Onset and cause of plate tectonics

4 problems

• When did modern plate tectonics begin on Earth?

  Contested

  Proposed start dates range from more than 4 billion years ago to under 1 billion years ago. Old rocks are scarce and can be read in more than one way.

• How do new subduction zones start?

  Contested

  Plate tectonics needs plates to sink back into the mantle at subduction zones, but a strong intact plate resists being bent and broken to start sinking. Whether new subduction zones begin by themselves or are pushed by neighboring plates is debated.

• Why does Earth have plate tectonics while Venus keeps a single lid?

  Open

  Hot convecting planets naturally grow a stiff outer shell that does not break into plates. Something weakens Earth's shell enough to let plates form, and it is unclear what that is.

• Can lab-based rock rheology produce plate-like convection?

  Open

  Computer models only produce Earth-like plates if the outer shell is assumed several times weaker than lab rocks. Whether a realistic weakening process closes that gap is unknown.