Jökull - 01.01.2015, Page 33

Grain characteristics of tephra from Katla and Hekla eruptions
The SiO2 content of the first erupted magma in-
creases with the length of the preceding eruption inter-
val. The magma composition also changes with time
during an eruption. This is e.g. seen in the charac-
teristics of tephra erupted after long repose when the
tephra layers have a greyish brown base and a brown-
ish black top (Thorarinsson, 1967, 1968).
Several Hekla eruptions have sent tephra over-
seas, in 1947 (Finland, Ireland), in 1845 (Faroe Is-
lands, Orkney Islands) in 1510 (Scotland, Ireland), in
1158 (Norway, Ireland) and in 1104 (Norway, Ireland)
(Wastegård and Davies, 2009 and references therein).
Figure 4. Isopach map of the Hekla-1947 tephra layer
(adapted from Thorarinsson, 1954). Sampling loca-
tions 2013, violet circles: (V-Ha) Vestan Hafrafells,
(Fl) Fljótsdalsheiði, (Ha) Hamragarðaheiði. Sampling
locations 1947, red circles (Thorarinsson, 1954): (Há)
Háahraun, (La) Langvíuhraun, (Ár) Árkvörn, (Hv)
Hvammur, (EHv) 4 km east of Hvammur and (Ve)
Vestmannaeyjar. – Þykktarkort af Heklu-1947 gjósku-
laginu. Fjólubláir hringir eru sýnatökustaðir 2013
og rauðir hringir sýnatökustaðir 1947, af nýfallinni
gjósku.
The Hekla 1947 tephra
The 1947-Hekla eruption started at 06:41 on March 29
in the summit of Hekla volcano. At 07:08 the eruption
plume had reached 30 km height (Þórarinsson, 1968).
Pumice fall began in Fljótshlíð, 30 km to the south,
around 07:10. The colour of the pumice was grey
brown and rather coarse. About half an hour later
the colour changed to brownish black and finer grain
size. The boundary between the grey brown pumice
(SiO2 64–61%) and the brownish black pumice (SiO2
58–56%) was sharp (Þórarinsson, 1968; Larsen et
al., 1999). A fine black tephra was deposited to-
wards the end. The maximum measured tephra thick-
ness was 100 cm some 1–2 km south of Hekla (top
crater). The 10 cm isopach extended into the farming
areas in Fljótshlíð about 30 km to the south and the
1 cm isopach extended off the south shore (Figures 4
and 5). In Vestmannaeyjar the thickness was 0.13 cm
(Thorarinsson, 1954, 1968).
Figure 5. Hekla-1947 tephra layer in soil section at
Hamragarðaheiði. – Hekla-1947 gjóskulagið í jarð-
vegi á Hamragarðaheiði (Ljósm./Photo. Edda Sóley
Þorsteinsdóttir, 2013).
The explosive phase can be classified as a Plinian
eruption (Þórarinsson, 1970). The total volume of
pumice and tephra was calculated to be ∼180 mil-
lion m3. About 3130 km2 of land were covered by
tephra and the volume on land has been calculated to
be ∼120 million m3 (Þórarinsson, 1968). Tephra fell
on ships 280 and 820 km from Hekla and reached Fin-
land 51 hours after the onset of the eruption (Thorar-
insson, 1954; Salmi, 1948).
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