Gray body temperatures and emittances of silica glass under shock compression between 10 and 30 GPa are determined. Observed radiative temperatures are higher than computed continuum temperatures for shock-compressed silica glass; however, below ~26 GPa observed emittances are <0.02. This suggests that fused quartz deforms heterogeneously in this shock pressure range as has been observed in other minerals. Between 10 and 16 GPa, radiative remperatures decrease from 4400 K to 3200 K, whereas above 16--30 Gpa, gray body temperatures of ~3000 K with low emittances are observed. The emittances increase with pressure from 0.02 to 0.09. The pressure range from 10 to 16 GPa coincides with the permanent densification region, while the 16--30 GPa range coincides with the inferred mixed phase region along the silica glass Hugoniot. The differing radiative behaviors may relate to these modes of deformation. Based upon earlier shock recovery experiments and a proposed model of heterogeneous deformation under shock compression, the temperatures associated with low emittances in the mixed phase region probably represent the melting temperature of the high-pressure phase, stishovite, which can be expected to crystallize from a melt in hot zones. Above 20 GPa the melting temperature of stishovite would therefore be 3000 K¿200 K and almost independent of pressure to 30 GPa. The effects of pressure on melting relations for the system SiO2-Mg2SiO4 are considered together with the proposed stishovite melting curve and suggested maximum solidus temperatures within the mantle of ~2370 K at 12.5 GPa and ~2530 K at 20.0 GPa. Using the proposed stishovite melting temperatures Tm and estimates of upper mantle temperatures T, the effective viscosity, which can be considered a function of the homologous temperature T/Tm, appears to remain nearly constant from 200 to 600 km depth in the Earth. ¿ American Geophysical Union 1989