The Glorious Geology of Iceland's Golden Circle by Agust Gudmundsson

Author:Agust Gudmundsson
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

11.7 Vertical Section Through Two Lava Shields

For the sixth stop (6) you need to find a good parking place; such a place is not so easily found as at the previous stop. Provided you find a good parking place for your car, the view to the north shows something really remarkable. You recall that there are many lava shields in the active volcanic zones of Iceland. In particular, you saw the lava shield Thrainsskjöldur on the way from the Keflavik to Reykjavik on the first day (Chap. 2) and the large Skjaldbreidur lava shield north of Thingvellir on the Golden Circle (Figs. 5.​14, 6.​1, 6.​2 and 6.​7; Chaps. 5 and 6). If you make the excursion to the Reykjanes Peninsula (Chap. 13) you will see more lava shields . The shields are all composed of pahoehoe lava flows . Exceptional section through young, that is, 9000-year-old, pahoehoe lava flow is provided by the west wall of Almannagja (Figs. 5.​5 and 5.​6).

However, looking north at the sixth stop you see the mountain Thyrill (Þyrill) which provides a vertical section through not one but through parts of two lava shields (Figs. 11.19, 11.20 and 11.21). The contact between the two shields is very sharp and easily seen, partly because the lower shield is much darker in colour than the upper shield (Figs. 11.20 and 11.21). The flow units can be distinguished in close-up (Fig. 11.21). The flow units of the younger shield, the light-grey one on the top (the upper shield), are clearly thicker than those that constitute the lower and older shield. The flow units are here much more altered than those in the walls of Almannagja, simply because the units in Thyrill have been the channels of groundwater and geothermal water for one or two million years whereas the Almannagja units are only 9–10 thousand years old. Because of the circulation of geothermal water through the rocks, there are numerous secondary minerals , particularly zeolites , in the rocks. In fact, there are secondary minerals in the rocks on both sides of the innermost part of Hvalfjördur, and you can even see them as white spots in the rocks at the present stop as well as when you drive on after this stop.

Fig. 11.19Thyrill (Þyrill) is a mountain composed primarily of two lava shields. The rocks of the upper one are grey, whereas those of the lower one are dark grey. In addition, there are some different layers, including tuff and breccia layers (hyaloclastites) , in the uppermost part of the mountain. View northeast, the contact between the main lava shields is shifted up and down across normal faults (compare Figs. 11.20 and 11.21)

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