Micromorphology of soils derived from volcanic ash in Europe: a review and synthesis

Summary An analysis of the available literature on European volcanic ash soils has been made. Most has been published in Congress proceedings and Journals of national societies, less than half in English. More than half of the papers deal with soils on the Canary Islands. Many papers focus only on one aspect, and complete descriptions are found only in recent publications. Often the use of vague micromorphological terminology and the absence of analytical data make interpretation and comparisons difficult or impossible. Nonetheless, some general features do emerge. In the least‐weathered soils, the micromass forms first coatings around the coarse constituents, later granules, finally giving rise to a loose granular microstructure in the topsoil, and a compacted granular one in the Bw horizons of typic andic materials. The b‐fabric of the micromass is undifferentiated. With increasing weathering, clay coatings appear, the microstructure becomes more blocky and the b‐fabric becomes speckled or even striated. In Vertisols and Aridisols, micromorphological characteristics are not much different from those in the comparable non‐volcanic soils, except generally for the tendency to granular (intrapedal) microstructures. In Icelandic soils, microstratification, lenticular microstructure and preservation of plant residues as a result of cryic conditions and permanent volcanic and aeolian activity are observed. Weathering of volcanic glass in well‐drained conditions yields allophane alteromorphs similar to palagonite, which are easily subject to fragmentation by pedoturbation. The possible influence of surface transport and burial on the formation of Bw‐horizons in andic materials is discussed.