Hot spots
PT predicts that volcanism on earth is either associated with divergent (MOR or rift) or with convergent (DOT or mountains) plate boundaries
There are about 20 locations where volcanism does not match this pattern; volcanism occurs within a plate or with unusual character for plate boundary setting
These so-called hot spots correlate with regions of mantle upwelling; plumes of warm mantle rise, melt, and cause intra-plate volcanism
Mantle plumes are relatively fixed, i.e. they move slowly relative to each other & we can use them to fix the ‘absolute’ motion of plates