This accumulation is apparent over small time ranges, but should be almost undetectable over longer periods. Significant differences may occur in velocities for individual plate pairs. A prime example of this is the motion between the Pacific and North American plates in the Gulf of California. The reason for the increased velocity is unclear. A more likely cause is accomodation of plate motion outside of the Baja rift since it began to spread 3.2 Ma.
Hotspots appear to be caused by deep mantle plumes at or near the earth's surface.
According to this view, hotspots move relative to one another, but remain somewhat fixed in their locations. The largest relative motions, according to quasi-fixists, are between Pacific hotspots and those found in the Atlantic and Indian Oceans. Evidence indicates that this circuit is faulty, however. The preferred explanation for the motion is movement between East and West Antarctica. Also, this circuit fails a paleomagnetic test.