Physiology: Why does metabolic rate scale with body size?

used to promote plate tectonics on Mars. The classification of “andesite” rocks at the Mars Pathfinder landing site is tenuous: this name (actually icelandite) was originally assigned on the basis of its major-element chemistry, noting that a sedimentary origin or weathering rind could not be ruled out without textural or mineralogical data. A re-analysis of a-proton X-ray spectrometer chemistry indicates that Pathfinder rocks may have a high water content, which supports a non-igneous classification. Formation mechanisms on a global scale are required to explain the extensive distribution of type-2 materials in the northern lowlands. The weathered-basalt interpretation would predict a large body of water or sedimentary depocentre in the northern lowlands, but does not account for local occurrences of type-2 materials in the southern highlands. These might be either andesites formed by igneous fractionation or the result of local weathering processes. The production of fractionated magmas of intermediate composition is an inefficient process unless it is promoted by dissolved water, so the occurrence of vast amounts of andesite would probably require a wet source region and efficient transport of less dense magmas. The intermediate-volcanism interpretation requires large-scale melting of the thin northern crust and flooding with high-silica volcanics, as well as isolated melting pockets in the thick and ancient southern highlands. However, the mechanism for rejuvenated mantle melting, the degree of magma fractionation and crustal assimilation, and the creation of local hotspots are all unresolved issues. Furthermore, there is a poor correlation between the transition of surface type-1 and type-2 materials with the transition from thick to thin crust, which suggests that the distribution of materials is a result of surficial processes rather than crust–mantle ones. Michael B. Wyatt*, Harry Y. McSween Jr Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, USA *Present address: Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-6305, USA e-mail: [email protected]