Soil Surface Coatings at Costa Rican Recently Active Volcanoes

Abstract

AbstractUnvegetated slopes of recently active volcanoes in perudic Costa Rica have locally thin, hard, laminated coatings at the soil surface. These coatings, which cover both ash deposits and boulders, are actively forming in some locations and eroding in others. We investigated the chemistry and formation of these coatings. Optical analysis of the coatings shows that they consist of anisotropic ash particles embedded in isotropic, translucent, fine‐textured amorphous material. Scanning electron microscope analysis clearly shows secondary coatings covering and cementing primary mineral grains and rock fragments. In situ microchemical analysis shows that the coatings consist predominantly of silica. Primary mineral grains in well‐developed coatings may also be replaced by silica. The internal fabric of the coatings suggests that accumulation of secondary silica and deposition of (reworked) ash are simultaneous processes and that all coatings were formed in various episodes. Precipitation of secondary silica from solution may occur where rapid weathering of primary minerals (volcanic ash) combined with high deposition of strong acids (HCl, H2SO4) from fumarolic and solfataric activity results in high dissolved Si concentrations. This process is further enhanced by evaporative concentration at the soil surface. Low pH precludes formation of secondary aluminium silicates and gibbsite. Erosion of the coatings seems to be largely due to episodically transported water.