tephra SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total Ref Hekla 4 Tephra 75.69 0.11 12.78 2.01 0.05 0.02 1.40 4.22 2.80 99.10 853 ------------------------------------------------------------------------ 853: Dugmore, A.J., Larsen, G, and Newton, A.J. (1995) Seven tephra isochrones in Scotland. The Holocene 5/3, 257-266.