We compare Arecibo dual-polarization radar image data for Venus to Magellan images and emissivity data to investigate the physical properties of volcanic deposits. Radar waves can easily penetrate smooth mantling layers such as ash, aeolian and crater-derived deposits. If a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales, the vertical component of the wave will be preferentially transmitted, resulting in a net linear-polarized echo component. Arecibo polarimetry data were used to create maps of the degree of linear polarization in the radar echo. We find that some volcanic fields in plains regions on Venus are associated with enhanced linear polarization. These fields sometimes have nearby windstreaks which suggest fine-grained surface material, and we infer that the radar wave is penetrating into mantling deposits that are a few centimeters to ~1 m thick. Enhanced linear polarization values are also correlated with specific lava flows. These lava flows have emissivity values of 0.80 to 0.84, similar to many other flows on Venus. The enhanced linear polarization may be produced by penetration of the radar wave into very smooth lava flows with internal air gaps. High-reflectivity, low-emissivity areas near the summits of Theia and Tepev Montes also have a linearly polarized echo component consistent with surface penetration by the radar wave. The cause of the high linear polarization in summit regions remains uncertain, but perhaps the radar wave is able to penetrate into high dielectric material in limited cases of very smooth surface texture.