Plots of potash content against depth from volcanoes to inclined seismic zones for active magmatic arcs reveal slightly different relations for (a) continental margin arcs containing full or excessive thicknesses of continental crust overlying presumably old continental lithosphere, and (b) migratory intra-oceanic arcs that bound interarc basins of marginal seas and lack any known continental basement rocks. Detached intra-oceanic arcs containing continental basement rocks and stationary intra-oceanic arcs where back-arc spreading is absent display intermediate or equivocal relations. The significance of the difference in K-h correlation for continental margin and intra-oceanic magmatic arcs is uncertain but should be taken into account by theories for the petrogenesis of arc magmas.

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