Plate Tectonics is the process of creating new crust at mid-ocean ridges and recycling older, denser crust back into the interior at subduction zones. Subduction is driven by two main mechanical processes: slab-pull and ridge-push.

RIDGE PUSH

High ridges are produced by the positive buoyancy of material in regions of localized heating. Ridge-push utilizes the potential energy of the height of these ridges to force slabs to subduct.

SLAB-PULL

Slab-pull depends on the negative buoyancy of the cold subducting slab to pull itself deeper into the warm interior. Of the two, slab-pull typically dominates.

Quantifying the ability of the lithosphere to subduct through these two mechanisms is important to understanding the ability of the crust to recycle. For example, Europa has a very young surface, but the capability of the cold, brittle ice to subduct into the warmer ice asthenosphere is still hotly debated^1,2.

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1. Howell, Samuel M. & Pappalardo, Robert T., 2019. Icarus.”Can Earth-Like Plate Tectonics Occur in Ocean World Ice Shells?” 
2. Selvans, M. 2014. Nature. “Plate Tectonics on Ice”