Chapter Five Weathering: The Breakdown 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. MECHANICAL 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 ACTIVITIES 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 WATER DISSOLVES HALITE AND GYPSUM   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 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

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 SYSTEM a) BASED ON MANY PHYSIAL & CHEMICAL CHARACTERISTICS b) EXAMPLES OF SOIL TYPES 1) ENTISOL 2) VERTISOL 3) OXISOL 4) 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