Soil & Weathering
- Lichens can be an agent of biological weathering.lichen image by Harvey Hudson from Fotolia.com
Weathering processes are diverse and are classified by the general agents of breakdown. Mechanical weathering involves physical forces that enervate rock masses, from freezing and thawing water to precipitated salts. A transformation of the chemical structure of rock materials is the result of chemical weathering. Action by living organisms that helps breaks apart rock -- like the penetration of tree roots, the nutrient withdrawal of lichens or the rooting of fossorial animals -- constitutes biological weathering. All of these factors can contribute to soil formation or modification. - Weathering can initiate the release of minerals that help build and enrich soil. For example, chemical weathering on a face of granite can produce particles of clay (hydrated aluminosilicates) or silt. The quartz of granite contributes sand crystals. Relative proportions of these granular materials, differentiated partly by size (sand being largest, clay being smallest, with silt in between), are some of the defining characteristics of soils, helping to determine their drainage and nutrient composition.
- Rock bits loosed by frost wedging can be incorporated directly into soil.the summit image by Oneworld-images from Fotolia.com
Weathering also contributes large fragments of rock that contribute to a soil's physical structure. Rock materials are sloughed off larger masses through the cumulative pressure of repetitively freezing and thawing water penetrating cracks and fissures -- a kind of mechanical weathering. Exfoliation, which may be related to a release of pressure on once-buried rocks at the Earth's surface, peels away whole slabs and "rinds" from boulders. These chunks of rocks may be incorporated directly into the soil. Bedrock or rock fragments submerged in soil may be subject to chemical weathering, as acidified rainwater, for example, percolates through the soil and comes in contact with them. - Talus accumulated at the base of cliffs may eventually develop soil layers.silverthorne,dillon,frisco,rocky mountains,mountai image by Earl Robbins from Fotolia.com
Cracks in rock masses that are widened by tree roots -- a version of biological weathering -- may begin to collect organic matter and eventually develop pockets of soil. These pockets, in turn, may support a new plant community. Soil may also develop on aprons of talus. Talus refers to angular chunks of rock gathered at the bases of cliffs and steep slopes, the product both of weathering and "mass wasting" (the slump of materials due to gravity). Over time, soil may accumulate in such a boulder field due to weathering, accumulation of organic detritus and colonization by opportunistic plants like slide alders.
