Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 149
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the eruptions have started as fissure eruptions as they are
situated in a rift zone.
The fissure eruptions seem to begin after the shield vol-
canoes have ceased activity. This seems to indicate a certain
periodicity in the volcanic activity. Some of the eruptive fis-
sures have been active in historical times. Most likely Eld-
vörp are of historical date and possibly Stampar too. The
lavas from the fissures seem to contain less olivine than the
lavas of the shield volcanoes, however, some olivine is found
in most of these flows. The fissure eruptions are closely con-
nected with the tectonic features and in several places the
lava has poured out of the fissure without forming craters.
Gahbroic nodules (inclusions) have been found in many
places in the Reykjanes area, though not in the area main-
ly dealt with here. These inclusions occur in postglacial lavas
as well as in palagonite tuffs and breccias and in dykes.
The most remarkable localities include Hrólfsvík by Grinda-
vík, Graenavatn in Krýsuvík, Sveifluháls, ögmundarhratm,
Búrfellshraun, Hólmshraun, Litla Eldborg, Selhraun and
several others.
Olivine nodules have not heen found so far.
The most characteristic feature of the Reykjanes Penin-
sula is the rift zone (Fig. 2). It seems to he justified to say
that it is truly a part of the rift zone of the Mid-Atlantic
Ridge itself. The Reykjanes rift zone is about 30 km in width.
It is characterized by numerous fissures, faults and narrow
grabens running NE—SW. The faults are normal tensional
faults. The vertical displacement is as a rule small, 10 me-
ters or less, however displacements of 60—70 m are also
found in the Reykjanes area, e.g. in Hjallar east of Hafnar-
fjördur.
In the above mentioned graben south of Vogastapi, the
tectonic movement has been active for a long time. The inter-
glacial lavas are cut by faults and the vertical displacement
in them is 12—15 m. The oldest postglacial lavas are cut by
10