Small particles in volcanic eruption clouds

Abstract

Small (<25 µm diam) particles directly sampled in volcanic eruption clouds of three Central American volcanoes consist chiefly of two kinds: dilute acid droplets and angular silicate ash. The acids are mainly H,SO, and HCI. The silicate ash is mantled with dilute acid which may have accumulated by condensation and by scavenging from the volcanic aerosol. The ash itself has a bimodal size distribu­tion with peaks at 2 to IO µm and 0.2 to 0.8 µm. The larger ash particles are mainly glassy, while the smaller population is mostly dilute sulfuric acid droplets and crystal fragments. We infer that the crystal fragments are produced by fragmentation of crystals with well developed cleavages. A major conclusion of the study is that crystal· rich magmas may produce more submicron-size ash than aphyric magmas.