Progressive changes in volcanic chemistry along a traverse across the Arabo-Ethiopian swell suggest a shallowing of the zone of melting with concomitant crustal thinning from plateau to rift. The gradational nature of this thinning seems to exclude a predrift fitting of the Red Sea coastal escarpments and accords with geological evidence for extension of continental crust beyond the escarpments into the rift zone. Na/K and, to a lesser extent, Ti are the most sensitive discriminants in relating volcanic chemistry to tectonic site. A strong coupling of crust and upper mantle, despite horizontal plate movement, is inferred.

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