Alfred T. Anderson, Jr. Professor Emeritus, Department of the Geophysical Sciences and the College Editor, Journal of Geology Department of the Geophysical Sciences 5734 S. Ellis Ave. HGS 369 Chicago, IL 60637 USA Phone: 773/702-8138 E-mail: canderso@uchicago.edu Publications Solid Earth and Planetary Sciences

A main interest is in how volcanoes work. Why is a volcano a hill? Why does magma erupt rather than solidify at depth? Are volcanoes rooted in big bodies of buried magma? Do plutons outgas mainly through volcanoes? Undergraduate and graduate students and a postdoctoral associate have recently worked with me on rocks from Kilauea, Hawaii, the rhyolitic Bishop Tuff, California. Studies of New Zealand rhyolites are just beginning. Our work uses physical and chemical analysis of pumice and melt inclusions in volcanic phenocrysts to assess the pressure of crystallization and the amount of gas in preeruptive magma. In this way we can estimate its bulk density and infer whether buried magmas accumulate gas. Preeruptive exsolved gas in magma is probably at least as important as dissolved volatiles in governing the preeruptive as well as eruptive behavior and evolution of magma.

Our studies of Kilauea Volcano, Hawaii focus on the way gas and crystals separate from melt beneath the volcano. We are investigating the idea that shield volcanoes form because new magma that rises into a subvolcanic magma body is dense. The subvolcanic magma body probably is neutrally buoyant relative to porous and fractured rock walls. The level and pressure of neutral buoyancy is controlled by the crushing strength of rock. For load pressures greater than about 2000 atmospheres, voids and cracks are crushed shut and the rocks become denser than magma. This rock densification determines the pressure at the bottom of the subvolcanic magma storage reservoir. New melt that enters from below, although relatively hot, is compositionally dense. If it is poor in gas, it is not buoyant and will remain at depth and be blanketed by overlying rock and magma. Upward accumulation of gas through such a reservoir seeks its highest point. Therefore, the location of the main eruptive vent remains fixed even though the tectonic source of magma migrates. This may be why, with steady migration of the Pacific plate over the Hawaiian hot spot, a succession of individual volcanoes forms rather than a continuous ridge or line.

So far we have established that the new, dense, picritic magma which erupted from Kilauea's summit magma storage reservoir in 1959 was able to do so because it was gassy and, therefore buoyant. Consequently, it erupted rather than ponding at depth. Ongoing studies of other Kilauean magmas aim to establish their preeruptive gas contents, depths of emplacement into the reservoir, and consequent crystallization.

Work on rhyolitic magmas has focussed on the Bishop Tuff, California. Graduate students Chris Skirius (now at OMNI Labs, Houston) and Fangqiong Lu (IBM, Austin) analyzed H2O and CO2 (using infrared spectroscopy) and trace elements in melt inclusions (using the ion microprobe with the help of Andy Davis). Their work demonstrated that later erupting Bishop magma was poorer in H2O, richer in CO2 and that phenocrysts sank into less evolved magma.

Post-doctoral associate Paul Wallace established the amount of gas that was in early-erupted Bishop magma. Big bodies of silicic magma may commonly contain large amounts of gas that have accumulated from huge (batholithic) reservoirs of magma beneath them. This concept has important implications for stratospheric injections of volcanic sulfur as well as for some ores and the outgassing and rigidification of new continental crust.

Our ongoing work in 2001 includes studies of cathodoluminescent zoning in some of the same phenocrysts of quartz fromthe Bishop Tuff. Different zoning features re correlated with different stratigrafic units and reveal that these magmas did not mix after the crystals grew their outermost 20 to 60 micron thick rims. Former undergraduate student Bret Peppard (now at Michigan, Ann Arbor) has written a manuscript on this work that is now being revised for publication.

Education:

• Princeton University, Ph.D., 1963