The rotation pole is the point on the Earth’s surface about which the spreading direction (marked by the trend of transform faults, which are slightly curved on a surface view of the spherical Earth) forms a small circle. Thus a line from the rotation pole to any transform is of equal length at the points along that transform. Accordingly, to accommodate such geometry, the amount of spreading must increase away from the rotation pole.
Key questions remain unanswered despite the alacrity with which scientists have greeted plate tectonics. Perhaps most important is the question of what forces drive the plate motion. Thermal convection in the Earth’s mantle is a popular explanation. But how exactly does that work? In how many “cells” of convection and how deep in the mantle does it take place?
Theoretical studies tend to predict regular-sized, symmetric cells while plate tectonics apparently requires irregular-sized and
asymmetric cells. A host of scientists are pursuing this problem. Geophysicists, who study variation in the Earth’s gravity field, are looking for sensitivity to the convecting mantle material. Geochemists, who study compositional anomalies in igneous rocks, are looking for variations in the degree of mixing of material in the mantle.
Other problems are as follows. To what extent can plates be viewed as internally rigid? What causes the often spectacular vertical movements observed within plates? What exactly happens at the base of the lithosphere, and how is this boundary between the rigid uppermost mantle and the underlying, more malleable, mantle best defined? To what extent might the style of plate tectonics be episodic? For how long has it been occurring in Earth’s history? How have the continents evolved through time? What exactly happens during subduction?