INTRODUCTION

The rock weathering process alters the physical or chemical state of rocks at or near the surface of the earth, without without necessarily eroding or transporting the products of alteration. Weathering is an assemblage of rock-altering processes that are powered by exogenic solar energy. Weathering is distinguished from other destructive processes by the inclusion in its definition of the concept of in situ, or non transported, alteration. Mass wasting and erosion, which are considered in later chapters, always involve translocation or transportation material.

A common effect of weathering is the detachment of slabs, sheets, spalls, or chips from rock surfaces. Exfoliation is the general term for the loosening or separation of concentric shells or layers of rock. It is caused by chemical processes. Another weathering is mechanical weathering which is also called disintegration, implying that pieces of rock are taken apart or disintegrated without alteration. In contrast chemical weathering is also called decomposition, emphasizing the breakdown of the chemical composition of the mineral grains that make a rock.

MECHANICAL WATHERING PROCESSES

The most important processes by which rock masses are mechanically broken or disintegrated are 1) differential expansion with pressure release, 2) thermal expansion and contraction, including fire damage; and 3) growth of foreign crystals in cracks and pores. The mechanical pressures generated by growing and moving organisms might be important enough to rate a place on this list as well. Each of the process affects different rock types in different ways, and the second and third processes depend strongly on climatic conditions.

Plants and Animals as Agent of Mechanical Weathering

The action of growing plant roots, especially of trees, is often described as mechanical weathering. Two-dimensional networks or sheets of interlaced roots can be followed for many meters, along bedding planes or joints, deep into fresh rocks. It has been supposed that the growing roots exert a pressure on the rock and force cracks to open. However, the efficacy of roots as agents of mechanical weathering probably has been overestimated just as their importance as agents of chemical weathering has been underestimated. Roots follow the paths of least resistance and conform to each little irregularity of a crack, but they do not exert much force to the rock.

CHEMICAL WEATHERING

Chemical or mechanical weathering is generally credited with being about six times more effective than chemical weathering in preparing surface rocks for removal by erosion. However. few rock fragments or mineral grains that are disaggregated by mechanical processes escape simultaneous or subsequent chemical reactions with water, air and the biota. Through the history of the earth, chemical weathering has been a major buffer in the ocean-atmosphere-biosphere-lithosphere system, maintaining atmospheric oxygen and CO2 content and global temperature within narrow limits.

The weathering reactions of carbonation, the reaction of minerals with dissolved CO2 in water, hydrolysis, the decomposition and reaction with water; hydration, the addition of water to the molecular structure of a mineral, base exchange, the exchange of one cation for another between a solution and mineral solid, and chelation, the incorporation of cation from the mineral to organic compounds.

STRUCTURE, PROCESS AND TIME IN ROCK WEATHERING

Structural Influences in Rock Weathering

Lithology, the physical character of intact rock, exerts an obvious structural control on weathering. It is the first factor needed to define rock-mass strength. Some rocks are massive, homogeneous, and nearly isotropic in their lithologic character. Close in importance to the lithology of the intact rock are discontinuities that separate the rock mass into blocks of various sizes and shapes. Ranging from microscopic pores and micro cracks joints and open bedding planes, these discontinuities provide access for water, air, and organisms and allow weathering reaction products to be carried away.

Climatic Influence on Rock Weathering

The impact of climate is directly on rock weathering, in humid midlatitude climate with seasonal freezing, cold weather brings a dormant period for both vegetation and soil microorganisms. Trees  shed their leaves and the humus layer at the surface of the ground accumulates faster than microorganisms, worms, and insects can consume it. Clay minerals that accumulate in the soil prevent water from freely penetrating the grounds and encourage overland runoff. The resulting landscape develops broad, gentle, soil-covered slopes by soil creep and erosion. In the humid tropical and sub-tropical, hydrolysis and residual clay-mineral formation reach to depths of 100 m or more.