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Title: Why does Earth have plate tectonics while Venus keeps a single lid?
Plain: 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.
Precise: Convection with strongly temperature-dependent viscosity generically produces a stagnant lid. Identify the lithospheric weakening mechanism (water, grain-size reduction and damage, pore fluids) and the planetary parameters (surface temperature, water content, mass, interior temperature) that place Earth in the mobile-lid regime and Venus outside it. An answer is a regime diagram from a rheology calibrated on experiments that places both planets correctly.
Settled by: Three-dimensional spherical convection with lab-calibrated weakening rheology that reproduces plate-like Earth and lid-like Venus with no tuned parameters.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)