Analyzing the temperature trajectory during flash sintering, this study demonstrates that Joule heating alone is responsible for thermal runaway that is the most likely cause for rapid densification. The calculated onset temperature T on is in good agreement with the literature data. As predicted by the analysis the T on data follow a 1/ T on ~log(field) linear dependence, which affords an expeditious means to estimate the activation energy of resistivity/conductivity of the sintered ceramics.