“The height of the blobs is mostly controlled by how dense they are and the viscosity of the surrounding mantle.”
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In other words, a kind of buoyancy, if I take this correctly.
Slow moving fluid dynamics
I am thinking Lake of Fire.
It’s a battle of buoyancy vs gravity... two opposing forces that all matter on the planet seeks an equilibrium point between. That one blob is “unstable” means that it has not reached a stable equilibrium between the two and that is why it oscillates in height/depth. It may be oscillating towards an eventual equilibrium point, where it will become stable, or it may continue oscillating forever if the conditions keep changing so that it can’t balance out those two forces at any particular resting point.