SUMMARY: An appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal. Upwardly converging streamlines followed by a fluid escaping from a flat-topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre-eruption chamber to be simultaneously erupted. The volume erupted (V) is related to the maximum depth tapped (R) by the equation V = (2π/9) R3. Large ash-flow sheets (100 km3) may therefore sample magma up to 5 km below the chamber roof.

Analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption. Many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow-field during eruption, where a series of sub-spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta. Mixed magmas erupted late in some zoned ash-flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption. Flow banding in rhyolites can result from the same fluid dynamic processes.

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