What factors control mechanical erosion rates?

Mechanical erosion rates are important factors in understanding how contine nts evolve. Mechanical erosion is much faster than chemical erosion, especi ally for highly elevated regions of the Earth's surface. It is a principal way in which mountain ranges are removed, exposing deep metamorphic roots, which comprise much of the older portions of the continental masses. In add ition, there has to be a long-term balance between erosion and mountain bui lding. A new data set allows us to explore in greater detail some of the ma ny factors which control mechanical erosion rates. The most important facto rs are some expression of the average slope of a drainage basin, some measu rement of the amount of water available for erosion, some environmental mea surements, and also a measurement of basin length, for which we have no goo d explanation. The estimate of global mechanical erosion rate obtained here is considerably lower than those obtained by some other workers, some of w hom have concentrated on the fact that smaller river basins tend to get ero ded faster than larger basins, and it is mainly smaller basins which have n ot been measured and which are therefore not allowed for by simple arithmet ic averaging of observed erosion rates. It is shown here that although smal ler basins are eroded faster, this is mainly because they are steeper than larger basins. We also show that extrapolation of current data to smaller b asins does not work because the observed continental area which is draining to the ocean cannot be attained by the simplest extrapolation scheme.