Weathering

- Changes that occur in rocks and minerals that are exposed to the atmosphere and surface conditions

- almost all rocks break down or decay either physically or chemically

3 types of weathering

3 types of weathering:

1.)  Physical

2.)  Chemical

3.)  Biological

Physical Weathering

(Mechanical Weathering)

Physical weathering - mechanical changes in rock whichbreak it down to smaller sizes without changing minerology

EX: Thermal

      Freeze/Thaw

      Salt crystal growth

      Jointing/Sheeting

- Physical weathering promotes chemical weathering

(Breaking rocks expose greater surface area)

Thermal Physical Weathering

The expansion and contraction of minerals when they are heated (Expand) or cooled (shrink) in a rock

 - takes place in arid/semi arid regions

-Causes internal stress that can weaken the the cohesion between mineral grains resulting in breakage of the rock at its surface (exfoliation)

Freeze/Thaw Physical Weathering

(Ice Wedging)

Freezing or thawing of water in pores and cracks

- water expands 9% of its original body when frozen

Repeated cycles of freezing and thawing can result in mechanical dissagregation of the rock

- Most effective where freezing and thawing take place frequently

Salt Crystal Growth Physical Weathering

Salt crystals grow in the cracks of rocks and break them

Jointing/Sheeting Physical weathering

When rocks that were compressed come up to the surface and crack due to loss of pressure

- is accomplished by brittle rupture and formation of very small spaces along penetrative fractures termed expansion joints.

Chemical Weathering

Changes in mineralogy by reaction with the atmosphere and water

3 Types

1.  Dissolution- dissolve mineral

2.  Oxidation- reduction (Redox)- Minerals react to Oxygen (rusting)

3.  Hydration - Dehydration- putting water into/ taking water away

Chemical Agents

* Oxygen

* Water

* Carbon dioxide (CO₂)

        H₂O + CO₂ --> H₂CO₃  (Carbonic Acid- weak acid)

* Sulfur Dioxide (SO₂)

        H₂O + SO₂ --> H₂SO₄   (Sulfuric Acid - Strong Acid)

* Also humic acids from reaction with decaying vegetation (Organic Acids)

Dissolution Reactions

NaCl + H₂O -->   Na⁺ +  Cl^-  +  H₂O

  -  Salt + Water -->  Na⁺ and Cl^- ions in solution

CaCO₃ + H₂O -->  Ca⁺² + H₂CO₃

  -  Calcite + Water -->  Ca ion + Carbonic Acid

2FeS₂ + 7O₂ + 2H₂O --> 2Fe⁺² + 4H₂SO₄

  -  Pyrite + Oxygen + Water --> Fe ion + Sulfuric Acid

Hydration Reactions

CaSO₄ + H₂O --> CaSO₄ * 2H₂O

  -  Anhydrite + Water --> Gypsum

2KAlSi₃O₈ + 2H⁺ + H₂O --> AlSi₂O₅(OH)₄ + 2K⁺ + SiO₂

  -  Orthoclase+Acidic Water -->Kaolinite(Clay)+stuff in solution

Fe₂O₃ + 4H₂0 --> 2Fe(OH)₃ * H₂O

  -  Hematite + Water --> Goethite

Oxidation Reactions

4Fe + 3O₂ --> 2Fe₂O₃

  -  Iron + Oxygen --> Hematite

4FeS₂ + 8H₂O + 15O₂ --> 2Fe₂0₃ + 8H₂SO₄

 (ferrous)                            (Ferric)

4FeSiO₃ + O₂ + 2H₂O --> 4FeO(OH) + SiO₂

(Ferrous)                            (Ferric)

  -  SiO₂= Quartz - relatively stable/insoluable undermost conditions

  - Silicate minerals will react to chemical weathering and release silica into solution

Chemical Weathering of Marble column

Marble carving being dissolved by dissolution

Marble= Calcite

Biological Weathering

Changes produced by the action of plant or animal action on rocks

 - changes may be chemical or physical

EXAMPLES:

- Burrowing

- Root Growth

- Ion exchange with roots

- plant secreated acid

Algea Growth

(Biological Weathering)

Algae growth on rocks dissolve minreals to obtain nutrients

Mineral Responses to Weathering

(Fe Bearing Silicates, Feldspar, Quartz, Muscovite Mica, Calcite)

Original Mineral                         Weathering Product

Fe bearing silicates                      Clay minerals

(olivine, pyroxene, Amphibole,           (Iron Oxides)

Biotite)          

Feldspar                                     Clay minerals

                                                 (K, Na, Ca ions)

Quartz                                        Quartz

Muscovite Mica                            Clay Minerals

                                                    (K ions)

Calcite                                        Ca, Co ions

Feldspar

(Mineral Responses to Weathering)

Weathering Product

Clay Minerals

(K, Na, Ca ions)

Quartz

(Mineral Responses to Weathering)

Weathering Product

Quartz

(Does not change)

Muscovite Mica

(Mineral Responses to Weathering)

Weathering product

Clay Minerals

(K ions)

Calcite

(Mineral Responses to Weathering)

Weathering Product

Ca, CO ions

Granite

(Rock Responses to Weathering)

Granite --> Quartz, Clay, Minor Fe Oxide

  -  K-Spar, maybe biotite

Diorite

(Rock Responses to Weathering)

Diorite --> Fe Oxides, Clays, +/- Quartz

Gabbro

(Rock Responses to Weathering)

Gabbro --> Fe Oxides, Clay

  -  Lots of dark minerals

Slate

(Rock Responses to Weathering)

Slate --> Chemically stable, mechanically weak

  -  Metamorphic (clays = baked)

          -  not very strong

Limestone

(Rock Responses to Weathering)

Limestone --> Dissolves (residual insol.)

- will weather away

Sandstone

(Rock Responses to Weathering)

Sandstone --> Quartz unchanged

  -  stability depends on what holds it togethor

- Cements affected accordingly to solubility

      Silica Stable

      Fe Oxide Stable

      Calcite Soluable

Goldrich Stability Series

(Most susceptible to weathering)

Olivine                                                    Ca-rich

          Pyroxene                                  Plagioclase

                  Amphibole

                        Biotite         Na-rich

                                       plagioclase

K-spar

Muscovite

Quartz

(Least Susceptible to weathering)

Quartz/K-spar are most stable

Typical Climate Conditions

Polar- to cold to hold moisture

Tropic- hot and wet

Temperate- Warm (not super cold) with moderate rain

Arid- Very Dry

Cleopatra's Needle

Desert -> Humid

Result from weathering

Soils

Surface mantle of rock weathering products that support life ( microbes to trees and elephants)

Engineer's definition - surface mantle that can be excavated without blasting

Soils are important as:

1.  source materials of sediments

2.  layer in which plants grow

Residual Soils

Soils formed where they are found (formed in place)

Soil development is controlled by:

1. Time (the more time, the more soil erodes)

2. Slope (the materials wash out and get carried away)

3. Climate (controlls how long it takes for soil to erode)

4. Parent rock

5. Vegetation and Organisms

Ideal Soil

(Horizons)

O horizon - organic, alot of dying/dead plant material (black soil) may not be present. 0-2'

A horizon - Same organic material is in it, alot of quartz grains, catalyst mineral stuff, contains humus leached zone. 2-10'

B horizon- Brown, no organic material, rich in clays, brown to red, enriched in clay and/or Fe oxides. 10-30'

C horizon- Transition zone, bed rocks and pieces of bed rock, partially weathered out materials, light yellow brown. 30-48'

Additional Horizons

(not always present)

E Horizon- Only present in acidic soils (pine forest) usually white

K Horizon- Only present in arid soils, zone of caliche

Transported soils

Soils that result from deposition of weathered materials delivered from elsewhere

-  Many of the early agricultural centers were located on the flood plains and delta of major rivers. New soil is deposited with each flood

Polar and Subpolar

(Soils and Climates)

slight weathering; thin, rocky soils, low temp, cold all the time

Temperate

(Soils and Climates)

Moderate physical and chemical weathering; well developed pedalfers

Tropical

(Soils and Climates)

Strong Chemical weathering; clays removed; only Al and Fe oxides and hydroxides stable-- Laterites

Arid

(Soils and Climates)

weak chemical weathering, K horizon developed (Caliche) -- Pedocals

-  thin A and B horizons

Sheeting

When expansion joints segment bedrock exposures into numerous parrallel rock layers often reffered to as sheets

Caused by Unloading or

Pedalfer soil

Form in temperate climates where there is sufficient precipitation to flush out most soluable products of chemical weathering

- Rich in aluminum and iron and are generally fertile

Pedocal Soil

From in arid/sem-arid environments if there is insufficient precipitation to completely wash out soluable weathering products

- Typically precipitate within the B-Horizon and accumulate to form deposits termed caliche or hard-pan

Laterite Soil

Form in tropical and sub-tropical climates where chemically weathering is so extensive and precipitation so frequent that only the most insoluable elements remain

- Infertile and represented by bauxite the principle ore for aluminum

Iron Oxide are common at the surface of the earth because it forms by the weathering of....

A.  Dark Minerals of Bowen's Reaction Series

B.  Clay Minerals

C.  Feldspar

D.  Shale

Which of the following is most susceptible to chemical weathering by dissolution?

A.  Calcite

B.  Hematite

C.  Clay Minerals

D.  Quartz

Which of the following is most likely to dissolve away in semi-humid environments?

A.  Calcite

B.  Gypsum

C.  Aluminum Oxide

D.  Clay Minerals

Feldspars Weather to...

A.  Iron Oxide

B.  Calcium Carbonate

C.  Quartz

D.  Clay

In which region of the USA are pedalfers most likely to form?

A.  Southwest USA

B.  Southeast USA

C.  Extreme Northeast

D.  Utah

Pedocal soils have a thick...

A.  A horizon

B.  B horizon

C.  Caliche layer

D.  E horizon

A buried B-Horizon may eventually be preserved as a very thin...

A.  Shale

B.  Sandstone

C.  Limestone

D.  Coal