Pore-space topology is studied in recent compact angular blocks and scoria fragments from block-and-ash flow deposits of Merapi volcano, Indonesia. Connected porosity and electrical conductivity of cored clay-free samples are measured in the laboratory at varying fluid salinity. The electrical formation factor, tortuosity and cementation index are derived and the pore-space topology is found 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 fragments. 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. ¿ 2001 American Geophysical Union