Functions of weathering

Transcription

1 Weathering

2 Weathering the set of exogenic (physical, chemical and biological) processes that alter the physical and chemical state of rocks at or near the earth's surface intensity of most weathering decreases with depth, because variations in temperature and moistures decrease with depth therefore biochemical weathering is generally confined to the uppermost few metres of soil and rock

3 Weathering

4 Weathering occurs in situ (nontransported alteration), unlike erosion which removes soil and weathered rock; although the 2 sets of processes proceed simultaneously with positive feedback the 2 forms of weathering act simultaneously and affect the nature and rate of one another: disintegration produces an increase in rock surface area while changes in strength with changes in composition

5 Functions of weathering gives rock lower strength and greater permeability, rendering it more susceptible to mass wasting and erosion; reduces strength (cohesion and friction) and increases permeability of rock and therefore decreases resistance to fluid and gravitational stresses; precursor to erosion produces minor landforms, produces landforms in soluble rock (especially limestone) and otherwise creates microrelief (e.g. weathering pits)

6 Functions of weathering releases minerals in solution (e.g. iron oxides, silica, carbonates) which become concentrated to form hard coatings on rocks and hard resistant layers in soil (duricrusts) that inhibit seepage and resist erosion first step in soil formation; ultimately produces an unconsolidated mass of 1) minerals that resisted alteration (e.g. feldspar), 2) new minerals (e.g. bauxite), 3) organic debris

7 Chemical weathering chemical weathering is the decomposition of soil and rock (change in composition) by biochemical processes weathering pits form where water collects and accentuates rates of chemical weathering

8 Processes - Oxidation process by which an element loses an electron to dissolved oxygen iron is the most commonly oxidized mineral element Fe +2 (ferrous iron) --> Fe +3 (ferric iron) or 2FeO + O 2 --> Fe 2 O 3 other readily oxidized elements include magnesium, sulfur, aluminum and chromium gives altered earth material a characterisitic yellowish brown to red colour water table is boundary between oxidizing and reducing environments

9 Processes - Hydrolysis decomposition of minerals in water as hydrogen ions replace cations in minerals pure water is a poor H+ donor, however CO 2 dissolves in water to produce carbonic acid: CO 2 + H 2 O --> H 2 CO 3 (carbonic acid) --> H + + HCO 3 - (bicarbonate) soil air is greatly enriched in CO 2 by decay of humus (up to 30% of soil air as compared to 0.03% of atmosphere) biogenic CO 2 is the major source of carbonated groundwater solubility of CO 2 increases as water temperature decreases (warm beer is flat)

10 hydrolysis is the most important process in the weathering of silicate minerals the most common weathering reaction on earth is the hydrolysis of feldspars producing clay minerals; e.g. K-feldspar -- > kaolinte the other weathering products (silicic acid and ions) are in solution, so the residue is clay the soil water solution becomes more basic as H + is consumed Processes - Hydrolysis

11 Processes - Carbonation (solution) dissolution of calcium carbonate in acidic soil and groundwater CaCO 3 + H 2 CO 3 --> Ca HCO 3 - similar reaction as hydrolysis but the dissolution is congruent, that is, the products are ionic, there is no residue bicarbonate represents the largest constituent of the dissolved load of most rivers carbonation of limestone results in karst topography the insoluble minerals form soil

12 Processes - Cation exchange substitution of mineral cations in solution for those held by mineral grains & crystals changes spacing in crystal lattice but not molecular structure most effective in clay-textured sediments as cations adhere to the surface of negatively charged clay minerals organic matter and sodium zeolites (water softening salt) have a high cation exchange capacity (CEC) CEC = f(temperature, content and chemistry of interstitial water, types and abundance of ions) colloidal suspensions of clay and matter adsorb H + creating acidic soil and weathering environment

13 Processes - Chelation mineral cations incorporated into hydrocarbon molecules (complexing agents or chelates) chelating agents are produced by alteration of humus in plant acids and excreted by lichens; e.g. ethylenediaminetetracedic acid (EDTA) chelates in solution are stable at ph under which the incorporated cation would normally precipitate and thus they are leached in seeping soil water H + released during chelation from organic molecules is available for hydrolysis thus plants contribute to the decomposition of soil and rock waste at depths to the base of the root zone the dissolved load of runoff from barren basalt plateaus in Iceland suggest that the rate of weathering is 1/3 as fast as on lichen covered surfaces

14 Climate and Weathering weathering is an exogenic geomorphic process, i.e. climatically-controlled it is controlled by moisture, temperature and seasonality, the same parameters that define regional climates in tropical climates, high temperatures, large annual rainfall and continuous biological activity maintain high rates of chemical weathering water is the agent of all weatering, except stress release (although water erosion is a mechanism of unloading of rock) all chemical weathering occurs in solution

15 Physical weathering physical weathering is the disintegration of rock and soil aggregates, by physical (mechanical) processes acting primarily on pre-existing fractures (e.g. joints, cracks between mineral grains); reduces size of fragments according to rock and soil structure (producing grains, crystals, blocks, slabs, etc.), with no change in composition

16 Processes - Stress/pressure release disintegration of rock in parallel sheets as it expands in response to the removal of confining stress most common mechanism of stress release is removal of overlying rock by erosion; thus this process is controlled by erosion but subsequently controls erosion the dilation fractures conform to the surface topography and increase in spacing with depth (e.g. a few cm at surface; a few metres at 30 m in Yosemite National Park) most common in granite: rocks cooled deep in crust where pressure is high by overburden; as it rose to surface, pressure decreases stress release causes exfoliation: the separation of concentric layers of rock

17 Processes - Insolation thermal expansion and contraction the surface temperature of dark colored rock can vary from 0-50 o C between day and night, since rock (esp. jointed rock) has low thermal conductivity the differential stresses of expansion and contraction of the outer 1-5 cm of rock causes separation of concentric shallow layers (another form of exfoliation) called spalling or spheroidal weathering when it effects boulders

18 Processes - Insolation thermal expansion and contraction controversy about effectiveness re: the ability of solar radiation to generate sufficient heating and cooling rocks disintegrate after fires, especially rocks composed of minerals with varying coefficients of volumetric expansion (e.g. granite: volume of quarts increases 3X more than that of feldspar) dry granite heated and cooled from 30 to 140 o C for 89,400 cycles over 3 years (equivalent of 244 years of diurnal cycles) produced no perceptible change, even with microscopic examination (Griggs, J. Geol., 44: ); but: 244 years is small amount of geologic time

19 Processes - Freeze-thaw freezing of water in pores and fractures the specific volume (vol./unit mass) of water increases by 9% upon freezing producing stress that is greater than the tensile strength of all common rocks therefore the stress generated by the crystallization of ice is the most pervasive mechanism of weathering, effects all rocks however, the effectiveness of freezing water is influenced by lack of confinement: if more than 20% of pore space is empty, then the tensile stress may be less than the tensile strength, thus frost shattering is most effective in saturated rock decreased freezing point with increasing pressure & impurities (e.g. salt) necessary frequency and magnitude: extent of frost shattering is a function of the combination of frequency, duration and intensity (rapidity and degree) of freeze-thaw cycles

20 Processes - Freeze-thaw hypotheses to account for apparent effectiveness of frost shattering; hydrofracturing: thin (monomolecular) films of water do not freeze even at low temperatures, given strong capillary adhesion to rock; powerful molecular forces in these thin films of semicrystalline water draw water along microfractures opening and propagating them shallow freezing forces films of capillary water along microfractures, disintegrating rocks well below the depth of freezing; e.g. hydrofracturing in the Allegheny Mountains (southern Appalachians of West Virginia and Pennsylvania) extends to m, while frozen ground rarely extends below 1 m

21 Processes - Salt weathering (haloclasty) growth of foreign crystals, mainly hydrated salts which are water soluble at normal ranges of atmospheric temperature and humidity; they hydrate and dehydrate repeatedly generating considerable stresses in fractures and between grain boundaries in permeable rock mostly granular disintegration minerals are transported in solution and precipitate as soil and groundwater evaporate; thus most effective in desert landscapes where water tables are near the surface origin of salt: sea water, chemical weathering of marine or evaporite sediments, dissolved in snow and rain, precipitates in lakes

22 Processes - Hydration (slaking) wetting, swelling and disintegration of soil aggregates, layered and fine grained rocks also pressure of air drawn into pores under dry conditions and then trapped as water advances into soil and rock; suction or pore pressure (less than atmospheric) can exert considerable stress; e.g. biotite expands 40% by volume contributing to the weathering of granite Expansion of Clay Minerals by Volume Ca-montmorillonite45-185% Na-montmorillonite (bentonite) % illite15-120% kaolinite5-60%

23 Processes - Biology minor agent of weathering maintain cracks created by other processes roots may pry rocks apart when tall trees sway in a strong wind; root throw can break fragments away from bedrock as lichens expand and contract or are removed by abrasion they can pull small rock fragments loose

24 Summary Physical and chemical weathering act together to break down rocks Chemical weathering changes composition of parent material Oxidation: element loses an electron to dissolved oxygen Hydrolysis: decomposition of minerals in water as H + replace cations Carbonation: dissolution of calcium carbonate in acidic soil & groundwater Cation exchange: mineral cations in solution substitute for those in minerals Chelation: mineral cations incorporated into hydrocarbon molecules Physical weathering does not change composition of parent material Stress/pressure release: disintegration of rock as it expands in response to the removal of confining stress Insolation: thermal expansion and contraction Freeze-thaw: freezing of water in pores and fractures Salt weathering: growth of foreign crystals which hydrate and dehydrate Hydration: wetting, swelling and disintegration of soil aggregates Biological: plants help disintegrate rocks Both weathering agents essential for soil formation

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45 Weathering

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48 Bornhardt - Inselberg

49 Weathering

50 Weathering

51 Soils

52 Soil formation Soil - several ways to define Unconsolidated material overlying bedrock Material capable of supporting plant growth Regolith encompasses all unconsolidated material at the surface, fertile or not

53 Soil - is produced by weathering Involves chemical, physical, biological processes to breakdown rocks Climate, topograpy, source material composition, and time are factors Soil formation

54 Soil forming processes: Weathering Climate and water a major factor Mechanical Weathering: physical breakdown of minerals by mechanical action. No changes chemically. Chemical Weathering: breakdown of minerals by chemical reaction

55 Soil profiles and horizons A cross section of the soil blanket between bedrock and atmosphere usually reveals a series of zones of different colors, chemical compositions, and physical properties A Horizon Rock material is exposed to heavy leaching B Horizon Zone of accumulation (zone of deposition) Zone of leaching C Horizon Very coarsely broken-up bedrock Below this is R horizon: bedrock or parent rock material

56 Chemical and physical properties of soils Color: dark or light Dark soils tend to be rich in organic matter Light soils generally lack organic matter Texture: size of fragments Sand-sized ( mm) Silt-sized ( mm) Clay-sized (less than mm) Structure: tendency to form peds Ped forming soils resist erosion Finer soils may become loess

57 Soil structure Spheroidal: granular or crumb In surface horizon Plate-like Often inherited from parent material Block-like: angular or subangular B-horizons in humid regions Prism-like: columnar or prismatic B-horizons of arid & semi-arid regions

58 Broad chemical-based classification The effects of chemical weathering, and thus the climatic factor Pedalfer: soil of a humid climate More extensive leaching Less soluble material remains after leaching Iron and Aluminum oxides, hydroxides and clays Pedocal: soil of a dry climate Calcium Carbonate generally present Soil classification

59 Soil classification Soil Scientists use a more sophisticated classification Entisol Inceptisol Aridosol Mollisol Spodosol Histosol

60 Soil classification

61 Soil classification

62 Soil formation Soil formation starts with Primary succession:! formation of soils, development of vegetation, and assembly of microbial, plant, and animal community Lyman Glacier and Lyman Lake, WA; Mt. St. Helens, WA (1984)

63 Soil formation Soil formation continues with Secondary succession:! development of vegetation community, traditionally towards climax Fireweed dominates secondary succession after a 2004 wildfire in Alaska (2006).

64 Soil erosion Weathering is the breakdown of rock or mineral material Erosion is the physical removal of the material that has been weathered Rain strikes breaks up and softens soil Surface run and wind off picks up soil particles Faster moving water or wind will carry off large size particles and a greater load Soil erosion is not beneficial to humans

65 Soil erosion vs. soil formation Soil losses in U.S. amount to billions of tons per year about 0.04 cm per year Human activities, including farming, accelerate the loss of soil In general, soil formation is slower than soil erosion Important factors: Climate and time Nature of the source rock

66 Soils and groundwater Soil can filter toxins Leaching transports soluble elements out of the soil and into groundwater If soil is contaminated, leaching will contaminate groundwater Water saturated soil is exposed to significant chemical weathering

67 Soil as a global resource Soil degradation is a global issue Destructive processes exist such as: Desertification Erosion Deterioration of lateritic soil Contamination from pollution Chemical modification to soil by humans These processes combine to the loss of soil, loss of soil quality, and degraded acreage left to grow enough food for a hungry world. Land area is finite.

68 Geomorphic significance of soils Soils are used to establish relative ages of Quaternary deposits Problems are plentiful when working with paleosols Paleosols can be of three types Buried soils: developed on former landscape and subsequently covered by younger rock or alluvium Relict soils: were never subsequently buried but are at surface Exhumed soils: were buried at some point but haven been reexposed by erosion

69 Geomorphic significance of soils Paleosoil notches