ZONING OF MAGMAS BY VISCOSITY IN VOLCANIC CONDUITS

ONE clue to the relations between different magmas may be found in the spatial relationships of the products of igneous activity. Much attention has focused on chemical zonation in the products of large ignimbrite-forming eruptions. Such deposits typically exhibit a mafic-over-silicic zonation which is interpreted as an inversion of a silicic-over-mafic layering in the source magma chamber (see, for example, ref. 1). In contrast, lava flows exhibit a silicic-over-mafic zonation, opposite to that expected. Such flows are apparently fed from conduits having a core-annular zonation, in which thin mafic margins enclose a silicic core. We argue here that this zonation develops by viscosity segregation during the simultaneous flow of two distinct magmas in the conduit. This process, rather than the pre-eruption configuration of magmas, may control the zonation pattern of eruption products when two magmas are transported simultaneously. © 1990 Nature Publishing Group.