Chapter Five Weathering: The Breakdown of Rocks. CHAPTER 5: WEATHERING: THE BREAKDOWN OF ROCKS A) WEATHERING: PROCESS BY WHICH ROCKS AND MINERALS BREAK.

Chapter Five Chapter Five

Weathering: The Weathering: The Breakdown of RocksBreakdown of Rocks

CHAPTER 5: WEATHERING: THE

BREAKDOWN OF ROCKS

A) WEATHERING: PROCESS BY WHICH ROCKS AND MINERALS

BREAK DOWN AT OR NEAR THE EARTH’S SURFACE  BENEFITS OF WEATHERING: PRODUCES SOILS (MINERALS AND

ELEMENTS)  DETRIMENT OF WEATHERING: DESTROYS STRUCTURES WE BUILD    B) EROSION: PROCESS BY WHICH MOVING WATER, WIND, OR ICE

CARRIES PIECES OF ROCKS AND DEPOSITS C) SEDIMENT: LOOSE, FRAGMENTED SURFACE MATERIAL

II WEATHERING PROCESS:

A. A. MECHANICAL WEATHERINGMECHANICAL WEATHERING (BREAKS ROCKS INTO SMALLER PIECES)

  FROST WEDGING (EXPANSION OF CRACKS IN ROCK AS WATER IN THE CRACK FREEZES AND EXPANDS)

    SALT CRYSTAL GROWTH (FORCES CRACK’S WALLS FARTHER

APART)

    THERMAL EXPANSION AND CONTRACTION (ALTERNATE

ENLARGEMENT AND SHRINKING)

    MECHANICAL EXFOLIATION (FRACTURING AND REMOVAL

OF SUCCESSIVE ROCK LAYERS AS DEEP ROCKS EXPAND

U PWARD AFTER OVERLYING ROCKS HAVE ERODED AWAY)

 OTHER MECHANICAL WEATHERING

PROCESSES

o       GROWTH OF PLANT ROOTS (EXPANDS EXISTING CRACKS IN ROCKS)o       BURROWING ANIMAL ACTIVITIESo       ABRASION OF TRANSPORTED PARTICLES

Mechanical Weathering• Frost action

– Mechanic effect of freezing (and expanding)

water on rocks

• Pressure release– Removal of overlying rock allows

expansion and fracturing

• Plant growth– Growing roots widen fractures

• Burrowing animals• Thermal cycling

– Large temperature changes fracture rocks by repeated expansion and contraction

 Mechanical Weathering

Mechanical weathering-contd.

Mechanical weathering-contd.

Surface Area & Weathering

Frost Wedging

CHEMICAL WEATHERING (LARGELY

CONTROLLED BY CLIMATE) *ROLE OF WATER

*DISSOLUTION

1. WATER DISSOLVES HALITE AND GYPSUM 2. CARBONIC ACID DISSOLVES LIMESTONE

(CO2+H2O----HCO3)                    i.      CALCIUM CYCLE                   ii.      ACID RAIN

 pH Scale

Climate weathering

3.OXIDATION (REACTION OF CERTAIN CHEMICALS WITH O2)

    i.      IRON OXIDES   ii.      COPPER OXIDES 4. HYDROLYSIS (REPLACEMENT OF MAJOR POSITIVE IONS WITH PROTONS) OF POTASSIUM FELDSPAR INTO

i) CLAY: BECOMES PART OF SOIL

ii) SILICIC ACID: CEMENTS SEDIMENTS OR FORMS ANIMAL SHELLS AND SKELETONS

iii) POTASSIUM IONS: PROVIDE PLANT NUTRIENTS

Spheroidal weathering

FACTORS THAT INFLUENCE CHEMICAL

WEATHERING

1.CLIMATE

i)  MOISTURE

ii)    HEAT

iii)  VEGETATION 2. LIVING ORGANISMS 3. TIME

MINERAL COMPOSITION: RELATIONSHIP BETWEEN A MINERAL’S

TEMPERATURE AND PRESSURE OF CRYSTALLIZATION AND ITS

SUSCEPTIBILITY TO WEATHERING (HIGH TEMP CRYSTALLIZATION-----

LESS STABLE AND EASILY WEATHERED-EXAMPLE: OLIVINE &

PYROXENE) REGOLITH: LOOSE, FRAGMENTED MATERIAL THAT COVERS MUCH OF

THE EARTH’S SURFACE  SOIL: UPPERMOST ORGANIC-RICH PORTION OF THE REGOLITH 

Mineral composition

Rounded Boulder

D.SOME PRODUCTS OF CHEMICAL WEATHERING:

a) CLAY MINERAL

i)   KAOLINITE

ii)  SMECTITE

iii) PRACTICAL USES FOR CLAYS

b) METAL ORES

i)        FORMATION OF BAUXITE

ii)      OTHER ORES

Soil• Soil - a layer of weathered, unconsolidated

material on top of bedrock– Common soil constituents:

• Clay minerals

• Quartz

• Water

• Organic matter

• Soil horizons– O horizon - uppermost layer; organic material

– A horizon - dark layer rich in humus, organic acids

– E horizon - zone of leaching; fine-grained components removed by percolating water

– B horizon - zone of accumulation; clays and iron oxides leached down from above

– A horizon - partially weathered bedrock

Bedrock composition on soil.

Soils and Climate• Soil thickness and composition are

greatly affected by climate– Wet climates:

• More chemical weathering and thicker soils

• Soils in moderately wet climates tend to have significant clay-rich layers, which may be solid enough to form a hardpan

– Arid climates:• Less chemical weathering and thinner soils

• Subsurface evaporation leads to build-up of salts

• Calcite-rich accumulation zones may form, cementing soil together into a hardpan

– Extremely wet climates (e.g., tropical rainforest)• Highly leached and unproductive soils (laterites)

• Most nutrients come from thick O/A horizons

Vegetation and soil development

Typical Mature soil

CLASSIFYING SOILS 1. OLD CLASSIFICATION SYSTEM PEDALFERS PEDOCALS LATERITES

2. MODERN CLASSIFICATION SYSTEMa) BASED ON MANY PHYSIAL & CHEMICAL CHARACTERISTICS

b) EXAMPLES OF SOIL TYPES

1) ENTISOL2) VERTISOL3) OXISOL4) ULTISOL

 3. PALEOSOLS (“OLD SOILS”) 

 Typical Mature Soil

SUMMARY – CHAPTER 5: • DEFINITION OF WEATHERING AND EROSION-DIFFERENCE BETWEEN THEM• MECHANICAL WEATHERING• WEATHERING BY THERMAL EXPANSION & CONTRACTION• MOST IMPORTANT FACTOR AFFECTING CHEMICAL

WEATHERING• OXIDATION• HYDROLYSIS• MINERAL THAT IS MOST SUSCEPTIBLE TO WEATHERING• WET/WARM VS DRY/COLD• MINERAL’S STABILITY FOR WEATHERING• FORMATION OF CLAY MINERALS• REGOLITH AND SOIL• DIFFERENT SOIL HORIZONS