Jökull - 01.01.2005, Page 57

Holocene tephra stratigraphy and eruption frequency of Katla, S-Iceland
A principal difference between the activity in his-
torical and prehistoric times is that in the latter pe-
riod the Katla volcano produced dacitic (SILK) tephra
along with the typical basaltic activity. These SILK
layers were produced over a period of ∼6000 years,
extending from∼7600 to∼1600 years ago and corre-
sponding to∼6600 to∼1680 14C years BP (Larsen et
al., 2001).
METHODS
Field methods and criteria
In order to obtain a representative composite tephra
section, selection of suitable soil profiles is critical.
Firstly, such profiles have to be located at an appropri-
ate distance from the source volcano. Close to a vol-
cano the tephra is often deposited on barren ground,
increasing the likelihood of erosion and reworking of
the primary fall. This decreases the preservation po-
tential of individual layers and may also result in sig-
nificant contamination between adjacent tephra lay-
ers. On the other hand, a soil section too far from the
source volcano will not include tephra fall from low-
intensity eruptions due to their limited dispersal. Sec-
ondly, the topography has to be favourable. Tephra
layers are best preserved in topographic lows which
commonly act as sediment traps ensuring rapid burial
of the tephra. Thirdly, the sections should contain
tephra layers that show minimal post-depositional re-
working and disturbance, i.e. contain the primary fa-
cies of the tephra fall.
The macroscopic characteristics that are used to
distinguish individual tephra layers are a function of
several factors. The colour of the tephra is a di-
rect function of chemical composition, changing from
black to white with increasing silica. Properties such
as grain size and size distribution, sorting and clast
shapes are symptomatic of the nature of the explosive
activity that produced the layer. The type and content
of crystals and lithics are also diagnostic features in
some tephra layers (Table 2a, 2b). Felsic tephra layers
are readily identified in Icelandic soils by their light
colour and are therefore ideal key marker horizons for
correlation between outcrops. The soils of the study
area include over 20 felsic tephra layers formed by ex-
plosive eruptions at the Katla, Hekla, Torfajökull, and
Öræfajökull volcanoes.
Table 2a: Criteria used during field measurements. a)
Grain size classification. – Einkenni notuð við grein-
ingu gjóskulaga á vettvangi. a) Kornastærðir.
Grain size (mm) Nomenclature
>64 Blocks and bombs
32–64 Coarse lapilli
16–32 Medium lapilli
2–16 Fine lapilli
1–2 Coarse ash
0,064–1 Medium ash
<0,064 Fine ash
The principal historical marker tephra layers
found in the study area east of Katla volcano (Figure
1) originate fromHekla (H), Veiðivötn (V), Öræfajök-
ull (Ö) and Eldgjá (E). The historical part of the tephra
stratigraphy includes 7 key marker layers. From the
youngest to the oldest, these are: H-1845, a brown
andesite tephra consisting of very coarse ash; H-
1597, an andesite layer composed of bluish-black fine
ash; V-1477, a distinct basaltic tephra layer made
up of greyish-green to greyish-brown fine ash; H-
1389, a brown andesite layer comprised of coarse ash;
Ö-1362, a yellow-white (rhyolite) very-fine to fine
ash that is present as a sub-centimetre-thick discon-
tinuous layer; H-1206 is olive-grey fine ash of an-
desitic/dacitic composition; and finally the basaltic
tephra of E-934, a black to brown bedded layer of
lapilli and ash (Thordarson et al., 1998; Larsen,
2000). The Vatnaöldur tephra, the basaltic compo-
nent of the Settlement Layer, formed in a phreatomag-
matic fissure eruption in the Veiðivötn volcanic sys-
tem around 870 AD (Larsen, 1984; Grönvold et al.,
1995; Zielinski et al., 1997), is a prominent marker
layer that marks the transition from historical to pre-
historic times. It is readily distinguished in soil
profiles of the area studied by its greenish-grey to
greenish-black colour and its relatively high propor-
tion of plagioclase crystal fragments.
In all, seven prehistoric tephra layers found in
the study area have been dated by the radiocarbon
method, six felsic and one basaltic (RF4), and five ad-
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