Náttúrufræðingurinn - 1977, Side 52

sure swarm which is made up of 12 erup-
íion sites with tholeiite lavas covering 54
km2. The three largest lavas, those of the
Seydishólar, Kerhólar and Tjarnarhólar
craters, are roughly contemporaneous.
Vigorous explosive activity in Seydishólar
led to the formation of the Seydishólar
tephra (0,09 km3) which fonns a good
marker horizon (Fig. 2) for the age deter-
mination of other lavas. The C14 dating
of the Kerhóll lava gave 6220 ± 140 years
B.P. (half-value period 5568 years), which
is in good agreement with the previously
suggested age of 50Ö0—6000 y., based on
tephrochronology. The new C14 age sup-
ported by tephrochronological data (Fig.
3), shows that the Grímsnes volcanic fis-
sure swarm was formed ahout 5500—6500
y. B.P. (Fig. 4).
Acknowledgement
Henrik Tauber, of the C14-dating La-
boratory of the Danish National Museum,
is tlianked for performing the C14 age
determination.
162