Magmatic Intruders

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Science  14 Jul 2000:
Vol. 289, Issue 5477, pp. 219
DOI: 10.1126/science.289.5477.219a

Despite the apparent chaos of a volcanic eruption, the deposits of ash often present chemically distinct layers and tend to be silica-rich at the bottom and relatively silica-poor at the top. This chemical stratigraphy has been thought to reflect layering within the magma chamber; material near the top of the magma body should be ejected first and thus lie at the bottom of the ash deposits.

A variety of processes have been proposed for how magma bodies might produce and collect light, silica-rich magmas at the top of the chamber. Eichelberger et al. suggest, however, that magma chambers need not be layered or zoned. They argue that the influx of a silica-rich magma into an existing magma chamber results in layering in ash flow tuffs—and perhaps even in the violent volcanic eruption itself. Processes analogous to those that occur when mafic magmas intrude into silicic ones may lead to the formation of intermixed magmas, which are then preserved in intrusive rocks, and result in much less violent eruptions. — BH

Geology28, 603 (2000).

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