« on: 19/07/2019 21:23:21 »
alancalverd is correct about rates of movement changing over time. And while the theory of continental drift has been replaced by plate tectonics, continents do indeed exist and have been persistent over long periods of time. Continental crustal material (sial) is much thicker and lighter (due to magmatic crystallization and segregation) than the heavier, thinner oceanic crust (sima). So continents tend to "float" higher on the mantle- which also makes them more difficult to subduct or destroy. Parts of most of the continents are billions of years old, while the maximum age of almost all the oceanic crust is less than 200 million years.