In breaking down and excavating rock, weathering can have a profound affect on landscapes.
Weathering is the first step in the process of denudation, which involves the breaking down and transport of rock materials. It is often subtle in action, involving massive periods of time and such inconspicuous phenomena as the behavior of water or chemical reactions in rock joints. Nonetheless, it can have a profound impact on landscape features. Does this Spark an idea?
Setting the Stage
One way weathering affects landscapes is by essentially "preparing" such rock material for further steps in denudation. The rubble shattered and sheared off rocks worked upon by frost-wedging or exfoliation, for example, work their way down a slope or cliff by the force of gravity--a denudation phenomenon called "mass wasting." Transferred down into a gorge, such rock materials may then be carried by rivers in flood miles from their source, an example of active "erosion."
Breaking Apart
The effects of frost-wedging are often dramatically obvious in the high mountains.
The seepage of water into cracks in a rock mass might seem insignificant, but it has the potential power to rend these masses apart through a form of mechanical weathering called frost-wedging. When water freezes into ice, it expands. Water frozen in rock crevices for an entire season or simply during the night can be freed to penetrate further when it melts; then, freezing again, it continues to exert outward pressure. Over time, this freeze-thaw cycle can widen rock interstices, slough off chunks or split boulders wide open. Landscapes heavily affected by frost-wedging often include high mountain ridges and summits, where the freezing and thawing of water is a relentless, frequent phenomenon. Traveling up to timberline and above, you'll often see isolated boulders laced with fractures, frequently with bits of splintered rock lying in the vicinity.
Biological Weathering
Lichens are agents of biological weathering.
In the same high country, you may see evidence of biological weathering, which involves the work of living organisms. Hardy trees like pines can take root in cracks in rock faces. The penetration of these joints by their roots can force them open further. Lichens often colonize bare rock and extract nutrients directly from it, weakening its structure; their expansion and contraction as they moisten and dry can also work on rock in similar fashion as frost-wedging. Fissures opened by tree roots or lichen can accumulate the organic and geologic material that builds soil, encouraging colonization by other plants.
Chemical Weathering Effects
In layers of carbonate rock, the chemical reactions that proceed as rainwater and groundwater move through its interstices often result in what is called chemical weathering. A dramatic and widespread example is the dissolution of limestone by percolation of acidified water. This process may open up great cavities and tunnels in subterranean limestone expanses, creating entirely new landscapes drained by underground rivers and inhabited by a diversity of cave-dwelling organisms. Chemical weathering, which also takes other forms, can act as a soil-building agent, contributing materials from weathered bedrock to accumulations of organic matter.
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