In contrast to those deposits formed by the alteration of volcanic ash in saline, alkaline lakes discussed in the chapter by Hay and Sheppard (this volume), large volumes of tuffaceous sediments around the world have been transformed to zeolites and other authigenic silicate minerals by the action of percolating water in open hydrologic systems. These include systems that have experienced significant chemical and hydrologic exchanges with the surrounding environment. Tephra sequences exposed to open-system alteration commonly show a vertical zonation of zeolites and other authigenic minerals that reflects the chemical changes in meteoric water moving through the system. Flow in...

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