EARTH AND PLANETARY SCIENCES 101 Fall, 2002

THE WAY THE EARTH WORKS

WEATHERING PROCESSES; CHANGES TO THE EARTH'S SURFACE

I. Weathering; Breakdown (Chemical and Physical) of ROCK

A. Importance??

1. Our planet made suitable for human use

2. Production of soil, of many kinds, from rocks, via weathering

B. Types of Weathering

1. Mechanical weathering; physical disintegration

2. Chemical weathering; decomposition of rock, exposure AIR, WATER

C. Rates of Weathering

1. slow process

2. chemical weathering more important than physical

D. Effects of weathering

1. dissolution of rock outcrops

2. differential weathering producing variable landscape

II. MECHANICAL WEATHERING

A. Frost action; wedging (expansion of frozen water) and heaving (layers of rock lifted)

B. Abrasion--grinding of rock away during transportation

1. erosion physical process of actually removing rock material

2. then transportation-weathering processes continue during transportation.

C. Pressure release; unloading; sheet-jointing; (exfoliation)

III. Chemical Weathering

A. Rock decomposition; reflecting a new environment and the attempt to be in equilibrium with that environment.

B. Different minerals weather chemically at different rates

C. Role of oxygen; oxidation of reduced cations (e.g., Fe)

D. Role of acids; produced by rain falling through the atmosphere

E. Solution weathering

1. complete dissolution of particular minerals

F. Important Example!!!!! Chemical weathering of feldspar

1. volumetrically critical example of complicated weathering process

a. products are clay minerals, interaction with acidic waters

b. H+ ion attacks the feldspar structure. K+ carried in solution

c. all silicon cannot fit into the clay minerals, some carried away

d. Ca-feldspars give off Ca⁺⁺ and HCO3-; Ca⁺⁺ eventually precipitated as calcite in sedimentary rocks

e. large quantities of dissolved rock may be carried away

f. quartz is relatively safe--weathering by mechanical aggregation only--FERROMAGNESIAN SILICATES USUALLY VERY DIFFERENT

IV. SOILS

A. layer of weathered, unconsolidated material on top of bedrock

1. mature, fertile soil is the product of centuries of development.

2. topsoil/subsoil clay minerals holding the water in the soil; quartz grains keep the soil relatively loose, and porous.

3. HORIZONS layers of soil distinguished by particular properties

a. A horizon zone of leaching, plant humus collects on top

b. B horizon zone of accumulation-material leached from A horizon-clay rich and abundant hematite (Fe⁺⁺⁺ oxide)

c. C horizon incompletely weathered parent material (transition to fresh, intact "bedrock"

B. Residual vs. transported soils

1. residual soils are developed due to weathering of rock directly beneath the soil.

2. transported soils include foreign material, introduced by running water, wind, glacial ice, etc.

a. best example as loess--wind-blown silty material, related to glaciation

C. Soils, Parent Rocks, Time

1. importance of quarts in the parent rock, to eventually yield a sandy soil

2. some parent materials will never produce a sandy soil, because no particular mineral will remain in the weathering environment.

a. e.g., mafic rocks weather to clays, and nothing but.

3. significance of time as a variable; note that weathering of a granite will first produce a very sandy soil which, as the result of alteration to clay minerals, will become more and more clay rich.

D. Soils and Climate--rather provocative topic, esp. with respect to the geologic past, or paleosols.

1. Humid regions, such as the eastern U.S., are characterized by downward movement of abundant water; effective leaching, producing “pedalfers” (from Al and Fe).

2. Arid regions experience little leaching by precipitation, and “pedocals” are produced--water is actually drawn upward through evaporation and capillary action.

a. leads to the evaporation of salts in the soil (e.g., calcium carbonate) and we term such soils alkaline in character (like in central New Mexico!)

3. Hardpan

a. hard layer of “earth” difficult to drill or excavate

b. specifically a clay rich layer, produced by the cementation of soil particles, particularly by calcite

c. caliche-large concentration of calcium carbonate.

4. laterites

a. tropical regions, with very high rainfall, highly leached soils result, called laterites

b. deep and intense weathering; soils are usually very red and consist of insoluble iron and aluminum oxides.

c. important sources of Fe and Al

d. not capable of sustaining abundant plant life; in tropical environments, the plant life is sustained by the overlying humus.

SOME IMPORTANT REACTIONS IN CHEMICAL WEATHERING:

1. solution of carbon dioxide in water to form acid:

CO₂ + H₂O = H₂CO₃ = H⁺ + HCO₃ ^-

2. solution of sulfur trioxide in water to form sulfuric acid:

SO₃ + H₂O = H₂SO₄ = H⁺ + HSO₄ ^-

3. solution of calcite:

CaCO₃ + CO₂ + H₂O = Ca ⁺⁺ + 2HCO₃ ^-

4. solution of potassium feldspar:

2KAlSi₃O₈ + 2H⁺ + 2HCO₃ ^- + H₂O =

(in granite) (rainwater)

Al₂Si₂O₅(OH)₄ + 2K⁺ + 2HCO₃ ^- + 4 SiO₂

(kaolinite, clay) (soluble) (insoluble or soluble)