This paper tests three hypotheses for the origin of the tectosphere. Continental collision cannot explain the low metamorphic grade of crust that predates the tectosphere. Halfspace cooling and buoyant underplating can both fit the diamond age data, although underplating by buoyant subduction is the favored model. Thermal models provide a further test. If halfspace cooling formed the tectosphere, diamonds from 150 km depth will be at least 200 m.y. younger than diamonds from 190 km. If buoyant subduction formed the tectosphere, diamonds from 150 km depth will be the same age or older than diamonds from 190 km. ¿ American Geophysical Union 1991