| Summary: | Paleomagnetic measurements were used to estimate the buildup time of hyaloclastite mountains such as tuyas. Tuyas are in general built up of a pillow lava under hyaloclastite, both formed in an underwater eruption. Above the hyaloclastite is a flow-foot breccia and a cap lava formed in a subaerial eruption. From the difference of measured paleomagnetic directions in the lava formations it is possible the estimate the minimum build-up time of the mountain. The research was done on the tuya Hlöðufell in Icelandic West Volcanic Zone, south of Langjökull. Results showed a considerable difference of paleomagnetic directions, or a 36° cumulative change from the pillow lava to the highest cap lava. Considering that the likely maximum 5° change of magnetic direction in one century, this indicates a minimum of 700 years for the formation of Hlöðufell. However, the results are not unambiguous since most of the difference occurs between the pillow lava at the base to the lowermost cap lava and from there to the higher cap lavas. Other explanations for the changes in measured paleomagnetic directions are possible, such as rotation of the lava blocks. More paleomagnetic measurements could help in narrowing the range of possible scenarios. However, the inaccessibility of the unmeasured lavas would make such surveys very difficult. Prior to this research, only two lava benches had been identified. In this research a clear lava bench was identified on the north terrace of the mountain underlain by what appears to be a flow-foot breccia. Several other, not as clear, smaller lava benches were also identified. This includes the south terrace where detailed analysis of the formations is required to decide on the true nature of these suspected lava benches. Í þessari rannsókn voru bergsegulmælingar notaðar til að varpa ljósi á hve langan tíma myndun móbergsstapa getur tekið. Móbergsstapar eru almennt byggðir upp af bólstrabergi og móbergi þar ofan á sem myndast við eldgos undir vatni. Ofan á móberginu er hraunfótsbreksía og ...
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