>Continents consist of light material that has floated to the outer portions of the solid Earth and been incorporated into the crust. This material is too buoyant to be subducted into the interior of the planet to any significant degree, so once continental material had completely segregated, it remained at the surface as a kind of scum resting atop the churning cauldron of Earth's interior fluid motions. The continental material is constantly being pushed around, broken up and re-arranged in a process known as continental drift.
>The Earth is the only planet in the Solar System that has continents in this sense, but it is likely that extrasolar rocky planets of a sufficient size to retain the heat that drives internal motions, and having a surface temperature similar to Earth, would also exhibit the dichotomy of drifting continents vs. areas of rapidly recycled mantle material (analogous to Earth's sea floors).
>It is at present unresolved whether a water ocean is necessary to maintaining this state of affairs.
Why? What does water mean, on the scale of plate tectonics?
>The Earth is the only planet in the Solar System that has continents in this sense, but it is likely that extrasolar rocky planets of a sufficient size to retain the heat that drives internal motions, and having a surface temperature similar to Earth, would also exhibit the dichotomy of drifting continents vs. areas of rapidly recycled mantle material (analogous to Earth's sea floors).
>It is at present unresolved whether a water ocean is necessary to maintaining this state of affairs.
Why? What does water mean, on the scale of plate tectonics?
