Saturday, 15 July 2006

Genesis of young volcanic soils from pyroclastic material in South Central Italy.

Claudio Colombo1, Vincenzo Michele Sellitto1, Giuseppe Palumbo1, and Fabio Terribile2. (1) Dip. SAVA Molise University, Via De Sanctis, Campobasso, Italy, (2) DISSPA UNIVERSITA' DI NAPOLI FEDERICO II, VIA UNIVERSITA' 100, PORTICI, Italy

The Phlegraean Fields were formed by about 30 different volcanic events, beginning approximately 30 ka ago in South Central Italy. The Phlegraean Fields have a very complex history and volcanic evolution, and consist of heterogeneous pyroclastic rocks. They were formed by two major collapses related to Campanian Ignimbrite. The eruptions were mostly phreatomagmatic, with subordinate magmatic explosions, and produced tephra fallout and pyroclastic materials, mostly surges, in a large area of about 50-100 km2. All the phreatomagmatic explosions generated large amounts of ash. The texture of the deposits of such explosions varies according to distance from the vent, consisting of ash to coarse-ash, surge beds and subordinate fallout layers. The eruptions formed large deposits of pyroclastic products such as the geologic formation called “Neapolitan yellow tuff”. The aim of this work was to study the mineralogy and geochemistry of four young volcanic soils developed on pyroclastic material located in the Phlegraean Fields. They are characterized by three different land uses: chestnut forest (Castanea sativa) (F1 and F2); downy oak forest (Quercus pubescens) F3 and fruit trees (F4). The four profiles were described and sampled both for chemical, mineralogical and micromorphological analysis. The thin sections were analysed by optical microscopy (OM) and selected areas were also observed by scanning electron microscope (SEM) and microanalysis (EDS) to detect the differences in the chemical composition. All grain size fractions were analysed by X ray diffractometry (XRD). The pedogenetic environment of the volcanic soils studied is particularly complex due to the heterogeneity of the pyroclastic materials stratified during the intense volcanic activity in the last 5000 years. The profiles have a good degree of soil development and their pH ranges from moderately acid to neutral. The soils shown very weak andic properties, low phosphate retention, low concentration of Al and Si extracted with ammonium oxalate, high values of CEC and exchangeable K. The DRX data of the clay fraction show that the soils have a very low content of phyllosilicates. Halloysite (hydrated and dehydrated phases) was detected in small amounts, its content generally increasing towards the surface horizons with a small and very broad XRD reflection at 0.78–1.1 nm; this indicates a poorly ordered phase. The clay fraction also showed detectable amounts of analcime and phillipsite. The specific peaks of zeolites (0.72 nm and 0.56 nm) were very sharp in the lower horizons while in the surface horizons they were present as small, broad XRD reflections. The sand fraction also contains biotite, leucite, sanidine and very small amounts of phillipsite and analcime. The intensity of the reflections of such zeolites is much lower than those of the same minerals in the clay fraction. This difference in intensity of the DRX patterns could be related to the great variability in crystal structure, grain size, Si/Al ratio and exchange-cation composition of zeolite minerals. Micromorphological analysis showed the rare presence of clay coatings in the C horizons along with pumice having a different degree of weathering. Chemical analysis obtained by SEM - EDS on the same sections showed that silicon had accumulated in the external parts of the pumice; such areas appeared optically isotropic to the OM while aluminium has a very different distribution from the silicon. In addition, other element mapping of the pumice in the thin sections chiefly showed a similar distribution of alkaline cations, indicative of moderate weathering. The pedogenesis of the young volcanic soils in the Phlegraean Fields appears to depend primarily on the short time-span of pedogenesis and then on the kind of pyroclastic material. More specifically, in the deep porous soils formed by fresh younger volcanic ash the most important pedogenic process is the formation of low amount of short-range-order aluminosilicates with the development of non-allophanic soils. Volcanic soils in the Phlegraean Fields show the abundant presence of primary minerals, glass, pumices, scoria, and unconsolidated tuffs with lesser amounts of allophane and zeolite in the fine fraction. A small amount of allophane and halloysite could be related to a young soil environment and strong drainage that removes Si from the soil solution, limiting the neogenesis of short-range-order aluminosilicate minerals The loss of bases during leaching of soils could be continuously replenished by the steady supply of bases from the zeolites. This could provide a chemical environment that prevents the formation of poorly ordered aluminosilicates.

Back to 1.3PA Andisols and Related Soils - Poster
Back to WCSS

Back to The 18th World Congress of Soil Science (July 9-15, 2006)