Pelapukan and Soil

7/25/2019 Pelapukan and Soil

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Chapter 6: Weathering and Soils


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Key Questions

How does rock change as it weathersphysically?

How does rock change as it weatherschemically?

What factors influence the intensity ofweathering?


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bauxite caliche

carbonic acid

dehydrate dissolution

exfoliation

frost wedging ()goethite

hematite

hydration

hydrolysis

joints


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kaolinite laterite

leaching

paleosol

regolith

secondary enrichment

soil horizon

soil profile

spall spheroidal weathering

weathering rind


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Integration of physical and chemical processes.

A major process to the formation of soil.

Weathering

Mechanical breakdown and chemical alteration

of rocks or sediments in situ when exposed toair, moisture and organic matter.


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Mechanical breakdown as a result ofchanges in pressure or temperature.

Plate tectonic movement. Loading and unloading of glaciers.

Heating and cooling

Wedging by ice or plants.

Crystal growth.

Activity of organisms.

Physical Weathering


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Development of Joints

Joints occur as a widespread set or sets of parallelfractures.

Rocks break at weak spots when they are twisted,squeezed, or stretched by tectonic forces.

Removal of the weight of overlying rocks releasesstress on the buried rock and causes joints to open

slightly, thereby allowing water, air, and microscopiclife to enter.

When dikes, sills, lava flows, and welded tuffs cool they

contract and formcolumnar joints

(joints that splitigneous rocks into long prisms or columns).


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Crystal Growth

Water moving slowly through fractured rockscontains ions, which may precipitate out of

solution to form salts. The force exerted by salt crystals growing

can be very large and can result in the

rupture or disaggregation of rocks. The effects can often be seen in deserts.


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Frost Wedging

Wherever temperatures fluctuate about thefreezing point, pore water periodicallyfreezes and thaws.

As water freezes to form ice, its volumeincreases 9 percent, forcing rocks apart.

Frost wedging probably the most effectiveat temperatures of -5o to -15oC.

Frost wedging is responsible for most of

the rock debris seen on high mountain.


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Daily Heating and Cooling

Surface temperatures as high as 80oChave been measured on desert rocks.

Daily temperature variations of more than40o have been recorded on rock surfaces,

No one has yet demonstrated that daily

heating and cooling cycles havenoticeable physical effects on rocks.


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Chemical Weathering

Chemical reactions transform rocks andminerals into new chemical combinations.

Dissolution.

Hydrolysis. Leaching.

Oxidation.


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Dissolution

Chemicals in rocks are dissolved in water.

Halite (NaCI) is a mineral that can be

removed completely from a rock bydissolution.

Calcite (CaCO3), if carbonic acid is present,

dissolves rapidly in rainwater


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Hydrolysis

Any reaction involving water that leads to thedecomposition of a compound is a hydrolysis

reaction. Hydrogen ions produced by the ionization of

carbonic acid most commonly cause hydrolysis.

For instance, hydrogen ions decomposepotassium feldspar and create kaolinite.

Hydrolysis is one of the chief processes involved

in the chemical breakdown of common rocks.


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Leaching

Leaching is the removal, by water solution, ofsoluble matter from bedrock or regolith.

Soluble substances leached from rocksduring weathering are present in all surfaceand ground water. Sometimes their

concentrations are high enough to give thewater a distinctive taste.


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Oxidation of Iron

Oxidation involves the removal of electrons from an

atom, increasing the oxidation number of an element.

In the presence of oxygen, a ferrous ion (Fe2+). is

oxidized, by giving up an electron, to a ferric ion (Fe3+).

The incorporation of water in a mineral structure is

called hydration.

The hydrolysis and oxidation of ferrous iron

compounds will form ferric hydroxide (Fe(OH)3).


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Oxidation of Iron (2)

The ferric hydroxide may dehydrate, meaning it will

lose some water, to form goethite (FeO.OH).

Goethite may dehydrate still further to form hematite(Fe2O3).

The colors of ferric hydroxide, goethite, and hematite,ranging from yellowish through brownish red to brick

red, can provide clues to the degree or intensity ofweathering.


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Combined Reactions

Chemical weathering often involves morethan one reaction pathway.

Dissolution plays a part in virtually allchemical weathering processes.

The effects of dissolution, hydrolysis, andleaching of carbonate rocks are widelyseen in the landscapes underlain by

carbonate rocks.


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Chemical Weathering Reactions


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Biological Weathering ?

Indirect Action by Plants and Animals (Chemical)

rotting vegetation releases chemicals which are leacheddown to rocks underground. The chemicals attack the rock.

animal urine is washed away by rainwater - the acids in thiscause chemical weathering with the rocks it comes intocontact with.

Direct Action by Plants and Animals (Physical)

plant roots invading cracks in rocks. As the plant grows, itsroots expand, putting pressure on the rock. This causespieces to break off. A photograph of this is shown below:

animals burrowing.


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Factors Influencing Weathering (1)

Mineralogy

The resistance of a silicate mineral toweathering depends on:

Chemical composition of the mineral.

Extend to which the silicate tetrahedrain the mineral are polymerized.

Acidity of the waters with which themineral reacts.


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Order of Stability of Common Mineralsunder Chemical Weathering

Ferric oxides and hydroxides. - Most stable

Aluminum oxides and hydroxides.

Quartz.

Clay minerals. Muscovite.

Potassium feldspar.

Biotite.

Sodium feldspar (albite-rich plagioclase).

Amphibole. Pyroxene.

Calcium feldspar (anorthite-rich plagioclase).

Olivine.

Calcite. - Least stable


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Mineral stability

The stability of minerals can be predicted using the Bowen's reaction series,however, in the case of the weathering series this is known as the GoldichDissolution Series:

OlivineMg Pyroxene Calcic Plagioclase

Mg-Ca Pyroxene Calcic-Alkalic PlagioclaseAmphibole Alkalic-Calcic Plagioclase

Biotite Alkalic PlagioclasePotassium Feldspar

MuscoviteQuartz

Those less polymerized minerals that crystallize at higher temperatures will bethe least stable at the surface. It is obvious that quartz will be the most stablemineral in the weathering environment, and will be a dominant constituent ofsediments and sedimentary rocks.


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Factors Influencing Weathering (2): Rock type and structure

Differential weathering: differences in the composition andstructure of adjacent rock units can lead to contrasting rates ofweathering.

Examples: hillcrest, cape. Differential weathering can etch awayerodible mudstone or shale from between layers of harder siltstoneor sandstone.


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Slope angle.

On a steep slope, solid products of weatheringmove quickly away, continually exposing fresh

bedrock to renewed attack.

On gentle slopes, weathering products are noteasily washed away and in places may

accumulate to depths of 50 m or more.


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Climate: moisture and heat promote chemical reactions.

Weathering is more intense and generally extends togreater depths in a warm, moist climate than in a cold,

dry one.

In moist tropical lands, like Central America and

Southeast Asia, obvious effects of chemical weatheringcan be seen at depths of 100 m or more.

Limestone and marble are highly susceptible tochemical weathering in a moist climate and commonlyform low, gentle landscapes; In a dry climate, however,they form bold cliffs as little carbonic acid is present to

dissolve carbonate minerals.


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Burrowing animals.

Large and small burrowing animals bring partlydecayed rock particles to the land surface.

Although burrowing animals do not break downrock directly, the amount of disaggregated rockthey move over many millions of years must be

enormous.


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Time

Weathering processes are speeded up byincreasing temperature and available water, andby decreasing particle size.

The rate of weathering tends to decrease withtime as the weathering profile, or a weathering

rind, thickens.


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Key Questions

How are weathering and soil formationrelated?

How have human activities acceleratedthe rate of soil erosion?

What evidence can geologists use to

infer a relationship between weatheringand plate tectonics?


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Soil: Origin And Classification

Soils support the plants that are the basicsource of our nourishment.

Soils store organic matter, thereby influencinghow much carbon is cycled in the atmosphereas carbon dioxide.

Soils are derived from (1) physical andchemical weathering processes, (2) decayof dead plants and animals.


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Soil ProfileAs a soil develops from the surface downward, a succession ofhorizontal weathered zones, called soil horizons, forms.

The uppermost horizon may be a surface accumulation of

organic matter (O horizon). The A horizon is the mixture of dark

humus (decomposed residue of plantand animal tissues) with mineral

matter.

The B horizon is enriched in clayand/or brownish or reddish iron andaluminum hydroxides.

The C horizon is the deepest

horizon and consists of rock in

various stages of weathering


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Soil Types

Different soils result from the influence of six formative

factors:

Climate.

Vegetation cover.

Soil organisms.

Composition of the parent material.

Topography.

Time.


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Mineral Deposits Formed by WeatheringLimonite and Hematite: Iron-rich Laterite

Formed by leaching of other minerals from iron-rich sedimentary rocks under warm and rainytropical climate.

Limonite (FeO.OH) and hematite (Fe2O3), theleast soluble minerals, form an iron-rich crust of

laterite during chemical weathering. Secondarily enriched deposit of limonite and

hematite contains high concentration of iron.


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Mineral Deposits Formed byWeathering Bauxite: aluminous laterite

Formed by leaching of clay minerals.

Silica is removed in solution and a residue of

Gibbsite (Al(OH)3),, remains. Gibbsitic bauxites are the major mineral

deposits of aluminum.


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Rate of Soil Formation (1)

In southern Alaska retreating glaciersleave unweathered parent material.

Despite the cold climate, within a fewyears an A horizon develops on the newlyexposed and revegetated landscape.

As the plant cover becomes denser,

carbonic and organic acids acidify the soiland leaching becomes more effective.


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Rate of Soil Formation (2)

After about 50 years, a B horizon appearsand the combined thickness of the A andB horizons reaches about 10 cm.

Over the next 150 years, a mature forestdevelops on the landscape and the OHorizon continues to thicken (but the Aand B horizons do not increase inthickness).


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Soil Erosion

It may take a very long time to produce awell-developed soil but destruction of soilmay occur rapidly.

Rates of erosion are determined by:

Topography.

Lithology Climate.

Vegetation cover.

Human activity.


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Erosion on Slopes

On a 1 percent slope, an average of 3 tons ofsoils are lost per hectare each year.

On a 5 percent slope loses 87 tons per

hectare. On a 15 percent slope, 221 tons per hectare

per year are lost.

Terracing can reduce the loss of soil onfarmed slopes.


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Soil Erosion Due to HumanActivity

In many third world nations, population growth has forcedfarmers onto lands of steep slopes or semiarid regionsthat foster rapid soil erosion.

Planting of profitable row crops that often leave the landvulnerable to increased rates of erosion.

Deforestation has accelerated rates of surface runoff anddestabilization of soils due to loss of anchoring roots.

When the O and A horizons are eroded away, the fertilityand the water-holding capacity of the soil decrease.


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World Crisis on Soil Erosion

Farmers in the United State are now losing about 5tons of soil for every ton of grain they produce.

In India the soil erosion rate is estimated to be more

than twice as high. The worlds most productive soils are being depleted

at the rate of 7% each decade.

Increased farming in the region above a dam inPakistan has reduced the dam life expectancy from100 to 75 years.


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Global Weathering Rates and HighMountains (1)

These following three rivers deliver about 20percent of the water and dissolved matterentering the oceans:

The Yangtze River, which drains the TibetanPlateau of China.

The Amazon River, which drains the northern

Andes in South America. The Ganges-Brahmaputra river system which

drains the Himalayas.


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Global Weathering Rates and HighMountains (2)

High mountains forces moisture-bearing winds to rise,resulting in large amounts of precipitation, high rates ofrunoff and erosion.

Evidence from ocean sediments points to an increaseof dissolved matter reaching the oceans in the past 5million years.

Sediments shed from the rising Himalayas coarsenfrom silts deposited about 5 million years ago togravels about 1 million years old.

Rivers draining the mountains gained increasing energy.

Mountain slopes or stream channels became steeper.


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