Products of Weathering

Terms: regolith, overburden, soil, karst, inselberg, tor, bornhart, exfoiation dome, corestone, saprolite, granular disintegration, tafoni, weathering pits, scree, felsenmeer, talus, fin, natural arch, hodoo, detrital sediment, transported soil, residual mineral, soil horizon, zonal, intrazonal, azonal, soil order, clay, Kaolinite, mollisol, aridisol

Products of weathering

1. Landforms created by weathering

• Disintegration landforms
• Solution Landforms: Karst

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.

Disintegration landforms

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

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).

Figure 1a.  Deeply weathered granite in the Sierra Nevada Mts., CA.  Weathering proceeds fastest along joints.  The unbroken rock between forms less weathered corestones which when exhumed form tors, such as those in figure 1b.	Figure 1b.  Granite tors in the Alabama Hills along the eastern flank of the Sierra Nevada Mts., CA. These bouldery knobs were most likely exhumed by recent uplift.

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

• http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16839
• http://www.ayersrockresort.com.au/geology/

weathering pits and cavities

Weathering Pits

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.

Figure 4.  Weathering pits in alkali granite, Peabody, MA. This glacial pavement is potmarked with weathering pits formed where water collected, perhaps in what were originally glacial gouged.   The granular disintegration of the granite forms grus, a coarse imature sediment composed of quartz, feldspar and rock grains.

Tafoni (small caverns) Tafoni is a unique type of pitted surface formed by alveolar (honeycomb) weathering,  a style of weathering resulting from salt weathering, and differential solution and precipitation of solutes in sandstones having a soluble cement. The weathering creates pocket-like caverns on rock surfaces, usually sandstones with a calcareous cement.  For other excellent pictures and good explanation go to Dawn Enicos page on tafoni http://www.flickr.com/photos/candiedwomanire/84148980/

Figure 5. Tafoni in the Triassic Moenkopi Formation exposed in the Wopatki national monument north of Flagstaff, Arizona.  Note also the exfoliation. Select image to enlarge.

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:

Felsenmeer: chaotic block fields typically developed above the tree line scree: general term for chaotic blocky rubble Talus: a scree slope formed at the base of a cliff Talus cone: conical scree slope
	Figure 6.  Talus (scree slope) beneath frost shattered rocks in the Picos de Europa, Spain.

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. 

Fins Fins are residual walls of rock that remain after adjacent rock has been removed along systematic parallel joint sets.  Such joints are typically formed by tensional stresses along anticlinal or monoclinal fold axes. The joints leading to the creation of the Courthouse Towers (fig. 7) in Arches National Park formed a thick sandstone (Slickrock Member of the Kayenta Fromation) and are attributed to the deformation and subsequent collapse of salt-cored anticline.

Figure 7. Fins form these walls of the Courthouse Towers in Arches National Park, Utah.

Hoodoos Hoodoos are rock pinnacles formed by differential weathering and erosion of vertically-jointed horizontal beds of shale interbedded with thin resistant beds of limestone, sandstone, or conglomerate.  The best examples and type locality of hoodoos occur in Bryce Canyon, Utah. (hoodoos, Bryce Canyon National Park, NPS, URL: http://www.nps.gov/brca/geology_hoodoos.html) Figure 7.  Hoodoos at Bryce Canyon National Park, Utah eroded into the Tertiary Claron Formation. These dissected intermontane lake sediments are composed of shales, sandstones and thin beds of limestone, each with a characteristic style of weathering noted by the indentations and ridges along the column flanks.   The protective caprock and horizontal ridges are composed of  more resistant limestone and sandstone. (Select image to enlarge.)

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.

Figure 8. Alcove used by the Sinaqua Indians for their cliff dwelling.  Note the smaller aloves formed along the cliff face. Motezuma's Castle, AZ. Click to enlarge.	Figure 9. North Window arch at arches National Park.  Arches form along the contact of the Slickrock Member of the Entrada Sandstone and the Dewey Bridge Member of the Carmel Formation.  Note the exfolation joints along the crest of the arch.  Click to enlarge.	Figure 10. Alcoves, arches and hoodos developed in the Claron Formation, Bryce Canyon National Park.

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.

More Terms

• saprolite: rotten stone
• grus: immature sediment produced by the granular disintegration of granites
• detrital sediment (silt, sand and gravel): rock derived sediment
• new residual minerals: iron oxides and clay minerals and residuals from incongruent solution
• ions in solution: dissolved ions that later form chemical precipitates (salts and carbonates)

Soil

 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 life.
Soil Horizons

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/

1. O (organic layer: O1 vegetation, O2 Humus)
2. A (Leach and washed zone: A1 mixture of mineral material and organic matter, A2 Leach zone)
3. B (zone of accumulation: enriched in clay, iron oxides, caliche in arid regions, or other insoluble material)
4. C Generally unweathered material (unweathered sediment or mechanically broken rock)
5. R Bedrock

 Factors that affect the formation of soil and soil characteristics (similar to those that affect weathering)

1. time: may soils in glaciated regions have had less than 12,000 years to develop
2. climate: precipitation, temperature, and vegetation
3. parent rock: some rocks may dissolve completely (e.g. congruent solution of limestone) or weather at very slow rates (e.g. silicic sandstone).
4. topography: steep slopes are transport limited and retain little weathered mantle
5. drainage: efficient throughflow promotes weathering

Figure  7. Depth and composition of weathered mantle in different climatic settings.  After Strakhov (1967)

Soil Classifications

base on:

1. The degree to which the soil reflects local climate and vegetation (soil orders: zonal, intrazonal, azonal)
2. color/composition http://soil.gsfc.nasa.gov/pvg/color1.htm
3. structure  http://soil.gsfc.nasa.gov/pvg/prop1.htm, consistence http://soil.gsfc.nasa.gov/pvg/consist.htm
4. texture http://soil.gsfc.nasa.gov/pvg/texture1.htm
5. percent organic matter (< or > 25%)
6. development of B horizon (accumulation of oxides and hydroxides)
7. presence of clay
8. water content

Additional Sites

• Soil Characterization Protocols: A Step by Step Guide, NASA's Goddard Space Flight Center, http://soil.gsfc.nasa.gov/pvg/chartoc.htm

Bibliography

Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N.J., 482 p.

Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.

Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.

*Goldich, S., 1938, A study of rock weathering. Jour. Geology 46:17-58.

Hart, M.G., 1986, Geomorphology pure and applied: George Allen And Unwin, Boston MA, 227 p.

Jones, N.W., 1998, Laboratory manual for physical geology: WCB McGraw-Hill, Boston MA, 303 p.

Linton, D.L., 1955, The Problem With Tors: Geographical Journal, v. 121, p. 420-487.

*Ollier, C.D., 1975, Weathering: Longman, London, 304 p.

Prick, Angelique, 2004, Frost and frost weathering: in Encyclopedia of Geomorpphology, A.S. Goulde (ed), Roultledge, New York, NY, p. 412-414.

Ritter, D.F., Kochel, C.R., and Miller, J.R., Process Geomorphology (3rd Edition): Wm.C. Brown Publishers, Dubuque, IA, 544 p.

Strakov, N.M., 1967, Principals of Lithogenesis, Edinburg, Oliver & Boyd.

Summerfield, M.A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p.

*Wampler, J.M., 1997, Mythical influences of crystallization temperature and pressure on the suceptibility of minerals to weathering: Journ. Geol. Edu., v. 45, p. 74-76.