Hmm, I'm not sure that's true. If you generate a noise function that dictates where the "plates" are, then the location of the "plates" is already predetermine

Hmm, I'm not sure that's true. If you generate a noise function that dictates where the "plates" are, then the location of the "plates" is already predetermined from the construction of the function. Then the output of that function could be used to alter the mountain/terrain functions. Thus, no need to pregenerate everything. You pregenerate all of the functions in advance, but not all of the outputs.