Research School of Earth Sciences
Hotter mantle, no plates?
If the mantle was hotter, it would produce more melt at mid-ocean ridges and thicker oceanic crust. This might inhibit subduction, and require the upper thermal boundary layer to do something other than plate tectonics, such as these two possibilities.
In this scenario only the mantle part of the lithosphere is subducted (lower panel). In the modern mantle (upper panel) the thin oceanic crust is pulled down as well, by the cold, dense lithosphere, even though the crustal layer is less dense.
Soft, dripping boundary layer?
The thermal boundary layer is warm and deformable, yielding a stretching-and-compressing form of surface tectonics, instead of rigid plate tectonics.
Tectonic activity might also have been episodic.
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