Abstract. Pore-space topology. is studied in recent compact angular blocks and scoria fragments •¾om block-and-ash flow deposits of Merapi volcano, Indonesia. Connected porosity and electrical conductivity of cored clay4¾ee samples are measured in the laboratory at varying fluid salini,ty. The electrical •brmation fiactor, tortuosity and cementation index are derived and the pore-,space topology is •bund to be porosity-dependent. The electrical flow pattern is controlled by crack-like paths in compact angular block, and by vesicles geometry in more porous scoria •¾agments. Because Merapi lavas are highly viscous and crystal-rich, we infer that the development of pore connectedness and vesicle coalescence is promoted by shear strain. Along with ascent rate considerations we conclude that degassing of Merapi magma occurs mostly in the conduit during ascent.