SURFACE-COATINGS AT COSTA-RICAN RECENTLY ACTIVE VOLCANOS

Unvegetated 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 fo rming in some locations and eroding in others. We investigated the che mistry and formation of these coatings. Optical analysis of the coatin gs shows that they consist of anisotropic ash particles embedded in is otropic, translucent, fine-textured amorphous material. Scanning elect ron microscope analysis clearly shows secondary coatings covering and cementing primary mineral grains and rock fragments. In situ microchem ical analysis shows that the coatings consist predominantly of silica. Primary mineral grains in well-developed coatings may also be replate d by silica. The internal fabric of the coatings suggests that accumul ation of secondary silica and deposition of (reworked) ash are simulta neous 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 depo sition of strong acids (HCl, H2SO4) from fumarolic and solfataric acti vity results in high dissolved Si concentrations. This process is furt her enhanced by evaporative concentration at the soil surface. Low pH precludes formation of secondary aluminium silicates and gibbsite. Ero sion of the coatings seems to be largely due to episodically transport ed water.