Geophysical explanation for the disparity in spreading rates between the Northern and Southern hemispheres

Abstract

An interesting characteristic of the Earth, often noticed by schoolchildren, is that it seems out of balance: Oceans dominate the Southern Hemisphere, while continental crust seems to be crowded into the Northern Hemisphere. The cause of this inequity involves the greater number of mid‐ocean spreading ridges and significantly faster spreading rates in the Southern Hemisphere, but no explanation has ever been suggested for this hemispheric disparity in spreading ridges and rates. An examination of east‐west spreading rates and plate motion indicates that the imbalance is due to simple spherical geometry. In the Southern Hemisphere, the system of spreading ridges encircling Antarctica is pushing all of the neighboring plates northward, away from Antarctica. As these plates approach the equator, they begin to occupy larger latitudinal circumferences. This large‐scale northward motion of latitudinal bands of crust to lower latitudes requires significant emplacement of new crustal material to fill up the larger circumferences, resulting in extremely high sum total east‐west spreading rates along those latitudes. Contrariwise, in the Northern Hemisphere the general northward motion of plates away from the equator and toward smaller circumferences has led to a steep drop in spreading rates and the number of active spreading ridges. Sum total east‐west spreading rates along latitudes are correlated with the rate of latitudinal circumferential extension to a significant degree (r = 0.68). The geophysical result of this correlation has been the asymmetric globe that we see today, mostly young and oceanic in the south, mostly old and continental in the north.