1) Unless the island arc is subducted it will be accreted to the continental-margin orogen previously formed along the edge of the continent.

2) Where oceanic lithosphere is subducted beneath another oceanic part of a plate the associated volcanic activity and produce an intra-oceanic island arc.

3) This suggests that the subducted sediment somehow survived as a chemically and physically distinct region large enough to avoid obliteration by diffusion.

4) Alternatively, subducted oceanic crust may be able to pull adjacent continental crust down into the asthenosphere.

5) From these observations, Hofmann and I proposed that plumes consist in part of deeply subducted oceanic crust and sediment.

6) The border between the N American and Eurasian Plates does not subduct.