If only the crust displaces, where do we derive the anisotropy necessary to substantiate the 175 sudden cataclysm traditions? The crust, oceans, polar ice, etc., are not even 1% of the way to sufficiency in this regard.
And if the Earth’s architecture involves one integral object with no decoupling potential, how would we ever observe two rotational states to begin with, as the monuments so clearly demonstrate?
The geophysical answer is constrained to somewhere between those bookends, and only the core-mantle boundary contains all five geophysical factors necessary for a decoupling.
This decoupling mechanism must also explain, and function in concert with, rapid climate change.
Thus one can perceive the utter elegance of ECDO Theory. It resolves every current geophysical, cultural, and climate challenge at an optimal logical saddle point.