As you walk into Morsárdalur beyond Rauðhellrar an amazing area with the name Kjós begins to appear toward west. It is circular with the shape of a funnel. To better realize circumstances here one can take a look at the photogallery under Kjós. It is from the Kjós area that the light colored material in Morsárdalur originates. Once inside the Kjós you will notice a sharp change in color. The lower strata are dark and made up of basalt lavas whereas above these the rock is almost white, consisting of palagonite rhyolite. Both the dark and light colored rock is also seen in Rauðhellrar but in Kjós the rhyolite domintes by far and extends to the mountain tops. The rhyolite has formed as a result of repeated eruptions below ice in a central volcano.
Central volcanoses have a life span of some 1 million years and in their core a depression often forms. Such depressions are referred to as calderas and one such is present in the Öræfajökull volcano near by. However, in the Kjós volcano a caldera is not noticed and my be absent. This can be seen if one looks at the dark basalt layers at the base. It so happens that the dark basalt layers mark the base of the central volcano and one can here examine an unusual part of a big volcano. The weight of volcanics within the volcano was either not very great or the roof of the volcano did not give in when the rhyolite erupted, only subsided mildly. Haukur Jóhannesson, an Icelandic geologist, has described central volcanoes of this type, as one of four that is used to define such volcanoes in Iceland. The stratigraphic cross sections for Kjós are shown here below. Noteworthy is how the basalt lavas dip toward the Kjós center but being able to examine volcano interiors like this is not common.
In the Kjós inner part the ratio of intrusion is rather high. These are near vertical dykes, some of which are feeders to lavas on what used to be the surface here. Also, we note dipping “cone” dykes and near horizontal sills. Major intrusions are also present, their composition is probably of intermediate composition, or andesite (between basalt and rhyolite).
The interior of the Kjós volcanic center seen from air (photo Oddur Sigurðsson).
It appears that rhyolitic activity in the Kjós center began with explosive volcanism. It is therefore logical to ask whether the explosion caused caldera formation. From what can be seen in Kjós a process of sagging down through clear faults of the central part does not appear to have taken place. Still, the entire center is not exposed and therefore it is possible that further to the nortwest some caldera collapse may have occurred but this is regarded unlikely. The numerous thin inclined sheets (dark streaks) cutting the rhyolite are likely outlining the magma chamber roof but vertical fault movements along these are not observed.