The study of global Earth structure by surface waves can now resolve wavelengths of about 1000 km. Phase velocity maps are detailed enough to test some plate tectonic models. Our results for Love waves indicate (1) continuous thickening and/or velocity increase with age of oceanic plates older than 100 million years, (2) failure of boundary layer scaling laws seen in the rate of thickening of oceanic lithosphere, and (3) asymmetric thickening of plates about ridges. The latter two points suggest that there is a wide variety of differences in the thermal states of oceanic lithosphere. Thermal anomalies, possibly caused by hotspots, are identified in 'residual' maps. ¿American Geophysical Union 1990