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Weathering and soil

Chemical weathering• Primarily done by water that is acidic.• Rain picks up material from the atmosphere

to become acidic.– Carbon dioxide Carbonic acid– Sulfur oxides Sulfuric acid– Nitrogen oxides Nitric acid

• Once rain enters the ground it usually picks up more carbon from decaying organic material and becomes more acidic.

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Chemical Weathering of Feldspar

The reaction of carbonic acid with groundwater and feldspar is acommon weathering reaction in most temperate climates (as in Colorado).

Potassium feldspar hydrogen ion water4KAlSi3O8 + H+ + 2H2O

Potassium ion kaolinite silica4K+ + Al4Si4O10(OH)8 + 8SiO2

Kspar

Kaolinite

Chemical Weathering of Mafic Minerals• The chemical weathering of mafic (dark

colored) minerals like pyroxenes, amphiboles, biotite, etc. (all contain Fe +/- Mg) releases iron which can then form iron oxide (Fe2+O) and goethite (Fe3+)(OH).

• These minerals give the weathered rock a red (“rusty”) color.

50% quartz

33% K-

feldspar17% Fe Silicates

Typical “Fresh”Igneous Rock

8% Rust17% clay

75%quartz

pore space or loss to solution

Products of Weathering

Mechanical Weathering

• Forces act to break rock apart without chemically altering it.

–Impact

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Mechanical Weathering

• Forces act to break rock apart without chemically altering it.– Impact

–Abrasion

Balanced rock from sand abrasion in the desert

Mechanical Weathering

• Forces act to break rock apart without chemically altering it.– Impact– Abrasion

–Frost Wedging

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Mechanical Weathering

• Forces act to break rock apart without chemically altering it.– Impact– Abrasion– Frost Wedging

–Root Wedging

Mechanical Weathering

• Forces act to break rock apart without chemically altering it.– Impact– Abrasion– Frost Wedging– Root Wedging

–Salt Wedging

Mechanical Weathering

• Forces act to break rock apart without chemically altering it.– Impact– Abrasion– Frost Wedging– Root Wedging– Salt Wedging

–Unloading

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Soil vs. Regolith• Regolith = the layer of rock and

mineral fragments at the Earth’s surface.

• Soil = a combination of mineral and organic matter, water, and air.–The portion of regolith that can

support rooted plant growth.

Soil Horizons• The most important characteristics

used to define soil horizons are:1. Color2. Texture3. Structure4. Organic Matter Content5. Moisture Content

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Soil Horizons - Color• Organics turn soil black to dark

brown• Ferric Iron turns the soil yellow-

brown to red• Ferrous Iron turns soil dark grey-

green• SiO2 or CaCO3 turns the soil light

gray to white

Carbon rich

Reduced (ferrous– Fe+2) iron rich

CaCO3 rich

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Soil Horizons - Texture

Soil horizons - structure

• Shape of aggregates of soil particles called peds

• In clay rich material the spaces between peds may be the only way water can travel down the profile

Soil Horizons – Organic Matter Content

• Litter (leaves and branches)• Humus (decomposed litter)• Both tend to be produced more at higher

temperatures, with an optimum of about 25° C

• Provides chelates, increase water absorption ability, and makes CO2 to create carbonic acid

Soil Horizons – Moisture Content

• Total quantity of water that can be held in a soil is the available water capacity (AWC)

• Field capacity• Permanent wilting point

SOIL PROFILESSOIL PROFILESFive Basic Horizons:Five Basic Horizons:

OO HorizonHorizon = Loose and Partially= Loose and PartiallyDecayed Organic MatterDecayed Organic Matter

AA HorizonHorizon = Loose Rock and Mineral Matter= Loose Rock and Mineral Mattermixed with Partially Decomposedmixed with Partially DecomposedOrganic Matter (Humus)Organic Matter (Humus)

EE HorizonHorizon = Fine= Fine--grained Mineral Particlesgrained Mineral Particlesthat have been carried down by waterthat have been carried down by water(Eluviation & Leaching)(Eluviation & Leaching)

B B HorizonHorizon = Accumulation of clay= Accumulation of claytransported from abovetransported from above

CC HorizonHorizon = Loose and Partially= Loose and PartiallyAltered Altered ““ParentParent”” Rock MaterialRock Material

Parent RockParent Rock

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O horizon

• Mostly undecomposed or partially decomposed organic material

• Mineral content minor (<50%)

A horizon

• Mineral horizon with decomposed organic material where clay and mobile components are leached downward

• Black, nutrient-rich topsoil• Zone of eluviation

Clay tends to get leached from the A and E horizons and carried down to the B horizon due to the electric

property of water molecules interacting with the negatively charged surfaces of clay particles.

E horizon

• Intense leaching that removes ferric iron or organic coatings from mineral grains leaving the layer bleached grey or tan

B horizon

• Transitional between E and C where material brought down from above accumulates

• Zone of illuviation• Commonly has reddish hues, iron and

aluminum concentrations, stable primary minerals, and a high clay content

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C horizon• Underlying, unconsolidated parent

materials that have been unmodified/slightly modified, by soil forming processes

R horizonSolid bedrock K horizon

• A calcite rich layer generally found in arid region

• Caliche

Soil Forming Factors• Climate• Topography (slope)• Parental material• Organics• Time

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Soil erosion (of bauxite soils) in Madagascar: clearly unsustainable.