Jökull - 01.12.1972, Síða 28

crater row now named Tröllagígar by the
authors, and formed the lava flow now namecl
Tröllahraun.
The crater row Tröllagígar stretches from
the Mókollar hills towards SW, nearly to the
peak Dór (cf. the map Fig. I). The distance
between the NE and SW ends of the crater
row is 16 km, but as seen frorn the map it is
far from continuous, but consists of 10 separate
sliorter rows designecl A—J on the map, and
with an aggregate length of 6 km. The total
nurnber of separate vents is about 60, the
largest craters are found within the rows A, B
(Figs. 2 and 3) and I (Fig. 7), which also have
been the main producers of lava. The crater
row series A—F and G— J havé, each of them,
the average direction N 50° E, but between
them is a sinistral en echélon arrangement.
The lava flow Tröllahraun has a total area
of 28 km2. Its volume is estimated at 0.3 km3.
Since the Lakagígar eruption of 1783 only
three eruptions in Iceland have produced more
lava, viz. Hekla 1845 ancl 1947 and Surtsey
1963—67. The production of tephra in the
Tröllagígar eruption was small.
Although basically a helluhraun (pahoe-hoe)
lava when leaving the craters great parts of
the Tröllahraun lava are of the apalhraun (a-a)
type, with a very clinkery surface. The rnain
vegetation on the lava flow is the lichen Stereo-
caulon vesuvianum which lends to the lava a
characteristic light grey colour (cf. Fig. 8).
Characteristic for the 1862—64 eruption was
a bluish haze and fine dust in the air affect-
ing grass and grazing animals. Although it did
not do much harm it was somewhat reminiscent
of the notorious haze accompanying the Laka-
gígar eruption of 1783.
Chemical analyses of the Tröllahraun lava
are presented in Table 1 along with analyses
of other recent lavas from Iceland for com-
parison. The Tröllahraun lava has a relatively
primitive tholeiitic chemistry, with low titania,
potassium and iron, but relatively high mag-
nesia. As compared to the Tröllahraun lava,
the Lakagígar lava farther south on the eastern
volcanic zone is more differentiated, as shown
by higher iron and potassium content and
lower magnesia, and the same applies to the
1961 Askja lava which has undergone a rather
extreme iron enrichment, possible due to pre-
26 JÖKULL 22. ÁR
cipitation of forsteritic olivine. On the south-
ernmost extreme of the eastern volcanic zone
alkali lavas have been produced in recent times
as shown by the analyses of a lava from Surts-
ey. A gradual change in chemistry frorn alkali-
basalts to tholeiites going north from Surtsey
was first observed by Jakobsson (in prepara-
tion) as well as the occurrence of alkali basalts
on the Snæfellsnes peninsula. The western
volcanic zone, on the other hand, has produc-
ed only tholeiites in postglacial times. The
chemistry of the Tröllahraun lava thus falls
naturally into what appears to be a regional
distribution-pattern of basaltic lavas in Iceland.
It is, however, more primitive in character
than other known tholeiitic lavas produced in
historical tirnes in Iceland.
The Tröllahraun lava is a fine grained rock
with abundant microphenocrysts of olivine,
diopsidic pyroxene and plagioclase latlis and
sparse macrophenocrysts of plagioclase. The
texture is granular with fully crystallized
ground rnass of the above minerals and ore.
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Jakobsson, Sveinn: Ritgerð í undirbúningi.
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