Types of recent weathering crusts and their global distribution

Abstract

Recent weathering crusts are defined as those formed and in process of formation under present or closely similar climatic conditions. Such crusts are classified on the basis of their composition and profile form into four types: nonleaching, weak periodic leaching, strong leaching, and freezing. Each but the last of these types is divided into a pair of subtypes according to the intensity of eluvial processes. All subtypes are divided into lithogenic groups reflecting the composition and properties of the parent materials. Recent weathering crusts of the nonleaching, weak periodic leaching, and freezing types are the most extensive and widespread over the world. Crusts of the strong leaching type, especially of the extra-humid subtype, are being formed only to a limited extent at the present time, although they were formed extensively in past geologic periods. Sedimentary parent materials with high proportions of quartz-kaolinite and goethite-gibbsite-kaolinite minerals, inherited from earlier periods, are common in many regions with arid and semi-arid climates at the present time. The potential for intensive weathering under humid and extra-humid, tropical climates has been realized in few recent weathering crusts because of the presence on the land surface of former weathering crusts that were subjected to strong hypergenic processes in the past. Where fresh parent rocks are exposed in small areas under such climates, profound weathering does now occur with the formation of new minerals.