Fróðskaparrit - 01.01.1998, Side 189

HOLOSENAR TEFRAFLÁIR f FØROYUM
195
attention to a cautionary event during the
1947 AD eruption of Hekla (Thórarinsson,
1954). Tephra from the 1947 AD eruption
fell over a large area of southern Finland,
where contemporary reports of dust fall
were backed up with analysis of the fall-out
that showed it to be volcanic glass (Salmi,
1948). As the eruption cloud was passing
Denmark there were also reports of fallout
from Jutland. Analysis of this material
showed it to be mineral sand and not vol-
canic ash, showing the caution that has to
be used when interpreting dust fall records
even when they coincide precisely with ei-
ther an eruption or the likely passage of an
eruption cloud.
In the case of the seventeenth and eigh-
teenth century eruptions of Katla no physi-
cal evidence in the palaeoenvironmental
record has yet been found to support distal
fallout. The historical records of black dust
smelling of sulphur are most suggestive
(Mitchell 1757 in Thórarinsson 1981 b), but
may be due to other factors. It is possible
that the Holocene basaltic tephra eruptions
in Iceland, while the most frequent form of
volcanic activity and, on occasion, most vo-
luminous, may not have lead to any signif-
icant distal tephra fall. Basaltic eruption
columns generally lack the height and in-
tensity of more silicic events, so may not
have crossed the critical atmospheric
thresholds permitting large scale dispersal.
Extensive studies of mid to late Holocene
peat profiles in Scotland by Dugmore et al.
(1995a) only produced records of tephra
layers with a silicic content >57% (‘ande-
sites’ according to Le Maitre, 1989). Data
that strongly suggests basaltic tephras have
not been deposited on any significant scale
over the British Isles during the last 7000
years. There are two notable Holocene ex-
amples of far-travelled basaltic tephras
from Iceland, but they are likely to repre-
sent special cases, unrepresentative of ex-
plosive Holocene basaltic eruptions in gen-
eral. The basaltic Saksunarvatn tephra does
reach NW Europe, but it was erupted on an
enormous scale during a major deglaciation
of Iceland, and because of this timing it is
unlikely to be similar to any later Holocene
event. Traces of the Eldgjá eruption of mid
930s AD have been found in the Greenland
icecap (Zielinski et al., 1995). This erup-
tion is also special as it is the largest known
basaltic fissure event in the world in record-
ed history, producing c.16 km3 lava and
tephra (Miller, 1989). Despite its unusual
size, in distal areas traces of fall-out may
only be detectable in ice cores.
Even though there is little evidence for
the repeated, wide-spread dispersal of
basaltic tephra from Iceland during the
Holocene, one possibility is that basaltic
tephra may have spread as far as Faroe (up
to 500 km from the source areas) but not as
far as mainland Europe (>1,000 km from
the source areas). However, until physical
evidence of tephra fall from the Katla erup-
tions in 1625 AD, 1660 AD, and 1755 AD
are found outside Iceland, the case for the
distal fallout, whether in the Faroe Islands
or elsewhere, must remain rather uncertain.
The key to resolving the status of histor-
ical-age tephra-falls in the Faroes lies in the
development of Persson’s pioneering
(1968) approach as recently undertaken in
the British Isles; the first step being the ex-