the Settlement Layer and
the last one measured was the Hekla layer H4 (Table
1a and Figures 4a, 4b). The subsequent time period is
well preserved in an outcrop about 1 km farther south
(HA) (Figure 3c), where a ∼280 cm-thick soil profile
contains 49 tephra layers between the chronostrati-
graphic markers H4 and A6 (Table 1). The remaining
part of the prehistoric tephra stratigraphy is preserved
in a soil profile at Rjúpnafell (RF),∼4 km to the south
of the Atley location. There, a ∼180 cm-thick soil
profile contains 43 layers between the marker layer
A6 and A13 (Table 1). Many of these oldest layers
are partly indurated (Figure 3d). In total, we identi-
fied 190 tephra layers in the prehistoric soil profiles
and 18 layers in the historical part, or 208 tephra lay-
ers in total. Of those, 183 layers were sampled and
126 were analysed for major-element composition.
Chemical analyses
Major-element composition can often be used to de-
termine the source volcano for a given tephra layer
because several volcanic systems in Iceland are char-
acterised by magmas of distinct composition (Jakobs-
son, 1979).
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