<Li> Third, there was the problem of why some parts of the Earth's surface (crust) should have solidified while other parts were still fluid . Various attempts to explain this foundered on other difficulties . </Li> <P> 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Jack Oliver is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article "Seismology and the New Global Tectonics", published in 1968, using data collected from seismologic stations, including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 . </P> <P> It is now known that there are two kinds of crust: continental crust and oceanic crust . Continental crust is inherently lighter and its composition is different from oceanic crust, but both kinds reside above a much deeper "plastic" mantle . 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 . The theory of plate tectonics explains all this, including the movement of the continents, better than Wegener's theory . </P> <P> Evidence for the movement of continents on tectonic plates is now extensive . Similar plant and animal fossils are found around the shores of different continents, suggesting that they were once joined . The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus in rocks of the same age at locations in Africa, India, and Antarctica . There is also living evidence--the same animals being found on two continents . Some earthworm families (e.g. Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa, for instance . </P>

How long did it take for the continents to reach their current location