Geophysicist Jack Oliver is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article "Seismology and the New Global Tectonics", published in 1968, using data collected from seismologic stations, including those he set up in the South Pacific. The modern theory of plate tectonics, refining Wegener, explains that there are two kinds of crust of different composition: continental crust and oceanic crust, both floating above a much deeper "plastic" mantle. Continental crust is inherently lighter. Oceanic crust is created at spreading centers, and this, along with subduction, drives the system of plates in a chaotic manner, resulting in continuous orogeny and areas of isostatic imbalance.