Present Chemical Weathering of Basalt in Iceland

Abstract

Before the initiation of the water cycle on the 'early Earth' all the rocks were volcanogenic (e.g. Ronov, 1964). At present, volcanic rocks cover about 8 % of the land surface of the Earth and more than half of that is of basaltic composition (Meybeck, 1987). The present contribution of the weathering of volcanic rocks to the dissolved load of the world's rivers is less than 3%, the most important contribution comes from carbonate rocks (45%) and shale (20%; Meybeck, 1987). The high contribution of shale is primarily due to the weathering of small portions of the shale, i.e. calcareousand pyritic-shales. The chemical weathering of Ca-Mg silicate rocks is important for the atmospheric COz on a multimillion-year time scale and may therefore influence the temperature of the atmosphere (e.g. Berner, 1992). Thus, even to day, the weathering of basalt is an important factor in the net loss of CO2 from the atmosphere, and it's general importance increases with geological age because the ratio of basalt to sedimentary rocks increases with the age of the crust (e.g. Ronov, 1964). Iceland provides an opportunity to study the effects of the age of rocks, glaciation and vegetation on the nature and rate of chemical weathering of basalts. About 10% of the country is covered with glaciers, and one fourth is covered with vegetation. The interior of the country is unpopulated, providing pristine unpolluted catchment areas of uniform temperature and rock composition (basalt) with a variable glacier-and vegetation cover and variable runoff. The objective of this paper is to define and interpret, the source of supply for the dissolved constituents in Icelandic rivers, the relative mobility of the elements during weathering of basalt, the rate of chemical denudation in Iceland and the effect of age of rocks, and glacier/ vegetation cover on the chemical weathering of basalt.