Products of Weathering
Products of weathering
1. Landforms created by weathering
Removal of the weathered byproducts by erosion is an integral part of any landform development, and there are an infinite variety of landforms created by differential weathering and erosion. A few landforms, where the form is dominated by the style of weathering are presented here, others are covered later under other sections.
2. Sediment and soils
Weathering and erosion fine tunes the landscape. Tectonics and internal adjustments rough it out, but weathering and erosion etch it, cover it, and create many of the fine details. I use the term disintegration landforms for features created by mechanical and chemical weathering acting on rocks that are largely insoluble. These landforms reflect styles of disintegration and differental weathering. In contrast solution landforms are formed in soluble rock. These features are covered in the karst outline.
Inselbergs, tors, and bonharts and exfoliation domes
Inselbergs (german, island hill) are isolate rock hills created by long term weathering and erosion. Bonharts and tors are varieties of inselbergs. These hills are steep-sided and typically lack talus at the base suggesting that they were created by deep weathering and exhumation rather than dissection and slope retreat. Joints play an important role in their formation.
Tors (fig 1b) are residual rock masses that display as isolated piles of boulders. Although they typically form in granite they also developed in other litholgies. Linton (1955) theorized a two-stage process of formation. The first involves deep penetration of weathering along jointed bedrock, which produces a thick saprolite mantle intersperse with unweathered corestones(fig. 1a). The second stage is brought on by exhumation either by tectonic uplift or lowering of base level. The granular saprolite is quickly removed wind and water leaving behind the rounded corestone (fig. 1b).
Bornhart (Named after W. Bornhardt, circa 1900, the German geomorphologist who first coined the term "inselberg.")
Bornharts are distinct steep-sided, dome-shaped hills. Unlike tors they are composed of relatively unjointed rock, except for large curved surface joints. These large rounded monoliths are proported to formed by exfoliation following the removal of neighboring weak rock. Probably the two most famous bornharts are Half Dome in Yosemite Valley, CA, and Ayers Rock in Australia. Bornharts are for all intents and purposes large isolated exfoliation domes. Not all bornharts are necessarily considered inselbergs. Half Dome for example, no longer rises above a plain but rather a rugged glaciated alpine landscape. As with tors, exhumation is most likely is important in their development.
Ayers Rock sites
weathering pits and cavities
Weathering pits are depressions created where water ponds in irregularities on rock surfaces. The trapped water locallizes chemical weathering and granular disintegration. Wind and water removes the loosened grains and the depressions enlarge trapping more water in a positive feedback cycle.
Rubble fields and slopes
Fields and slopes of blocky rubble is common in alpine and periglacial setting where freeze-thaw dominates. Examples of such features include:
Caves in the While Mountains of New Hampshire, such as Polar Caves (www.polarcaves.com) and Lost River Caves, are founded in relict periglacial scree slopes. The "caves" are simply interconnected pores is an extremely coarse slope deposit.
Fins, Alcoves, arches, and hoodoos
These are unique features formed by varying styles of disintegration and erosion of horizontally bedded and vertically jointed rock. They are common features of the Colorado Plateau Province.
Alcoves and Arches
Alcoves and arches are weathering features common in the dissected horizontal strata of the Colorado Plateau. They form where chemical and physical weathering is concentrated along horizontal discontinuities where water and salts concetrate, such as the contact between a sandstone and underlying shale bed. Once formed, an alcove enlarges, often through exfoliation. Indians in the the American Southwest built cliff dwellings in natural alcoves, enlarging them by digging out any soft weathered rock and using jointed blocks to enclose them. Arches form where alcoves break through residual fins of rock created by erosion along joints.
Arches National Park, NPS, URL: http://www2.nature.nps.gov/geology/parks/arch/
Regolith, Overburden, Soil
General definition of the above terms: Disintegrated and decomposed mineral and organic matter occurring naturally on the surface of the earth.
Regolith and Overburden:
The terms regolith and overburden are general terms applied to any sediment overlying bedrock. They may be transported or formed insitu by weathering. Transported regolith includes glacial, fluvial, colluvial, lacustrine, marine, or aeolian deposits. With the exception of rare saprolite localities, the regolith in New England and the Upper Midwest is transported, mostly by glaciers and their meltwater by products. Insitu soils are found south of the glacial limit. Transported regolith also occurs in alluvial valleys and deserts.
Soil: The surface residuum
created by the disintegration, decomposition, and translocation of mineral
and organic matter over and extended period of time. Soil forms
by the interactions of water, rock and organic activity, and contain
layers of mixing, leaching and accumulation called soil horizons. Another
common requirement of soil is that it is capable of supporting plant
Soils typically develop distinct horizontal weathering zones call horizons. The five principle horizons are listed below. Not all horizons are present in all soils or are developed to the same degree. There are approximately 12 soil orders. (See USDA soil taxonomy Wikipedia, URL:http://en.wikipedia.org/wiki/USA_soil_taxonomy or the USDA NRCS The Twelve Orders of Soil Taxonomy for a high resolution printable poster, Univ. Idaho also has a good site). The orders are further subdivided into suborders, great groups, subgroups, families and series, which are beyond the content of this course. For examples of representative soil series go to USDA NRCS Representative and State Soils, URL: http://soils.usda.gov/gallery/state_soils/
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