348 PLATE TECTONICS Figure 10 Map of plate boundaries and absolute motion vectors Arrow lengths are proportional to the plate speed, with the longest (in the western Pacific) corresponding to speeds of about 100 mm per year Reproduced from Fowler CMR (1990) The Solid Earth: An Introduction to Global Geophysics, Fig 2.20 Cambridge: Cambridge University Press about 160 km per million years (or 160 mm per year) At the slow end, the North America–Eurasia plate boundary passes very close to its Euler pole in northern Siberia, where the relative motion becomes essentially zero Absolute Plate Motions So far, we have dealt only with relative motions, which are fairly easy to determine There is also interest in determining so-called ‘absolute’ plate motions, where the motions of all plates are related to some common reference frame The possibility of doing this arises from the proposed existence of mantle plumes, which rise as narrow columns of relatively hot rock from deep in the mantle, possibly from the core– mantle boundary They reach the Earth’s surface in so-called ‘hotspots’ where they are manifest by clusters of intense volcanic and seismic activity Wellknown examples occur in Iceland and Hawaii, but there are thought to be many tens of such hotspots and associated plumes Hotspots leave clear trails on the Earth’s surface, which comprise lines of volcanoes or volcanic seamounts and zones of thickened volcanically produced crust The Hawaii–Emperor seamount chain, trending north-west from Hawaii, is an excellent example, but there are many other subparallel seamount trails in the south-western Pacific which are thought to have resulted from plumes If points along the hotspot trails are dated (e.g., by radiometric dating of volcanic products), the relative motion between the plumes and the plate, and between given plumes, can be determined We can also calculate the motions of individual plates relative to the average plume motion When this is performed, the relative motion between many of the hotspots turns out to be quite small, and significantly less than the average relative motions between plates From this, and the fact that plumes are thought to rise through the mantle, it seems reasonable to assume that the average plume motion relative to the mantle is quite small, although it should be admitted that this has remained difficult to test in detail, and there are those who even question the existence of plumes Nevertheless, if it is assumed that the average motion of hotspots relative to the mantle is zero, plate motions can then be given in the supposedly fixed mantle reference frame These are referred to as absolute plate motions They can also be described in terms of Euler poles, and are shown in Figure 10 Mechanisms and Plate Driving Forces Plates as Parts of the Mantle Convection Cycle Plate motions are ultimately driven by the Earth’s heat energy, and they are intimately related to the