34. Jórugil - Feeder dykes of tindars

Jórugil. Photo Kristján Sæmundsson.Jórugil is a ravine with a perennial stream flowing over large fault steps down to Hestvík in the southwest of Þingvallavatn. Along its course, the stream has cut three hyaloclastite ridges down into their base revealing the feeder dykes. Two of the ridges are of the tindar type and belong to the youngest of three groups that make up the Dyrafjöll complex southwest to Mt. Hengill. The third, which belongs to the oldest group, has been smoothed by glacial erosion.

1) Let us look at this oldest hyaloclastite ridge first. It forms a low mound of aphyric olivine tholeiite along the high wall of the Jórukleif fault. The compound lava of the interglacial Hæðir lava shield to the north forms the base of the hyaloclastite. A dyke, about 1 m thick, alternately of columnarly jointed basalt and glassy tuff, where water entered the conduit, cuts up through the shield lava and merges with the hyaloclastite.

2) Further up, the stream cuts through a low ridge connecting two topographically prominent tindar (Jórutindur-Litla-Sandfell). A 1 m thick dyke composed of two tiers of columns arched up towards the margins, separated by a thin glassy parting in the middle, cuts up through shield lava and moraine and merges with a suprastructure of pillow breccia. The rock of both, dyke and tindar, is feldspar-phyric.

3) The third connection of a feeder dyke to its eruptive product is exposed lower down the stream. There the feeder dyke transects a low ridge extending from Hátindur (the ending tindur is appropriate) forming a bench along the Jórukleif fault. The stream has eroded a 10 m deep gorge downstream of the intersection with the dyke. Here there are again two tiers of columns and a glassy parting in between. Here the base of the tindar is aphyric breccia of the same type as that erupted by the first dyke. It is overlain by about 5 m of glacial and alluvial sediments. The dyke cuts right through the hyaloclastite but splits in the sediments that were probably not yet consolidated at the time of the eruption. As of the dyke, the material of the tindar is feldspar-phyric and consists of pillow breccia. 

Kristján Sæmundsson, 2010

Map of Jórugil - Feeder dykes of tindars.