Chapter 25 - Regoliths and Soils on Volcanic Ash

Abstract

Soils on volcanic ash have specific physical and chemical properties, as well as typical micromorphological characteristics. They are characterised by an extensive accumulation of short-range order Si-Al compounds, such as allophane and imogolite, formed during weathering. This results in a grouping at the highest taxonomic level as Andosols (WRB) or Andisols (USDA) and in the use of ‘andic’ designations at other levels. The microstructure evolves from coarse monic in fresh ashes over chitonic and enaulic to granular in Andosols. A weakly separated intrapedal granular microstructure may persist in Bw horizons with a blocky microstructure or with a lenticular (freeze-thaw) microstructure. In Vertisols on volcanic ash a blocky microstructure is observed. The coarse material generally includes large amounts of volcanic glass. Fresh ash can already be transformed by hydrothermal alteration, resulting in the presence of secondary products. In the first stages of pedogenic weathering, short range order Si-Al compounds are formed. Pedogenic weathering of basaltic glass can produce palagonite-like alteromorphs of allophane. Magnetite and ilmenite can persist even in strongly weathered soils. Phytoliths and fungal hyphae are generally very abundant. The b-fabric is undifferentiated as long as short range order clays and halloysite are predominant, and it becomes stipple- or mosaic-speckled when kaolinite or illite appears. Micromass coatings around coarse fragments, and internal hypocoatings in the case of pumice, develop during a fist stage of soil formation. Other textural pedofeatures, except for rare allophane coatings, are generally not present in Andosols, but appear in intergrades to Luvisols, often still composed of optically isotropic compounds. In certain duripans (Tepetate), clay coatings with opaline zones are observed. Gibbsite coatings and nodules occur in conditions with strong leaching. Opal nodules are common in many soils formed on volcanic ash.