The role of volcanic activity and climate in alluvial fan growth at volcanic areas: an example from southern Campania (Italy)

Volcaniclastic-rich alluvial fans developed in the southern Campanian Plain (Italy) during the late Pleistocene and Holocene in an area eastward of the Somilia-Vesuvius and Campi Flegrei volcanoes. Meanwhile, bedrock-rich alluvial fails developed in areas unaffected by pyroclastic deposition. Late Pleistocene and Holocene volcaniclastic-rich alluvial fails show some important differences: (i) late Pleistocene alluvial fails were dominated by hyperconcentrated flow deposits, whereas the Holocene ones were dominated by debris flows deposits; and (ii) late Pleistocene fails consist of several Superimposed sedimentary bodies, characterized by homogeneous volcaniclastic material, whereas Holecene fails show either volcaniclastic bodies with homogenous lithology or mixed lithology (i.e., juvenile fractions eroded from different tephra layers). These differences are not related to the amount of volcaniclastic supply in time, but Seem to be linked to changes in climatic condition between late Pleistocene and Holocene. Rapid remobilization of the pyroclastic material was favored by climatic and vegetation conditions of the study area during the late Pleistocene, when a semiarid setting dominated by steppe-like vegetation prevailed. During Holocene, the general increase in temperature and humidity favored vegetation and soil development and stabilization of the loose volcaniclastic materials. Thus, part of volcaniclastic material was stored in the catchments and was available for erosion a long time after an eruption. Shallow soil slips, active also today, generated volcaniclastic debris flows characterized by mixed lithology, of pumice and scoria. (C) 2004 Elsevier B.V. All rights reserved.