August 2008 Geology and GSA Today media ...(ahood Geological and Environmental Scien...)

At convergent margins, silicic magma chambers help to stabilize the continents by adding low-density, high-silica material to the upper crust. Silicic calderas are the volcanoes associated with these typically large magma chambers; the calderas result from the catastrophic collapse of the chamber roof following the explosive eruption of viscous magma. The primary goal of Hughes and Mahood's study was to determine which tectonic and crustal factors control the occurrence and composition of silicic calderas in volcanic arcs. They present the results of a global compilation study that examines the attributes of 91 Quaternary silicic calderas in 18 volcanic arcs. Hughes and Mahood show that the number of calderas in a given arc depends on the associated plate convergence rate, and that the age, thickness, and stress regime of the underlying crust determine the composition of caldera-forming eruptions. Better understanding of where and why these types of volcanoes occur will increase our understanding of how continental crust forms at margins and aid in interpreting the geological settings of ancient arcs.

Patrice Rey and Nicolas Coltice, School of Geosciences, Madsen Building F09, The University of Sydney, Sydney, NSW 2006, Australia. Pages 635-638.

Rey and Coltice present a working hypothesis that claims that circa 2.8 to 2.5 billion years ago, Earth's continents became progressively stronger, allowing for the formation of high mountain belts. The erosion of these mountains acted as a coupling agent between the continental geochemical reservoir, the ocean/atmosphere geochemical system, and Earth's mantle. One of the outcomes of this coupling is that phosphorus (a key element for DNA)
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