Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 151

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It appears that the width of the faults and fissures varies
with rock type, being more open in the case of postglacial
lavas and the palagonitic rocks. It is possible that they are
more open in the stratigraphically younger rocks and if so
what does that mean?
DISCUSSION
S. Thorarinsson: Are the shield volcanoes on Reykjanes
monogenetic, viz. each one built up by a single eruption?
J. Jónsson: I think that the shield volcanoes on Reykjanes
geologically speaking are formed in one eruption and the
eruption may have lasted for several years just as the Surts-
ey eruption.
S. Thorarinsson: This is then an olivine phase similar to
the one we are having in Surtsey?
J. Jónsson: Yes.
S. Thorarinsson: The difference between a fissure erup-
tion and a shield volcano eruption need not be great. Tephro-
chronology would be helpful in dating fissure systems and
finding out how fast they are formed. Does not nearly every
effusive volcano start as a fissure volcano?
J. Jónsson: Not necessarily. The Búrfell crater east of
Hafnarfjördur is a single (mono) vent although it lies in a
strongly faulted zone. (See Fig. 3.) Several craters of the
same type are known on the Reykjanes Peninsula, e.g. south
of Thórdarfell and SE of Sandfellshaed.
Trausti Einarsson: The main postglacial fractures were
formed soon after the active period of the shield volcanoes.
After that, fracturing has been much less. Thus the shearing
movement in postglacial time appears to have been spasmo-
dic, not continuous. Different reaction of lava fields and the
older palagonite rocks to the movement along principal frac-
tures is of interest for the interpretation of the movement.
S. Thorarinsson: If the volcanic activity has not been con-