Electrical conductivity and pore-space topology of Merapi lavas: implications for the degassing of porphyritic andesite magmas

Pore-space topology is studied in recent compact angular blocks and scoria fragments from block-and-ash flow deposits of Merapi volcano, Indonesia. Co nnected porosity and electrical conductivity of cored clay-free samples are measured in the laboratory at varying fluid salinity. The electrical forma tion factor, tortuosity and cementation index are derived and the pore-spac e topology is found to be porosity-dependent. The electrical flow pattern i s controlled by crack-like paths in compact angular block, and by vesicles geometry in more porous scoria fragments. Because Merapi lavas are highly v iscous and crystal-rich, we infer that the development of pore connectednes s and vesicle coalescence is promoted by shear strain. Along with ascent ra te considerations we conclude that degassing of Merapi magma occurs mostly in the conduit during ascent.