07. Weathering and soil 06 - Colorado School of Mines

[Pages:14]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

Potassium feldspar hydrogen ion

water

4KAlSi3O8

+

H+

+ 2H2O

Potassium ion kaolinite

silica

4K+ + Al4Si4O10(OH)8 + 8SiO2

Kaolinite

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

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.

Typical "Fresh" Igneous Rock

50% quartz

33% K-

feldspar

17% Fe Silicates

Products of Weathering

pore space or loss to solution

75% quartz

17% clay 8% Rust

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. Color 2. Texture 3. Structure 4. Organic Matter Content 5. Moisture Content

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Soil Horizons - Color

? Organics turn soil black to dark brown

? Ferric Iron turns the soil yellowbrown to red

? Ferrous Iron turns soil dark greygreen

? 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 PROFILES

Five Basic Horizons:

O Horizon = Loose and Partially

Decayed Organic Matter

A Horizon = Loose Rock and Mineral Matter

mixed with Partially Decomposed Organic Matter (Humus)

E Horizon = Fine-grained Mineral Particles

that have been carried down by water (Eluviation & Leaching)

B Horizon = Accumulation of clay

transported from above

C Horizon = Loose and Partially

Altered "Parent" Rock Material

Parent Rock

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