Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 196

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composition. The volume of rhyolitic tephra as fresh fallen
is four to five times and the volume of the basaltic tephra
about three times the volume of dense rock of the same com-
position.
Studies of Hekla (Thorarinsson 1954, 1967, Tómasson
1967) have revealed that the bulk of the tephra produced
by an eruption which may last a year or more is produced
during an initial, Plinian phase of the eruption, lasting usu-
ally only a few hours, and this tephra is to a great extent
rhyolitic or dacitic, whereas the tephra produced later in the
eruption is mainly andesitic.
The first historical eruption of Hekla, 1104 A.D., produc-
ed about 0.5 km3 of tephra (2.5 km3 as fresh fallen) and that
tephra was rhyolitic or nearly so. The second biggest tephra
production was in the 1300 eruption, 0.125 km3 (0.5 km3 as
fresh fallen). The 1766/68 eruption ranks as third with 0.1
km3. Altogether the 14 Hekla eruptions have produced a
little more than 1 km3 of tephra, whereof nearly 50% is
rhyolitic, the rest dacitic and andesitic.
We know less about the volume of Hekla’s historical lava
flows as lava of only 7 eruptions has been identified. The
total volume of the lava produced by the last three eruptions
is about 2.7 km3 or an average 0.9 km3 per eruption. This
is doubtless more than the average of all 14 eruptions (the
1104 eruption probably did not produce any lava) and the
total volume of lava produced by Hekla in historical times
is probably not far from 8 km3. A part of this lava is dacite,
but the bulk of it is andesitic. The biggest lava production
was by the 1766/68 eruption, about 1.3 km3, and the vo-
lume of lava and tephra of this eruption is the third biggest
in Iceland in historical times, the biggest ones being the Laki
1783 (12.5 km3) and Öræfajökull 1362 (2 km3).
Let us then turn to Katla, the second biggest producer. The
total number of historical eruptions in Katla or the Katla
area is according to Thoroddsen 14. Tephrochronological stu-
dies have revealed that Katla had a big eruption about 1000