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Some compari- Phys. 94,151 (1991). ics) and by the National Science Foundation's Physical sons are under way to determine the resolu- 16. R. Bleck and D. B. Boudra, J. Geophys. Res. 91, Oceanography Program under WOCE. The TOPEX sat- tion requirements in different applications, 7611 (1986). ellite data was provided by Caltech's Jet Propulsion 17. The FRAM Group, Eos 72, 169 (1991). Laboratory and the Goddard Space Flight Center such as climate versus forecast uses, and 18. F. 0. Bryan, C. W. Boning, W. R. Holland, J. Phys. through the support of the National Aeronautics and whether isopycnal coordinates are better Oceanogr. 25, 289 (1995). Space Administration. The computational resources for than fixed vertical levels. The two methods 19. A. J. Semtner and R. M Chervin, J. Geophys. Res. the simulations shown in the figures were provided by are probably comparable if the diffusion in 97, 5493(1992). the National Center for Atmospheric Research, Los 20. G. A. Jacobs et al., Nature 370, 360 (1994). Alamos National Laboratory, and the Pittsburgh Super- leveled models is oriented along isopycnals 21. P. D. Killworth, D. Stainforth, D. J. Webb, S. M. computing Center. or if both model grid sizes are well below the local radius of deformation. Hence, future www.sciencemag.org development will probably emphasize im- provements ofphysical components and pro- Numerical Models of cesses in models, such as the surface mixed layer and sea ice, and outflows and mixing Caldera-Scale from water-mass formation regions. Further- Volcanic Eruptions more, biogeochemical processes can be in- on Earth, Venus, and Mars cluded by using methods already tested in Downloaded from coarser grid physical models (33). An immediate goal of modelers is to Susan Werner Kieffer conduct longer term simulations, including some to full thermodynamic equilibrium. Volcanic eruptions of gassy magmas on Earth, Venus, and Mars produce plumes with Ocean models will not be fully proven to be markedly different fluid dynamics regimes. In large part the differences are caused by well formulated until they can reproduce the differing atmospheric pressures and ratios of volcanic vent pressure to atmospheric the equilibrium distributions of tempera- pressure. For each of these planets, numerical simulations of an eruption of magma ture, salinity, and other properties as well as containing 4 weight percent gas were run on a workstation. On Venus the simulated the modem spreading of dissolved anthro- eruption of a pressure-balanced plume formed a dense fountain over the vent and pogenic gases and radioactive tracers. The continuous pyroclastic flows. On Earth and Mars, simulated pressure-balanced plumes likelihood that the ocean is capable of dif- produced ash columns, ash falls, and possible small pyroclastic flows. An overpres- ferent overturning circulations if started sured plume, illustrated for Mars, exhibited a complex supersonic velocity structure and from different initial conditions needs to be internal shocks. examined. The natural variability of cli- mate needs to be determined, including the distinct possibility that limits of climate predictability are affected by oceanic turbu- Calderas are large craters formed by the more of magma (Fig. 1A). A large caldera lence. Finally, a number of past climates and collapse of the summits of volcanoes after eruption, with mass fluxes believed to be potential future climate (including those in- eruptions of tens of cubic kilometers or >108 kg/s, has never been witnessed on fluenced by activities of mankind) should be Earth. Such "caldera-scale" The author is in the Department of Geological Sciences, eruptions can investigated with the aid of high-quality University of British Columbia, Vancouver, British Colum- produce plumes that reach the stratosphere atmospheric models coupled to global ocean bia V7W 2H9, Canada. as well as ash flows (so-called ignimbrite SCIENCE * VOL. 269 * 8 SEPTEMBER 1995 1385 sheets) that cover thousands of square kilo- ment methodology. Such models can guide effects that exist on other planets (10-17). meters. A well-known example is the Valles field observations, and in turn there is an Going beyond the parametric models is caldera and its deposits that cover most of ongoing need for more observations and for conceptually and computationally difficult northern New Mexico (Fig. 1A). more ways to test the simulations. Planetary because planetary volcanism occurs in a Calderas have been observed on other observations can serve several purposes wide variety of geologic and atmospheric planetary bodies such as Mars (Fig. 1B) and here: The models can guide interpretations conditions (Table 1). Atmospheric temper- Venus. Indeed, on lo, a moon ofJupiter, the of remote sensing data, and the data in turn ature and gravity vary by an order of mag- Voyager spacecraft showed that calderas oc- serve as a new nonterrestrial database for nitude from one planet to another, but at- cur as landforms and that caldera-scale constraining the models. At the present mospheric pressure varies by 14 orders of eruptions are occurring at the present time. time, the terrestrial models referenced magnitude. It is not feasible to investigate Observations of these planetary analogs can above have not been extended to other the effect of individual parameters system- provide us with a rich database for compar- planets. Instead, parametric models and atically one by one, because an inordinate ison and contrast of the processes inferred steady-flow approximations have been amount of computational time would be for Earth. Why do some of the volcanic adopted in an attempt to understand com- required. Therefore, I chose to examine landforms look so similar to our terrestrial plex volcanic plumbing systems, eruption how the fluid dynamics of a relatively gas- ones, and why do some look so different parameters, and geologic and geomorphic rich eruption on Earth would change if this (Fig. 1, A through C)? The present study sought to understand the relation between eruption dynamics and global parameters (such as gravity and atmospheric pressure and temperature) and to compare and con- trast the dynamics of eruption on the dif- ferent planets.