An integrated three-year program including geology, geophysics and geochemistry is planned for the purpose of studying the active volcano Nevado del Ruiz, Colombia. Since the November, 1985 eruption, the PI has been engaged in a diverse set of preliminary projects at Ruiz which suggest that previously held beliefs about the volcano may be in error. In order to improve our understanding and to contribute to the general conceptual model of Andean volcanism a four-task program has been developed: 1) A detailed geological study of the volcanic complex including analyusis of the age of volcanism, its evolution, eruption dynamics, petrology, and volcanic hazards. 2) A study of recent behavior of the volcano as recorded in the nine known tephra layers distributed widely over the region. Eruption volumes, dynamics and evolution will be a primary focus. 3) A detailed gravity study to evaluate and model the subsurface structure of Ruiz. Most of the questions about the unusual aspects of the form and behavior of Ruiz could be resolved if an unrecognized caldera were actually present. 4) A continued analysis of the geothgermal system and its response to the mesurgence of magma. An effort will be made to more completely sample the system, analyze the alteration products, and determine their age and relationship to the geological evolution. Additional isotopic studies will be undertaken to understand the nature of the deep reservoir and the origins of the main and minor constituents. The goal of the research is to produce an integrated model of the major northern Andean volcanic center which could then be applied to and compared with those developed at other circumPacific volcanoes.