Three roughly circular regions on Venus about 1000 km across are ident
ified as potential Hawaii-scale plume sites on the basis of their grav
ity (70-90 mgal) and topography (1.6-2.0 km) anomalies, and signs of m
elt generation and rifting. Axisymmetric isoviscous convection models
are used to reproduce gravity profiles across these plumes and the lin
e-of-sight acceleration of the Magellan spacecraft as it passes over t
hem. The best fitting models have a conductive lid thickness of less t
han 150 km, mantle viscosity of 8.9 x 10(19) to 9.6 x 10(20) Pa s and
a basal heat flux of 15-25 m W m(-2). The lid thickness is constrained
by requiring a modest amount of melt generation and a potential tempe
rature of about 1300 degrees C. The high mantle viscosity relative to
that of Earth is probably a consequence of the absence of water in the
mantle, and may help to explain the current absence of plate tectonic
s on Venus. The low heat flux suggests that the thermal evolution of V
enus has differed from that of the Earth.