32. Tjarnarhnjúkshraun við Ölfusvatnsá - Xenolithes

Map of Tjarnarhnjúkshraun við Ölfusvatnsá - Xenolithes.The scoria cone of Tjarnarhnúkur is rather unique being situated on the crest of a tindar type volcanic range and being the only postglacial member.

It erupted a highly porphyritic lava, which extends about 5 km north along river Ölfusvatnsá on to a lake terrace of juvenile Þingvallavatn at 60 m above the present lake level.

The lava is significantly weathered and broken up by frost action at high ground and covered by outwash and thick soil at lower ground. Ölfusvatnsá has cut a 10-20 m deep and 1500 m long gorge into moraine and hyaloclastite. As soil seems to be lacking at its base, it is concluded that its age is early Postglacial, probably around 11,000 years. The lowest part of the gorge is cut into the pillow lava of Mælifell. The rock type is picrite crowded with olivine phenocrysts, which are enriched in the lowest part of the pillows from crystal settling. Crystal settling of plagioclase xenocrysts (bytownite) can also be observed in a porphyritic pillow lava in the same gorge north and south of the Mælifell unit. The lava is rich in gabbroic xenoliths, which are best seen in the section along Ölfusvatnsá. Their mineralogy varies depending on which primary phase predominates, but most common are feldspar- and olivine-rich ones. Volcanic bombs of the crater often contain gabbro xenoliths as their cores. Most likely the solidification of the magma body was well on its way when it was remobilised and erupted.

Kristján Sæmundsson, 2010