Materials that issued from an unusual Venusian volcano produced (1) a complex domical structure about 100 km across with thick, broad flow lobes up to 41 km wide, (2) an extensive sheet of thick flows, and (3) radar-bright surfaces that extend to 360-400 km from the volcano. Altimetry indicates that the relief of the domical structure is about 0.5--1.1 km. The lobes and flows have prominent regularly spaced ridges ~686--820 m apart. Thick flows with large ridge separations and broad lobes are rare on Venus. We suggest that the viscosities these flows were larger than those of most lava flows on Venus. Comparisons of the dimensions of the volcano's lobes with lava flows on Earth suggest that the Venusian lavas may have large silica contents. Radar-bright surfaces around the volcano may represent the result of an explosive eruption or very thin deposits of low-viscosity lavas. Thus we suggest that the radar-bright surfaces and lavas of the volcano were derived from a magma that differentiated within the crust or mantle of Venus. The differentiation produced (1) a gas-rich low-viscosity phase, (2) high-viscosity lavas, and (3) a residual primary magma. ¿ American Geophysical Union 1992