Geochemical evolution of the acid crater lake of Poas volcano (Costa Rica): Insights into volcanic-hydrothermal processes

Abstract

This thesis describes the evolution of Laguna Caliente, an acid crater lake at the summit of Po:is, a persistently active volcano in central Costa Rica. The appearance, volume, temperature and chemical composition of the lake have continuously changed over the entire known period of its existence. Occasionally, the lake completely disappeared, which occurred for the last time in 1953-1954 and 1989-1994. These intervals were accompanied by eruptions, which at Po:is are usually restricted to expulsion of steam, ash and lake bottom sediments. Such eruptions may also go through the lake, as was the case in 1968-1980, 1987-1994, and most recently in 2006-2008. The latest eruption of fresh magma occurred in 1953-1954, a phenomenon that is rare for this volcano. The periods of eruptive activity alternate with relatively quiet intervals. They go together with changes in the altitude of the volcanic plume and the amount of gases emitted into the atmosphere. In addition, gas and particles are also emitted almost continuously from vents in the crater area, which vary in vigour and location with time. Because the gas contains large amounts of sulphur and chlorine, and is transported through the air, there are also variations in the environmental effects produced by acid rain, gases, and particles around the volcano. Hence, the intermittently strong activity not only pose direct dangers in the crater area at the summit, but may also be harmful to the health of residents and livestock, and may impact agriculture, infrastructure and ecosystems on the flanks ofPo:is. In order to better predict the nature and timing of future eruptive events, it is necessary to know the particular processes that operate in the interior of the volcano and that are responsible for its activity at the surface. The accompanying movement of fluid materials and heat effects are rapidly transmitted upward by rising vapour and circulating groundwater. The crater lake intercepts these signals and transforms them into measurable chemical and physical properties. The results of this study show that Laguna Caliente provides a detailed record of ongoing processes at depth. The lake-water temperature and a large number of dissolved chemicals show systematic but not equal variations over longer and shorter periods of time, which makes their interpretation a challenging exercise. A comprehensive inventory of new and existing data is presented, covering the history of the lake between 1978 and 2008.