Náttúrufræðingurinn - 1984, Side 65

SUMMARY
Icelandic rock types III
Tholeiite
by
Sveinn P. Jakobsson
Icelandic Museum of Natural History
P. O. Box 5320
125 Reykjavík
The article describes the rock type
tholeiite, with the 1783-84 Lakagígar
(Laki) lava flow in S-Iceland as an exam-
ple. The eruption site is the 24,5 km long
crater row Lakagígar. The lava (Fig. 2) is
about 12 km3, but its chemical composition
is homogeneous. It is porphyritic with
plagioclase, augite and olivine. The
amount of phenocrysts varies somewhat,
plagioclase being about 3-5 percent,
augite 1—2 percent and olivine about Vi
percent. The groundmass consists of pla-
gioclase, augite, and euhedral ore. A
chemical analysis of the Lakagígar lava
(Table I) indicates a Si02 content of 50.37
percent and a MgO content of 5.70
percent.
Tholeiite is the most common rock
type in Iceland. It has been formed in all
known tholeiitic volcanic systems in Ice-
land and has a distribution similar to that
of olivine tholeiite (Sveinn P. Jakobsson
1984). It is estimated that about 30 percent
of the volcanic rocks produced during
Recent times in Iceland are tholeiite. If,
however, all volcanic formations since
Miocene are considered, some 50 percent
are probably tholeiite.
59