Fjölrit RALA - 15.06.2004, Page 24

hydroxides point to a mobility of iron. Isotropic allophane coatings and anisotropic clay
coatings are indications of a more advanced stage of weathering and soil formation.
The profiles of Tenerife (N10-N12) are clearly polygenetic. The microstructure grades from
granular in the surface horizons to angular blocky in the solum, except when layers of loosely
packed lapilli are intercalated. The glass is commonly weathered to an orange isotropic
material. In general, rock fragments and minerals show a considerable degree of weathering.
The micromass, sometimes dominant, still has an undifferentiated b-fabric, which locally
becomes weakly stipple speckled, pointing to the formation of phyllosilicates.
With progressing weathering and soil formation, microstructure evolves from single grain
over granular to blocky. In general a high biological activity is noticed, except in the profiles
of Iceland, where microstratification is preserved throughout the profiles. Only in an
advanced stage of weathering the undifferentiated b-fabric changes to a speckled one. The
orange alteromorphs of volcanic glass have an allophane like composition and strongly
influence the results of selective oxalate extractions of Si, A1 and Fe. The coatings of
micromass around pyroclasts observed in most profiles are typical for these volcanic ash soils.
In several profiles indications are found of a polygenetic soil formation and truncated
palaeosols.
References
Stoops, G. 2003. Guidelines for the Analysis and Description of Soil and Regolith Thin Sections.
SSSA. Madison, WI., 184 pp
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