Iona, your assumption that I am making a "uniformitarian assumption" is incorrect.

On some parts of the earth, the action of plate tectonics is a gradual, relatively smooth process. The separation of the continents at the mid-Atlantic ridge, for example. For the most part, this is an up-welling of magma which pushes the two sections apart. And this happens at a fairly predictable (and predicted) rate.

There are other parts of the earth, however, when the shifting of plates is sudden and violent. The Fukushima earthquake is one example. Also, there is evidence that within recent history, the western side of the San Andreas fault suddenly shifted some fifteen feet to the north, relative to the rest of California (That must have been a lollapalooza!).

I could cite example after example of sudden (NON-uniformitarian) changes of position in the earth's crust, often followed by a long period of relative quiescence—due to the fact that the stress built up over time has released the tension, and it takes time for the slowly moving plates to build up tension again.

It is these sudden "nonuniformitarian" shifts that cause earthquakes.

I was citing the AVERAGE rates of continental drift, which includes BOTH gradual movement and sudden shifts.

I'm sorry, but if you think you have refuted my argument, then you are mistaken.

By the way, your Niagara Falls erosion argument does not relate to matters of plate tectonics. The two are quite different geological phenomena.

Don Firth