Rock Weathering and Soil Formation PDF

WEATHERING , EROSION & SOIL FORMATION The Earth’s surface is ever changing………(There are evidences of paleogeographic changes through out the geologic pasts) No two parts of the earth’s surface are same………… (Different places are dominated by different geologic agents and their...

WEATHERING , EROSION & SOIL FORMATION The Earth’s surface is ever changing………(There are evidences of paleogeographic changes through out the geologic pasts) No two parts of the earth’s surface are same………… (Different places are dominated by different geologic agents and their style of working is different)  Different agents of earth are continuously working to make the surface smooth…….(Some times seasonal- Flood, Glaciations) Some are commonly slow (water/wind/chemical agents/gravity but exceptions are there) and some are fast….. (Land slide/Tectonic forces) This laid to the change in geography of any region………. (starts from small scale and become large with time) 1970 Place remains the same but geography changes with time It starts from disintegration and decomposition of a small grain of rocks to change of geography The processes by which the rocks of the earth get disintegrated/decomposed are called weathering…. Except for minor surface roughening, weathering alone does not make landforms. Instead, it provides altered or broken rock from which landforms are shaped Weathering is an assemblage of rock- altering processes that are powered by exogenic, essentially solar, energy. The depth of weathering is thereby restricted by the depth to which exogenic-powered processes can operate Weathering is distinguished from alteration caused by hydrothermal liquids and gases. Although the chemical reactions may be similar, weathering can be expected to decrease with depth, whereas hydrothermal alteration might increase Rock weathering must always be studied in detail as a set of mineralogic or geochemical processes because rocks are assemblages of minerals, and every mineral has specific physical and chemical responses to the near-surface environment Numerous observations and experiments verify that in the presence of water, rock-forming minerals lose strength and are much more easily broken and deformed (Kirby, 1984, and references therein). These mineralogic studies become geomorphically significant with the demonstrations that meteoric water (derived from the hydrologic cycle) circulates to depths of 10 to 20 km  Fracture permeability permits groundwater to circulate to depths of 10 km or even 20 km along fault zones (Costain et al., 1987; Nesbitt and Muehlenbachs, 1989).  Circulation to depths greater than 3 km is a widespread phenomenon in the Appalachian provinces of the eastern United States (Tillman, 1980).  Water with the isotopic composition of rainfall circulated around Eocene-age plutons in Idaho to depths of 5 to 7 km (Criss and Taylor, 1983). Thus rocks can be altered, at least in part by exogenic process and therefore weathering, at depths of many kilometers and for millions of years before they are exposed by uplift and erosion to create a landscape. Basic structures of the rock mass, including bedding and other primary layered structures, granularity, and intrusive dikes or veins, are not obliterated until extremely advanced stages of weathering Weathering is distinguished from other destructive processes by the inclusion in its definition of the concept of in situ, or nontransported, alteration. Mass wasting and erosion, always involve translocation or transportation of material Weathering is the precursor of mass wasting and erosion Weathering continues even after a rock fragment has become dislodged from a hillside ledge or pried loose from the bed of a stream Weathering processes do not end until a rock fragment has been finally dissolved, transformed to a stable compound, or buried or submerged beyond contact with the atmosphere and circulating groundwater  corestone 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 Exfoliation is caused by chemical, thermal, and physical processes Many physical rock properties such as permeability, seismic velocity, electrical resistivity, elasticity, and strength are determined by the number, orientation, and interconnectedness of pores and fractures Whenever a rock mass is fractured, whether by internal or external stresses, the resulting fragments have sizes with a statistical distribution that follows Rosin's law (Bennett, 1936). Rosin distribution produces an excess of fine fragments, with particle abundance in inverse proportion to their size, following a fractal distribution Selby (1980) defined eight parameters for assessing rock-mass strength for geomorphic purposes (1) strength of intact rock (2) state of weathering of the rock (3) spacing of joints (4) orientation of joints with respect to the hillslope (5) width of the joints (6) lateral or vertical continuity of the joints (7) Infilling of the joints (8) Movement of water out of the rock mass Out of the eight parameters, all refer to either weathering or the fractures that divide the rock. Only the first refers to the strength of the rock material itself Weathering-The First Step in the Rock Cycle Weathering-The First Step in the Rock Cycle How rocks disintegrate – Weathering The chemical and physical breakdown of rock exposed to air, moisture and living organisms – Regolith A loose layer of fragments that covers much of Earth’s surface – Soil The uppermost layer of regolith, which can support rooted plants The rock in the photo has weathered in place with little erosion, forming soil  Weathering: Three Types Mechanical weathering The breakdown of rock into solid fragments by physical processes Chemical composition of rock NOT altered Mechanical weathering does not involve any significant transportation Chemical weathering The decomposition of rocks and minerals by chemical and biochemical reactions Biological weathering Weathering by biological process (plants, animals, human being etc.) Reasons of Mechanical weathering: 1. Pressure release on unloading: The rocks exposed on surface were at depth once. As they expose to a new environment of changed P&T condition, they are not stable On pressure release, rocks expand 0.1 to 0.8%. So the rocks at great depth once exposed to surface are more prone to weathering than the rocks at shallow depths Examples of Mechanical weathering by pressure release A. The Alps in Switzerland has suffered 30 km of denudation in last 30 M years, one of the most rapid rate reported so far B. Due to pressure release, distinctive sets of joints develop on rocks (sheeting joints/exfoliation). Sheeting joints are parallel to the land surface and form concentric shells or layers up to a few feet in thickness. Spacing in joints increase with depth. They rarely extend more than a few hundred feet below the land surface C. Sheetings are best seen in granite and massive quartzite Probably, sheeting joints develop only near the land surface because the geothermal gradient decreases to much less than normal in the outer 200 m of the crust and the temperature of the rock becomes nearly constant. The overburden pressure, however, continues to decrease at a rate proportional to the rate at which overlying rock is peeled off by erosion. No compensating contraction due to decreased temperature is possible at shallow depths, so the expanding outer layers of rock are bowed upward or outward toward the land surface, and sheeting joints develop If a rock mass is irregularly fractured at depth, the most fractured portions adjust readily as the superincumbent load is removed, but portions with few or widely spaced fractures store the stress of expansion until the overburden pressure is so low that failure occurs. Such failure is parallel to the land surface at a depth of 100 m or less, so the sheeting joints conform closely to local topography. The size of monolithic domes is determined largely by the spacing of faults and joints other than sheeting, but their shape is determined by sheeting In glaciated mountains, sheeting on cirque walls and floors is concave skyward, parallel to the bowl shape of the cirques, whereas the sheeting joints are convex on the adjacent convex summits. At the rims of cirques, the upland and cirque-wall joint sets intersect D. Due to quarrying/excavation, rocks expand into the opening and hence rock burst occur. Hence during tunneling, excavation cycles are followed To prevent further rock bursting in tunnel, Iron bars are supported at the tunnel wall and roof Hill slope sudden collapse due to removal of rocks E. A valley with small anticlines in Northampton, England. Here, brittle sandstone beds break apart due to anticlinal valley and the underlying claystone bulged upward Vanivilas Sagar Dam in Karnataka, Chitradurga schist belt Synclinal hills near Saketi, Himachal Himalayas 2. Growth of foreign crystals in cracks or pores: If water is confined in the cracks of rocks and freezes, expansion on freezing generates very high stress within the rock. Under atmospheric condition, water increases 9% specific volume (vol/unit mass). But the expansion ratio increases in a confined system to the same temperature A maximum pressure of 2100 tons/sq foot is observed at -22°C that is sufficient to crush the strongest rock Frost cracking and hydrofracturing is the process by which freezing water breaks the rock (generally in high hills and permafroast region) Certain minerals, such as pyrite (FeS2) in shale or slate, oxidize readily on exposure to oxygen-bearing groundwater or a moist atmosphere. The newly formed iron oxides are of lower density and larger volume than the original minerals, and the volume increase can be sufficient to split weak rocks or cause spalls or "pop-outs" in strong rocks. Of greater significance to weathering processes are the numerous salts that are water soluble (Evans, 1970). The force of crystallization around nucleii of precipitating salts is substantial. Halite (NaCl), for example, when precipitating over a range of temperature between 0°c and 50°C from supersaturated solutions, generates crystallization pressure ranging from 54 to 366 MPa (554 to 3737 kg/cm2) (Winkler and Singer, 1972). Such pressures exceed the strength of almost all rocks. Supersaturation is a necessary condition for the growth of salt crystals but is easily achieved either by cooling of a saturated solution or by evaporation Dendritic salt crystals, or even crusts, may form in cracks, along mortar joints in buildings, or on grain boundaries within a permeable but otherwise coherent rock Hydration pressure Hydration pressure is more effective than simple crystal growth because, as temperature and humidity change seasonally or daily, the salts may hydrate and dehydrate repeatedly (Winkler and Wilhelm, 1970). The hydration pressures of common salts range from 10 MPa to more than 200 MPa (100 to more than 2000 bar). In industrial areas, dilute H2SO4 attacks marble buildings and cemented floors and within brick joints and create cracks Mirabilite (Na2SO4·10H2O) which is unstable and quickly dehydrates in dry air, the prismatic crystals turning into a white powder, Thenardite (Na2SO4). In turn, thenardite can also absorb water and converts to mirabilite. If mirabilite present in building materials it creates cracks easily Abrasion The gradual wearing down of bedrock by the constant battering of loose particles transported by wind, water or ice B. Soils are heaved up due to freezing of pore water Thin film of water forms semi-crystalline ordered molecular structure in clay pores helps in developing micro fractures  3. Thermal expansion and contraction A. Boulders and cobbles expose to hot desert sun and are found broken somewhat like orange slices Desert travelers have described hearing rifle shot sounds of stone breaking in Sahara desert and are said to have gone to battle alerts at this sound Thermal expansion in hot days and cooling in cold evening have been claimed as cause of the cracked stones Differential expansion generates stress in the rock, i.e. quartz expands 3 times more than feldspars. In addition, quartz, feldspar, and many other common minerals expand under heat in a highly anisotropic fashion, with expansion along certain crystallographic axes being as much as 20 times greater than along other axes. Therefore, temperature changes create stresses both within and between mineral grains, sufficient to cause disaggregation. Thermal expansion is a prominent process of weathering in moon. Ground temperature in deserts can exceed 85°C with diurnal fluctuation greater than 50°C (Goudie, 1989, pp. 12- 13). Because the heating decreases rapidly with depth, during daytime the outer few centimeters of a rock expand relative to the deeper parts, but as the outer shell cools, it contracts faster than the still-warm interior. This lag effect (Warke and Smith, 1994, p. 63) creates diurnal alternating compressive and tensional stresses, especially in the outermost few millimeters of rock where the changes are most extreme. Thermal effects are not limited to low- latitude deserts. They are even more pronounced in regions of extremely low air temperatures, such as Antarctica. Presence of moisture and the various soluble salts that commonly precipitate on rock surfaces, terrestrial ranges of temperatures can fracture rocks. Effect of fire on rocks can cause rock burst Thermal shock generated by brush fires and forest fires has been observed to spall 5 to 50 percent or 70 to 90 percent of the surface areas of boulders. Spalls, ranging in thickness from a few millimeters to a few centimeters, remove areas of several hundred cm2 from boulders, mostly on their sides Presence of water in rock pores and fractures has an important but variable influence on fire spalling. On the one hand, by boiling away, it can delay and reduce the heating of the rock. On the other hand, the steam can be a potent mechanical and chemical reactant (Allison and Goudie, 1994) Physical or mechanical Through the history of the earth, weathering is generally chemical weathering has been a credited with being about six major buffer in the ocean- times more effective than atmosphere-biosphere- chemical weathering in lithosphere system, maintaining preparing surface rocks for atmospheric oxygen and CO2 removal by erosion (Lasaga content and global temperature et al., 1994, p. 2376). within narrow limits An important principle of chemical weathering is that mineral dissolution is concentrated along sites of excess surface energy such as crystal dislocations, cleavage planes, microcracks, and other imperfections Chemical weathering is governed by the kinetics of chemical reactions at these activated sites on mineral surfaces, and not by diffusion rates However, rapid chemical reactions require steep gradients and continued disequilibrium between the reactants (Brantley and Stillings, 1996). If reactants are not renewed and equilibrium is approached, the reactions slow down Dissolution can be either congruent, in which the components of the solid are equally soluble and occur in the solutiion in the same proportion as in the solid (NaCl dissolving in water is a simple example), or incongruent, if certain soluble components go into solution and other less soluble compounds form residual solids Water in rocks is in at least four physical states: (1) Water chemically combined in hydrated minerals (2) Hygroscopic, retained, or bound water adsorbed on mineral surfaces as ordered films on the order of 0.1 lm in thickness, with density and viscosity much greater than normal and not mobile under gravity (3) Capillary water, filling or wetting cracks and voids, intermediate in mobility between hygroscopic and gravitational water (4) Gravitational water that moves through larger pores and cracks under gravity and other pressure gradients (Bell, 1992b, p. 78; Pope et al., 1995). In each of these states, water facilitates weathering in a variety of ways: as a transport medium for reactants and solutes, as a solvent, by exerting mechanical pressure, as a chemical reactant, and as a chemical buffer (Pope et al., 1995) Chemical weathering process is exothermic and the products are less dense and more voluminous then their parent materials, that is opposite to the process acting at depth Experiment carried out by J.H.Feth (1964) suggests rapid reactions of water by tracing dissolved materials in it. (a) Fresh snow had dissolved material in a few ppm, consisting of CO2, NaCl and dust materials (b) As snow melt water is soaked in soil, mineral content increases 7.5 times. Silica increases 100 folds (c) After several months mineral content is doubled A. Oxidation Oxidation always takes place with water (rusting in iron). It is one of the typical volume increasing reactions between minerals and the wet atmosphere; especially common is the reaction of iron- bearing minerals with oxygen dissolved in water Reduction of iron to ferrous state takes place by some organic process in stagnant water. It imparts grey/dark grey colour to sediments and results in deposition of foul smelling organic rich mud. Oxidation-reduction reactions are inevitable/ with the result that instead of the mineral going into hydrous solution as ionic species, new compounds are formed, some of which are nearly insoluble Unprotected iron surfaces stay clean and bright in extremely dry air, as is proved by the condition of tools and equipment found in polar regions or in the Libyan desert decades after they were abandoned. https://inspire99.com/none-can-destroy-iron-but-its-own-rust-can-ratan-tata/ The oxides of iron and related metals are exceptionally stable chemical compounds/ even on the geologic time scale. Precambrian rocks rich in iron oxides are widely distributed on the continents, most of them dating from the time about 2.5 billion years ago or less when our evolving atmosphere began to be oxidizing (Pinto and Holland, 1988). They are important sources of iron ore today. B. Carbonation CO2 dissolves readily in water. Low T and high P encourages dissolution. CO2 in rain water comes from atmosphere. Decay of vegetal material produce CO2 in soil. So the CO2 content in soil is 10-1000 times in soil whereas in atmosphere it is 0.03%. Dissolved CO2 in water forms a weak acid (Carbonic acid). The reaction of carbonic acid with minerals is carbonation. Carbonate rocks such as marble, dolomite, limestone , chalk are most affected by this process. Cold water dissolves more carbon dioxide gas than does warm water; water under pressure also dissolves more gas Air that fills pore spaces in soil is greatly enriched in carbon dioxide by the decay of humus. Soil air drawn from the biologically active upper layer of a soil profile may have from 10 to 1000 times more CO2 than that in the free atmosphere; that is, CO2 may constitute as much as 30 percent of soil air compared to the 0.03 percent of dry normal atmosphere. As limestone dissolves, the impurities such as clay, quartz, sand and residual iron minerals laid down below called Terra Rossa In limestone terrain a typical erosional landform is found called Karst Topography Blind valley: In limestone terrain, due to chemical weathering, large underground channels are made. Rivers/streams flowing on the surface often follow the underground valley and emerges somewhere else. This valley is called blind Valley In Karst region Development of Stalactite and Stalagmite Are due to chemical weathering of limestone Hydrolysis: The most important chemical reaction of silicate mineral is Hydrolysis. For example olivine and water reaction Mg2SiO4+ 4H+ + 4 OH- 2Mg+++ 4OH-+ H4SiO4 Olivine Ionised water Ions in solution Silicic acid The results of such complete hydrolysis is that, the mineral is entirely dissolved in water assuming that excess of water is available to carry ions in solution CO2 dissolves in water creates concentration of H+ ion and hence makes the water acidic which promotes hydrolysis Hydrolysis of feldspar produce Kaolinite (clay) Alkali Aggregate Reaction: Fracture in Hirakud Dam Hydration: Weathering by hydration involves addition of molecules of water in interlayer space mainly in some clay minerals cause to expand. Some clays dehydrate and hydrate alternately depending upon the moisture available. After a heavy rain some clay minerals hydrate and swell up but after drying they shrink and create engineering problems  BIOLOGICAL WEATHERING Chelation is a significant process of biological weathering by which metallic ions are strongly bounded with hydrocarbon molecules Plant root maintain negative charge on its surface and a surrounding field is H+ ions. Organic compounds absorb and chelate essential metals fractions as they hydrolyse Experiments carried out by Lovering and Engel, 1967 shows that the growth of Equisetum and three other grasses remove silica from fresh crushed rock powder which is equivalent to removing of all silica from a basalt of 30 cm thick slab in 5350 years. Fungi grown in nutrient solutions of freshly crushed rock enhance significantly the amount of Si, Al, Fe and Mg. PH drops from 6.8 to 3.5 in 7 days and this enhance chemical weathering All chemical weathering is enhanced by microbial activities Cyanobacteria are representatives of the oldest known forms of cellular life on our planet. Their well-preserved remains have been found in early Archean rocks that are 3.3 to 3.5 billion years old (Schopf and Packer, 1987). The fossil record of oxygen-producing photosynthetic microorganisms (microbes) demonstrates that biologic weathering processes have been active for essentially the entire geologic history of the earth (de Ronde and Ebbesen, 1996). Microbes are now being cultivated and genetically engineered to leach commercial heavy metals from mine dumps, to immobilize radioactive and toxic wastes from leachates, and to increase food production by higher plants (Lindow et al., 1989). The importance of symbiotic mycorrhizal fungi and associated bacteria that live on the roots of higher plants and facilitate their mineral uptake has led to the introduction of the term rhizosphere for that part of the soil weathering profile around roots in which microbes are active (Berthelin, 1983, p. 249i Yatsu, 1988, pp. 366- 375, Robert and Tessier, 1992).  WEATHERING OF SILICATE MINERALS Laboratory experiment on chemical reaction between minerals and water was carried out to find out the relative order of rapidity of weathering of silicate minerals and weathering series constructed as follows: This is called Goldich stability series and is similar to Bowen’s reaction series Si-O bond is very strong. High Temp. minerals are low in Si:O and low stability. High Si:O indicates high bond strength and produce more resistant mineral except for biotite which weathers fast. Biotite weathers first because iron oxidizes and layered mica structures expand on hydrolysis. Relative chemical weathering can be studied the presence of citations of elements in ground water/river water: Na, Ca, Mg are the most mobile, K and Si are less mobile, Fe, Mg are least mobile (Feth’s cation mobility series) Colman (1982) studied weathering rinds in basalt and andesite and found sequence of mobility as: Ca> Na> Mg > Si > Al ≥ K > Fe> Ti  Factors affecting weathering Tectonic setting – Young, rising mountains weather relatively rapidly – The Himalayas weather rapidly than the Easternghats (Therefore, Himalayan terrain is more prone to landslide than the Easternghat) – Mechanical weathering is most common (Tectonic force is the dominant agent of weathering and fluvial and gravity are the next)  Why do major landslides occur near plate boundaries? Tectonics and mass wasting – World’s major historic landslides clustered near converging lithospheric plates High mountains undergo rapid weathering Earthquakes near plate boundaries can trigger landslides This massive slide was triggered by A magnitude 9 earthquake in Alaska near a subduction zone.  Rock composition – Minerals weather at different rates Calcite weathers quickly through dissolution Quartz is very resistant to chemical and mechanical weathering Mafic rocks with ferromagnesian minerals weather more easily  Rock structure Distribution of joints influence rate of weathering Relatively close joints weather faster Antiform weathers faster than synform  Topography – Weathering occurs faster on steeper slopes Rockslides Frequent rock fall in the Himalayan roads Bending of trees on hill slope  Vegetation has both positive and negative affect – Contribute to mechanical and chemical weathering – Promotes weathering due to increased water retention – Vegetation removal increases soil loss Vegetation can both hold water And increase weathering. If removed Rocks may also be vulnerable to abrasion  Biologic activity – Presence of bacteria can increase breakdown of rock Vascular plants and associated microbial communities affect the nutrient resources of terrestrial ecosystems by impacting chemical weathering that transfers elements from primary minerals to other ecosystem pools  Climate – Chemical weathering is more prevalent in warm, wet tropical climates Mechanical weathering less important here – Mechanical weathering is more prevalent in cold, relatively dry regions Chemical weathering occurs slowly here Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering Colour dots on map match colors on chart  Products of Weathering Clay – Tiny mineral particles of any kind that have physical properties like those of the clay minerals – Clays are hydrous alumino-silicate minerals  MINERAL DEPOSITS Khetri, Rajpura-Dariba Cu deposit in India, Rajasthan Beach Placer in Gopalpur, East coast VISAKAPATNAM  Bauxite Deposit in Easternghats Bauxite Deposit on Deccan trap Residual Concentration of Bauxite on Easternghat in Odisha and on Deccan basalt in Maharashtra https://www.sciencedirect.com/science/article/pii/S016913681300019X https://www.tf.uni-kiel.de/matwis/amat/iss/kap_a/advanced/ta_1_4.html 79  Sand – A sediment made of relatively coarse mineral grains Soil – Mixture of minerals with different grain sizes, along with some materials of biologic origin – Humus – Partially decayed organic matter in soil  https://serc.carleton.edu/NA GTWorkshops/health/case_s tudies/hydrofracking_w.html https://www.123rf.com/photo _14855761_cultivation-of- carrots-in-the-sand-in-a-field- in-normandy.html https://www.unicalce.it/en/pr oducts/glass/ http://ptedo.com/pakistan/infras tructure-construction/ 81 https://slideplayer.com/slide/ 4181831/ Pedology (soil science) and geomorphology have always been closely linked (Knuepfer and McFadden, 1990) The term ‘soil’ is used to describe the rock detritus at the surface of the earth that has been sufficiently weathered by physical, chemical, and biologic processes so that it supports the growth of rooted plants. (agricultural definition) For engineers all the loose, unconsolidated, or broken rock material at the surface of the earth, whether residual from weathering at that place or transported by rivers, glaciers, or wind, is soil Soils are characterized by horizons: distinctive weathered zones, approximately parallel to the surface of the ground, that are produced by soil-forming processes The five principal horizons, from the surface down into unaltered rock, are conventionally given the capital-letter symbols O , A, E, B, and C  (R) Below the five master horizons is the underlying parent material, such as bedrock, alluvium, or other material from which the soil has formed. It is sometimes referred to as the R horizon The horizons in a soil profile illustrate the principle that many mechanical, chemical, and biologic weathering processes operate simultaneously near the surface of the earth but at different rates and to various depths. Most geologists would observe the effects of mechanical weathering on the rock mass in the upper part of the R horizon. Its lower limit is not specified To pedologists, the R horizon or regolith usually implies the rock mass beneath the soil profile By colour, chemical analyses, grain size, and other diagnostic criteria, soil profiles are divided into horizons and subhorizons that record the intensity and duration of the various soil-forming, or weathering, processes Five key factors of soil formation have been long recognized by pedologists: Parent material Climate Topography Organisms Time Under certain conditions, each of the five variables can exert the controlling influence Three of the factors-climate, organisms, and topography-are likely to change during the progress of soil genesis Parent material is the dominant soil-forming factor. A residual soil cannot contain minerals that are not present in, or cannot be made by weathering of, the parent material. A Paleozoic limestone is composed of 90 % calcium and magnesium carbonate, 7 % quartz sand, and 3 % detrital clay. Weathering by solution of 5 m of this limestone produces a residual parent material 1 m thick. The residuum is 70 % sand and 30 % clay and hydrated iron oxides. Extreme example of the contrast between bedrock and residual soil is the great surficial deposit of lateritic bauxite Residual soils show considerable variation of engineering properties form top layer to bottom layer. The transition is observed gradual. Relatively finer materials are found near ground surface and they become coarser with depth to reach larger fragment of stone. Thus residual soils can be very different from their parent material, given the right combination of the other four soil-forming factors Nevertheless, structural or parent material control on weathering is as important in soil genesis as it is in landscape development Climate must compete with parent material for the status of "first among equals" in the list of soil forming factors  Climate – Chemical weathering is more prevalent in warm, wet tropical climates Mechanical weathering less important here – Mechanical weathering is more prevalent in cold, relatively dry regions Chemical weathering occurs slowly here Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering Organisms as a factor in soil formation have been reviewed in their role as chemical and biological weathering agents Organisms utilize easily digestible materials (like simple sugars and carbohydrates) found in the plant material, leaving more resistant materials (such as fats and waxes) behind. The material left behind is not easily decomposed; it comprises the humus found in soil. Humus acts as a gluing agent, essentially holding primary soil particles (sand, silt, clay) together to form secondary aggregates or ‘peds’. The topographic influence on soil formation is emphasized more by pedologists than by geologists If a slope is steep, runoff is rapid, erosion removes the soil as fast as it forms, little water enters the soil, and the profile is thin and poorly developed. Time: Soil chronosequences can provide valuable clues to the relative or even numerical ages of various landforms (geomorphic evolution) A basic issue of pedology is whether soils continue to evolve as some function of time, which might be linear, logarithmic, or exponential, or whether they reach equilibrium with land-surface lowering so that a "steady-state" soil profile and weathered zone move downward through the rock masses as the surface is lowered.................? Recent studies show that even soils several million years old continue to evolve (Machette, 1985; Harden, 1986-1988; Harden et al., 1991; Birkeland, 1992, p. 276; Markewich et al., 1994). Such long-term weathering must have been in progress during very significant late Cenozoic climate changes In soil chronosequences, a succession of coastal or river terraces, glacial moraines, alluvial fans, or other landforms of progressively greater age but with similar parent materials, vegetation, slopes and climate, are shown to have soil profiles with progressively greater development  10 12 14 16 18 0 2 4 6 8 OSLF FrP MHP EP-I EtEP-II OdPt AHF (SF GTFn-I GP-II GT1 GHT-II OGP-I GP-I PVF OGP OGYP YSLF GTFn-II Opt OdPt-I DAP GTFn-III FlPn KrTFn RTFn-II OP OdYPn-II OdYPn-I OdSjPn-I OdYPn-III GPC OdKaPn Ypa MaFTn OdYTFn KTFn WMF ShFn YgCgTFn-III YgCgTFn-II Ypt GSP YPdP YgCgTFn-I YP YSP KFn-I BTFn BGFP BGTFn-II YGP GHFP GFP GOFP Three sets of processes are seen in soil chronosequences: 1. In arid regions, especially, the addition of silt and clay to the soil by wind, with significant additions of Ca2+ from weathered silicate and carbonate dust, and Na+ from seawater 2. The transformation of minerals, especially the progressive alteration of clay minerals and silicates to stable hydrous oxides 3. Transfer of various soil components to progressively greater depths (Birkeland, 1992) Chronosequences in desert soils can be based on the extent of secondary mineral accumulation, especially of calcium carbonate Calcrete is a massive shallow accumulation of soil carbonate that may be a meter or more in thickness and may effectively control scarp formation A series of six stages of calcium carbonate accumulation in arid and semiarid soils has been established, ranging from a few percent CaCO3 that forms coating on pebbles (Stage I), through layers of coalesced nodules (Stage III), to massive cemented surface layers 0.5 to 2 m in thickness in which 25 to 75 percent of the soil mass is pedogenic calcrete (Stages IV to VI) (Machett, 1985) Stage I soils range from Holocene to late Plleistocene in age Stage IV and older calcrete layers are found on land surfaces that range in age from 100,000 years to several million years (a, b) Field photograph showing the paleosol horizon and the overlying and underlying litho units, (c) occurrence of calcareous nodule in the soil and faintly developed sub-angular blocky structure, (d) Nodules recovered from fractures in the paleosol from PG basin, India Most of the carbonate that has accumulated in dry soils is probably derived from windblown dust-size fragments of calcium-bearing minerals that were previously weathered by hydrolysis to calcium bicarbonate in solution, then precipitated as calcium carbonate and blown by the wind (Reheis et al., 1995). Lesser amounts are derived from weathering in place and from upward or laterally moving groundwater (Machette, 1985)  Soil Classification Each developed nation has a scheme of soil mapping that enables its agronomists and economists to plan the effective use of available land Modem soil classifications attempt to classify according to definable, observable chemical and physical properties of the horizons in soil profiles Soil classification proposed by U.S. Department of Agriculture in 1965 is the best and most comprehensive classification of soils and soil-forming processes yet devised and is being used world wide (Soil Survey Staff, 1975) In U.S. soil taxonomy (Soil Survey Staff, 1975) the hierarchy of terms, in descending order of rank, is: Order syllable Suborder ˈsɪləb(ə)l/ noun Great group A unit of pronunciation having one vowel sound, with or without Subgroup surrounding consonants, forming the whole or a part of a word; for Family example, there are two syllables in water and three in inferno. Series Eleven soil orders are defined, based on the presence or degree of development of the various horizons in soil profiles The name of each soil order ends in sol (L. solum, soil) and contains a formative element that is used as the final syllable in the names of taxa in suborders, great groups, and subgroups The formative elements are derived from Greek or Latin roots, are as short as possible, and are designed for use in any modern language By compounding the several ranks of formative elements, any soil type on earth can be classified Thus the name of each of the 55 suborders consists of 2 syllables, a formative element For example, a well drained soil formed in silty, calcareous, glaciolacustrine deposits of late Pleistocene age: It is classified at the subgroup level as a Glossoboric Hapludalf Decoded, using the formative elements It belongs to a subgroup of the great group Hapludalf, the suborder Udalf, and the order Alfisol Translating the formative elements of the name, the soil has a gray to brown surface horizon, moderate to high base (cation) saturation, and clay accumulation in a subsurface horizon (formative element alf; It is of the suborder of Alfisol that forms in humid regions (formative element ud) The compound adjective glossoboric precedes the great group name Glossic refers to minor but characteristic tongues of the clay-depleted A horizon that extend down into the clay-enriched B horizon If this tonguing were more pronounced, the soil would be a Glossudalf instead of a Hapludalf Boric is from the root boreal; as defined for this subgroup, the mean annual soil temperature is less than 10°C