Although the physical laterpretatlon of the model parameters a and 

 b is clear, an Important question remains as to the geological signifi- 

 cance of these terms. Why do certain areas of the sea floor have a par- 

 ticular representative spectrum, and why do all spectra seem to show 

 such a consistent power law form over large ranges of spatial frequency? 

 An obvious hypothesis is that the spectral form reflects the unique 

 interaction of the relief-forming processes and the materials being 

 affected. For example, the formation of new sea-floor crust at oceanic 

 ridge crests affects the relief of the new sea floor at all spatial fre- 

 quencies. If the relief-forming process is uniform over some geographic 

 region and interval of geologic time, there is no reason to suppose a 

 change in the statistics of the surface being constructed, although its 

 deterministic shape might change. Conversely, if there is a change in 

 the relief -forming process (such as the spreading rate) or material 

 (perhaps a change in the properties of the magma source), it is likely 

 that the resulting relief would also be affected. 



Many geological environments represent a composite of several 

 relief-forming processes (tectonic, sedimentary, erosional) and several 

 types of material. Such composite reliefs should result in an amplitude 

 spectrum reflecting the composite spectra of these several processes and 

 materials. If each style of relief is dominant over a different spatial 

 frequency band, and each component spectrum conforms to the power law 

 functional form observed in one-component cases, the composite spectrum 

 should appear as a set of straight line segments on a plot of log ampli- 

 tude versus log frequency. Examples of such composite spectra will be 

 shown in a later section. 



55 



