Water Resources of the Caribbean
Tectonic, environmental, and human aspects of weathering and erosion: A global review using a steady-state perspective
Robert F. Stallard
U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066, USA
We have long known that erosion and tectonics continuously shape the Earth's surface, yet our description of the innumerable associated processes is inadequate. Important aspects of landscape development are studied in such diverse disciplines as geology, geomorphology, geochemistry, hydrology, pedology, ecology, population biology, and agronomy. Data and models from each discipline contribute significantly to our understanding of the processes that control the style and rate of erosion, the composition of erosional products, and the dispersal of these products to the ocean or to sites of long- term burial on land. Now that human activities are reshaping the land, a better description of weathering and erosion is critical.
Problems of scale and episodicity make rigorous study of weathering and erosion difficult. Processes important to rate limitation operate on all scales from mineral surfaces to entire plates (Stallard, 1992). Some of the most significant phenomena function at the scale of hillslopes and lowest order stream channels. Although most erosional processes are singularly slow and nonspectacular, some, such as those triggered by rare meteorologic and tectonic events, are capable of enormous transformation of the Earth's surface. The recurrence times of many major erosional phenomena range from years to millennia. On top of this complexity, human activities have so modified the erosion cycle that it is difficult to distinguish between natural and perturbed effects.
Much action takes place within soil, an environment that is heterogeneous in space and time (Stallard, 1992).Organisms are important. The effects of the penetration of tree roots, tree throws, and burrowing organisms are hard to quantify. Soil is difficult to probe without inducing severe perturbations that thwart measurement (Litaor 1988). Heterogeneity is so great that even on simple hillslopes a large number of samples (200 to 1,000) are typically needed to confine the error to within 10%.
This review concerns the weathering process and how tectonics, geomorphic processes, and biology interact to control rates of denudation and the compositions of denudation products. In the first part of the review, I summarize several detailed syntheses of the tectonic and climatic aspects of chemical and physical denudation. Despite the use of parallel sampling methodologies and conceptual approaches, there have been few attempts to link studies of solute and solid erosion. Much of the review, therefore, uses the under-explored concept of steady-state erosion to link chemical and physical erosion. The objective is to establish a context for comparing chemical, biological, physical, and human aspects of erosion.
My choice of examples is affected by my field experience. I have worked primarily in neotropical watersheds, starting in the Amazon and Orinoco River systems. More recently, I have focused on small watersheds in Puerto Rico and Panama. I draw heavily from these data. In addition I use data from the Mackenzie River system and from studies of small, cold-climate watersheds, especially South Cascade Glacier in Washington. This combination of regional data sets allows comparison of similar tectonic environments in cold and warm climates. Tectonic environments include cratons, continental mountain belts, and island-arc mountains. Investigation of erosion in temperate zones is especially complicated because this zone encompasses the boundary of glaciated and nonglaciated regions. As such, it is occupied by extensive and young subglacial deposits, outwash deposits, and loesses. Moreover, the impact of human activities in temperate zones is substantial and ubiquitous.
Stallard, R.F., 1995, Tectonic, environmental, and human aspects of weathering and erosion: A global review using a steady-state perspective: Annual Review of Earth and Planetary Sciences, v. 12, p. 11-39.