The shape of seven large domes on the plains of Venus, with volumes between 100 and 1000 km3, is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface. Both the altimetric profiles and the horizontal projection of the line of intersection of domes on the synthetic aperture radar images agree well with the theoretical similarity solution for a Newtonian fluid but not with the shape calculated for a rigid-plastic rheology nor with that for a static model with a strong skin. As a viscous current spreads, it generates an isotropic strain rate tensor whose magnitude is independent of radius. Such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes. The stress induced by the flow in the plains material below is obtained and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes. The viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot and lies between 1014 and 1017 Pa s. Laboratory experiments show that such viscosities correspond to temperatures of 610° to 700°C in dry rhyolitic magmas. These temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite. These results show that the development of the domes can be understood using simple fluid dynamical ideas and that the magmas involved can be produced by wet melting at depths below 10 km, followed by eruption and degassing.
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