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Little is known about the impact of tephra deposits from explosive volcanic eruptions on soil formation and weathering processes in organic soils. The weathering of tephra of basaltic and rhyolitic origin and their impact on Icelandic histosols were studied through a combination of physical, chemical and mineralogical analyses. Two sampling sites were selected according to the presence of the light coloured rhyolitic tephra from the Or ae fajokull eruption in 1362 CE and a dark-coloured basaltic Veioivotn tephra from 1477 CE in the volcanically active area south of Vatnajokull in South East Iceland.

The determining factor of pedogenesis in the investigated histosols is the OM, but the influence of tephra and aeolian material from external sources must be taken into consideration. The soils are the result of altered plant residues and volcanic material (tephra and aeolian material). Plant remnants, as well as the soil itself, are protected from decomposition by the prevailing anaerobic conditions, a low soil pH and the repeated addition of inorganic matter. Clay formation is low, while metal-humus complexes are predominant. Fe-o/Fe-d ratios indicated a generally low degree of weathering, being higher close to the basaltic tephra.

The mineralogy was dominated by plagioclase and pyroxene, with quartz and zeolite as minor components. In contrast to previous research on Icelandic soils, our investigations revealed layer silicates at both sites. While we found evidence of smectite in the soils at Kalfafell, hydroxy interlayered minerals were found at Reynivellir. In the basaltic tephra, traces of layer silicates could be verified. In contrast, the rhyolitic tephra did not show any pedogenic minerals, suggesting that it had hardly altered since its deposition in 1362 CE. It is not the chemical composition of the inorganic parent material, but the location that may be an influencing factor on the formation of clay minerals in the investigated histosols.