DECIPHERING the geological evolution of the Earth’s continental crust requires knowledge of its bulk composition and global variability, The main uncertainties are associated with the composition of the lower crust, Seismic measurements probe the elastic properties of the crust at depth, from which composition can be inferred, Of particular note is Poisson’s ratio, sigma this elastic parameter can be determined uniquely from the ratio of P- to S-wave seismic velocity, and provides a better diagnostic of crustal composition than either P- or S-wave velocity alone(1). Previous attempts to measure sigma have been limited by difficulties in obtaining coincident P- and S-wave data sampling the entire crust(2). Here we report 76 new estimates of crustal sigma spanning all of the continents except Antarctica, We find that, on average, sigma increases with the age of the crust, Our results strongly support the presence of a mafic lower crust beneath cratons, and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions, followed by magmatic underplating, or a model in which crust formation processes have changed since the Precambrian era.