Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 164

CHEMISTRY OF THE POSTGLACIAL LAVAS
IN THE KRÍSUVlK AREA, SW-ICELAND
EINAR GUNNLAUGSSON
National Energy Authority
Reykjavík, Iceland.
ABSTRACT
The chemistry of postglacial lavas in Krisuvik-area based on 51 chemical
analysis is briefly described. The area is one of the most volcanically active
areas in Iceland. All the lavas are tholeiitic basalts and all except two, (shield
volcanoes), are from volcanic fissures. All the fissure lavas are similar in com-
position, but the shield volcano lavas are slightly different.
One of the most active volcanic areas in Iceland is the Krísuvík-
area, situated near the middle of the Reykjanes peninsula, the land-
ward continuation of the Reykjanes Ridge (see insert in fig. 1). The
area under discussion is about 180 km2. Postglacial lavas originating
from within the area cover about 200 km2 of which about 130 km2
are shown on the map.
The oldest rocks in the area are lavas from fhe last interglacial
period found at Rrísuvíkurberg. Subglacial volcanic activity dur-
ing the last glacial period resulted in the formation of steep palago-
nite mountains and ridges. Two parallel palagonite ridges, Sveiflu-
háls and Núpshlíðarháls, form the most pronounced topographic
relief in the area. They trend in NNE-SSW same as the general
direction of faults in the area.
The area has been volcanically very active in postglacial times.
In early postglacial time two shield volcanoes formed. These are
most probably monogenetic. The volume of these shield volcanoes,
Hrútagjárdyngja and Strandarheiði, is much larger than that of
the later fissure eruptions. The lava from the shield volcano
Hrútargjárdyngja flowed towards the south between Sveifluháls
and Núpshlíðarháls where it is mostly covered by younger lavas.
The lava also flowed towards the west and north and all the way
to the northern coast at Vatnsleysuströnd. There the lava coalesces
with lava from the other shield volcano, Strandarheiði, on the
margin of the area under discussion.