Global Environment Second Edition Chemical Weathering: Minerals, Plants, and Water Chemistry

Global Environment

Second Edition

Chemical Weathering: Minerals, Plants, and Water Chemistry

Introduction

The process of chemical weathering brings together interaction between the lithosphere, the atmosphere, the hydrosphere, and the biosphere, all major components of the global environment. It exerts controls on the chemical composition of soil water, ground water, and river water; the nature and abundance of vegetation on land; and, over longer timescales, the carbon dioxide level of the atmosphere, the chemical composition and biology of the oceans, and the formation of different rock types. In this chapter we will examine how these interactions occur. Because of its fundamental importance, we focus first on water and the hydrological cycle.

An idea of the paths that rainwater may take once it strikes the ground, is shown diagrammatically in figure four point one. Water that has been intercepted by vegetation and then drips off it is termed throughfall. Water infiltrating the soil is called soil water, and that passing directly into the nearest stream is referred to as surface runoff. Once in the soil, the water either passes downward or is taken up by plant and tree roots. In the latter case, the water is transported up through the tree and eventually evaporated from leaf surfaces. In this way the water is returned to the atmosphere, and the overall process is known as transpiration. Water trickling downward through the soil eventually encounters a level in the soil or underlying bedrock where all pore space is filled with water. At this point the water becomes groundwater, the rock or soil is said to be saturated with water, and the level where this occurs is known as the water table (figure four point one). Above the water table, pore space is filled by a mixture of air and water to form the unsaturated zone.

Groundwater flows underground until the water table intersects the land surface and the flowing water becomes surface water in the form of springs, rivers, swamps, and lakes. Surface water leaving its source is known as runoff. The continual contribution of groundwater to rivers, important between rainstorms, is known as base flow. Groundwater continues to flow due to a hydrostatic head built up by the recharge of new rainwater at the source. Because of diurnal, seasonal, and longer-term climatic changes, rainfall input and, consequently, the position of the water table can fluctuate, but the fluctuation and its timing can be considerably damped and delayed, depending upon the capacity of the subsurface rocks to store groundwater.

Water coming into contact with rocks (and derived soils) reacts with primary minerals contained in them. The minerals dissolve to varying extents, and some of the dissolved constituents react with one another to form new, or secondary, minerals. Dissolution is brought about mainly by acids provided by plant activity and bacterial metabolism (and, in areas of pollution, by acid rain), and the overall process is called chemical weathering. Besides biological factors, chemical weathering is also aided by physical processes that act to break up rocks and expose additional mineral surface area to weathering solutions. This is known as physical weathering, and the dominant process is the fracturing of rocks by expansion accompanying the freezing of water in cracks. Thus, physical weathering is most important at higher latitudes and elevations. Together chemical, biological, and physical weathering result in the breakdown of rock and the formation of soil.

Rock-water interaction can continue, albeit much more slowly, to great depths, resulting in the formation of a thick, weathered residue called saprolite, which preserves the texture of the host rock. The water table and underlying groundwater may reside within saprolite as well as within the host rock. The word soil is strictly defined as the shallow zone where plant roots and macro-fauna destroy the host rock texture via a process known as bioturbation. The formal term for the combination of soil and saprolite is regolith, but common usage, as will be employed in this book, will designate regolith simply as soil.

Besides soil formation, chemical weathering also results in a radical change in the composition of soil water and groundwater. These changes reflect both the composition of the primary minerals and the degree of biological activity acting to bring about mineral dissolution. Although limited dissolution can occur by reactions of primary minerals with pure rainwater, it is safe to say that most weathering and consequent change in water composition is brought about, directly or indirectly, by biological activity, and it is this intimate interplay between rocks, water, and life that we shall discuss first in this chapter. In essence, this is a "tree's eye" view of weathering.