The lunar steep-sided Gruithuisen and Mairan domes are morphologically and spectrally distinctive structures and appear similar to terrestrial extrusive volcanic features characterized by viscous magma. We use the basic morphologic and morphometric characteristics of the domes to estimate the yield strengths (∼105 Pa), plastic viscosities (∼109 Pa s), and effusion rates (∼50 m3/s) of the magmas which formed them. These values are similar to those of terrestrial rhyolites, dacites, and basaltic andesites and support the hypothesis that these domes are an unusual variation of typical highlands and mare compositions. The dikes which formed them are predicted to have had widths of ∼50 m and lengths of about 15 km. The magma rise speed implied by this geometry is very low, ∼7 × 10−5 m/s, and the Reynolds number of the motion is ∼2 × 10−8, implying a completely laminar flow regime. Estimates of emplacement duration range from one to several decades. These new calculations confirm the unusual nature of these features and support previous qualitative suggestions that they were formed from magmas with significantly higher viscosity than those typical of mare basalts.