Náttúrufræðingurinn - 1972, Page 23

NÁTTÚRUFRÆÐIN GURINN
99
followed by a caldera collapse and the formation of a rhyolite breccia tvhich is
found mainly within the caldera but locally also on the rims. The escarpments
of an outer collapse are morphologically identifiable. An inner collapse fault
was identified where a thick rhyolite breccia butts against the truncated edges
of tlie oldest rhyolite flows. During the last glacial period volcanism continued
and the caldera began to fill with subglacial rhyolites and to a less extent
witli basaltic hyaloclastites. Simultaneously fluvioglacial and morainic debris
was deposited among the volcanic products. The subglacial rhyolites consist of
very distinct rock varieties ranging from granulated glass matrix to columnar
lithic lobes encrusted with pitchstone. Filling of the eastern part of the caldera
(which may be older) almost stopped during the glaciation period and erosion
has since carved out the valleys of Jökulgil and its tributaries. In the western
part of the caldera, volcanism persisted longer and rhyolite domes were erupted
on the caldera filling from ring fissures along the south margin mainly. On
the outer slopes of the central volcano NE-SW fissure volcanism occurred
during the glaciation period and the rhyolite piled up subglacially to form
ridges having this trend. To the northeast and southwest of the central vol-
cano the same fissure swarm produced hyaloclastite ridges of basaltic com-
position.
During Postglacial Time volcanism proceeded along a NE-SW fissure swarm
which crosses the western part of volcano. Four eruptive events produced scores
of small intermediate and acid flows from linearly aligned cylindrical vents.
Lavas of different composition erupted simultaneously frorn different parts of
the fissures as in the case of Veiðivötn (basalt), Námshraun (dacite), Hrafn-
tinnusker (rhyolite) and centers west of Laufafell (andesite). The caldera is the
site of vigorous thermal activity. A gravity survey has revealed a positive Bouguer
anomaly of 10 mgal coinciding with the caldera. This indicates the precence of
dense rocks, probably a concentration of dykes and intrusions underneath,
acting as a lieat source to the thermal system.