Stereo and photoclinometry derived topography of shield-like volcanoes on Io indicate little relief (<3 km) and very low slopes (0.2° to 0.6°). Several shield volcanoes appear to be associated with broad rises of 1 to 3 km, but only 5 shield volcanoes have been identified with steep flank slopes (between 4° and 10°). These steep slopes are restricted to within 20-30 km of the summit, but where discernable, most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree. Despite their abundance, ionian shield volcanoes are among the flattest in the Solar System. The steepest volcanoes on Io are most comparable to large venusian shield volcanoes. Using simplistic Bingham rheologies we estimate the viscosity and yield strengths of ionian lavas. Yield strengths are estimated at 10 1-10 2 Pa, lower than most basaltic lavas. Viscosity estimates range from 10 3 to 10 5 Pa s, although these are probably upper limits. Actual values may have been as low as 10 0 Pa s. Viscosity is sensitive to flow velocity, which is poorly known on Io. The best constraint on flow velocity comes from observations of the 1997 Pillan eruption, which bracket the eruptive phase to 132 day maximum, and more probably less than 50 days. Low slopes, long run-out distances and our estimated rheologic properties are consistent with (but not proof of) a low silica, low viscosity, high temperature composition for ionian lavas, supporting arguments for low-silica lava compositions such as basalt or komatiite. We cannot eliminate sulfur on rheologic grounds, however.