Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 87

Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 87
85 Faint glacial stríae are found at a few places. On the whole the surface rises towards the centre of the cap but no traces of a crater are seen. The surface is cut by a SW-NE fault- line, with the east side a little uplifted, and the summit is formed hy the tectonically broken highest part of that wall. These facts, coupled with the described encroachment of the lavas on a low tuff-breccia mound, suggest that the lava cap did not originate from eruptions on the mountain. We then come to the following picture of the origin of Hrafnabjörg. On a plain with fluviatile and eolian sands, some 50 m or less ahove present sea level, was piled up vol- canic tuff-hreccia by explosive volcanism. Slumping in this mass was common and led to unstratified masses of hreccia alternating with undisturbed primary stratified tuff-breccias. At least some of the material originated within the area of the present mountain and tuff layers, flattened homhs, and lava tongues were deposited over the slopes left by slumping. Perhaps there was no central crater hut rather a number of orifices fed by the numerous dykes. In the end a roughly horizontal plain resulted. Before the uplift of the mountain this plain has been at an elevation of 300 m or less. In the next stage lava flows cover the surroundings and encroach on the slopes of a low mound of tuff-breccias, finally overriding and covering completely the mound. These lavas may afterwards have been much eroded and scoured by gla- ciers prior to the last stage in the development: the uplift of the present mountain by some 300 m or more, and more in the centre than at the periphery. It is likely that this lava phase corresponds to that of Ármannsfell and that both mountains were uplifted in the same tectonic phase. The lower tuff-breccias of Hrafnabjörg may possibly correspond to those forming the core of Ár- mannsfell but it seems more likely that they are younger and represent the filling material in the graben formed late to the east of Ármannsfell. AU the primary rocks in Hrafna- björg are of normal polarization and quite possibly belong
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Rit (Vísindafélag Íslendinga)

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