Lecture 4: Weathering

 

WEATHERING Is a fundamental process of landscape evolution and sediment production.

 

It involves the in situ decay or breakdown of rock. IT OCCURS BEFORE EROSION & MAKES EROSION EASIER.

 

HOWEVER Some weathering processes such as solution involve THE movement of some weathered material.

 

THERE ARE TWO TYPES OF WEATHERING

 

1)    CHEMICAL WEATHERING

 

WHICH CAUSES THE DECAY OR DECOMPOSITION (ROTTING) OF MINERALS & A CHEMICAL CHANGE. CAUSED BY INTERNAL DISRUPTIVE FORCES SUCH AS WATER REACTING WITH A MINERAL

 

AND

     

2)    PHYSICAL (MECHANICAL) WEATHERING

 

WHICH CAUSES THE DISINTEGRATION OF ROCKS WITHOUT CHEMICAL CHANGES OCCURRING. CAUSED BY EXTERNAL DISRUPTIVE FORCES SUCH AS THE GROWTH OF ICE OR SALT CRYSTALS IN A CRACK.

 

 

EMPHASIS HAS BEEN PLACED ON THE IMPORTANCE OF CHEMICAL WEATHERING. SOME ARGUE THAT SOME CHEMICAL PROCESSES MERELY MANIFEST THEMSELVES AS PHYSICAL CONSEQUENCES, E.G.

THE FREEZING OF WATER IN A CRACK.

 

TEXTBOOKS LIKE TO SEPARATE CHEMICAL & PHYSICAL PROCESSES BUT IN REALITY A NUMBER OF CHEMICAL & PHYSICAL PROCESSES CAN ATTACK A ROCK AT THE SAME TIME.

 

 

CHEMICAL WEATHERING

 

ROCK MINERALS USUALLY FORM UNDER EXTREMELY DIFFERENT CONDITIONS FROM THOSE AT THE EARTH’S SURFACE. PRIMARY MINERALS FORM IN EXCEPTIONALLY HOT & HIGHLY PRESSURIZED CONDITIONS IN MAGMA CHAMBERS.

 

MINERALS IN ROCKS REACT WITH WATER CARRYING ATMOSPHERIC & TERRESTRIAL CHEMICALS & FORM SECONDARY MINERALS SUCH AS CLAY MINERALS & OXIDES.

 

THE STABILITY OF A MINERAL OR ITS TENDENCY TO CHANGE IS RELATED TO THE TYPE OF CRYSTALLISATION ENVIRONMENT IN WHICH IT FORMED.

 

THE LEAST STABLE MINERALS USUALLY FORM IN EXTREMELY HOT (>>1000oC) & PRESSURIZED CONDITIONS, E.G. OLIVINE – A MAFIC, DARK GREEN IRON OR MAGNESIUM SILICATECRSTALLISES AT HIGH TEMPERATURES & PRESSURES & WHEN EXPOSED IS RAPIDLY ALTERED TO FORM CLAY MINERALS & SOLUBLE IONS.

 

QUARTZ CRYSTALLISES AT MUCH LOWER TEMPERATURES (~500-600oC)

& PRESSURES & IS VERY STABLE, REMAINING IN THE ENVIRONMENT FOR MILLIONS OF YEARS (QUARTZ CYCLING ~108 YRS.).

 

IT IS POSSIBLE TO CONSTRUCT A CONTINUUM OF MINERAL STABILITY.

-SEE OVERHEAD.

 

75% OF THE SURFACE OF CONTINENTAL AREAS IS COMPOSED OF SEDIMENTARY ROCKS (52% IS SHALE) & ONLY 25% IS COMPOSED OF IGNEOUS ROCKS YET MOST WEATHERING STUDIES HAVE BEEN DONE ON IGNEOUS ROCK TYPES. THIS IS BECAUSE IT IS EASIER TO SEE THE EFFECT OF WEATHERING ON PRIMARY MINERALS RATHER THAN SECONDARY MINERALS & CLASTIC ROCKS.

 

ROCK TYPE

 

THE MOST IMPORTANT INFLUENCE ON WEATHERING IS ROCK TYPE., BECAUSE IT DETERMINES:

 

1)    COMPOSITION

2)    TEXTURE

3)    POROSITY

4)    STRUCTURE

5)    BULK PROPERTIES – ELASTICITY, STRENGTH, HARDNESS ETC.

 

WATER

 

THE SECOND MOST IMPORTANT FACTOR IS WATER.

 

1)    THE AVAILABILITY OF WATER

IN WET OR HUMID ENVIRONMENTS GRANITES & OTHER ROCKS APPEAR ROTTEN BECAUSE OF THE WEATHERING OF FELSPARS & BIOTITES LEAVING QUARTZ GRAINS (GRANULAR DISINTEGRATION).

 

IN VERY DRY CONDITIONS ROCKS APPEAR FRESH WITH NO TRACE OF WEATHERING, E.G. ANCIENT EGYPTIAN BUILDINGS & STATUARY.

 

THUS WETTER = MORE WEATHERING, DRIER = LESS WEATHERING.

 

2)    THE TEMPERATURE OF THE WATER

 

VERY IMPORTANT IN TERMS OF CHEMICAL PROCESSES BECAUSE TEMPERATURE INFLUENCES THE RATE OF CHEMICAL REACTIONS. MOST REACTIONS DOUBLE WITH EVERY 10oC INCREASE IN TEMPERATURE. THUS CHEMICAL WEATHERING SHOULD BE (& IS) MORE INTENSE & RAPID IN HUMID TROPICAL AREAS THAN IN TEMPERATE OR POLAR AREAS.

 

 

3)    CHEMICALS IN THE WATER

 

PRECIPITATION & GROUNDWATER ARE NOT PURE. WHEN VAPOUR CONDENSES IN THE ATMOSPHERE IT ASSIMILATES CO2

FROM THE AIR. THE CO2 CONTENT PF THE PRECIPITATION INCREASES BY 10-30% BY THIS PROCESS (ATMOSPHERE 0.03%,

PPT 0.3-0.9%). THE CO2 DISSOLVES IN THE WATER & FORMS A WEAK CARBONIC ACID (H2CO). THIS PROCESS IS CALLED CARBONATION. ADDITIONAL CARBON DIOXIDE IS PRODUCED AS THE WATER INFILTRATES THROUGH DECOMPOSING ORGANIC MATERIAL (PEAT, BOREAL FOREST LITTER). BACTERIA IN THE TOPSOIL CONSUME ORGANIC MATTER & GIVE OFF CO2. IN MOIST AERATED SOIL BACTERIAL ACTIVITY CAN INCREASE CO2 IN THE SOIL AIR BY >10%.

 

WHEN CARBONIC ACID COMES IN CONTACT WITH CARBONATE ROCKS LIKE LIMESTONE (CALCITE – CACO3) IT REACTS & PRODUCES CALCIUM BICARBONATE (CA(HCO3)2) WHICH IS WATER SOLUBLE & IS REMOVED IN SOLUTION AS METEORIC WATER OR GROUNDWATER MOVES THROUGH THE ROCK.

 

ROCK (CACO3) +H2O+CO2 çè CALCIUM CARBONATE (CA(HCO3)2)

 

IN THIS WAY EXPOSED LIMESTONE SURFACES BECOME ETCHED AS THE JOINTS ARE WIDENED PRODUCING KARREN, LAPIES OR LIMESTONE PAVEMENTS (CLINTS OR PINNACLES & GRYKES OR CUTTERS). IF THIS PROCESS IS CONTINUED JOINTS WIDEN & SUBSURFACE CAVES OR CAVERNS & TUNNELS FORM & MAY COLLAPSE TO FORM SWALLOW HOLES, POTS OR DOLINAS. IN SOME AREAS MOST OF THE LIMESTONE HAS BEEN REMOVED EXCEPT FOR ISOLATED HILLS – COCPIT KARST, TOWER KARST (FENG LIN).

 

ON EMERGENCE FROM THE ROCK ON A CAVE WALL OR AT A SPRING THE REVERSE PROCESS OCCURS & CALCIUM CARBONATE IS DEPOSITED AS VARIOUS TYPES OF SPELEOTHEM OR DRIPSTONE, FLOWSTONE OR POOLSTONE (FLOOR STONE) – STALAGMITES, STALACTITES, STRAWS, COLUMNS, ORGAN PIPES, CURTAINS, TUFA OR TRAVERTINE.

 

SOLUTION WEATHERING IS VERY IMPORTANT IN OTHER ROCK TYPES TOO, E.G SWEDISH LAPPLAND NEAR TO THE ARCTIC CIRCLE IN METAMORPHIC ROCKS – 136,500 T/YR REMOVED IN SOLUTION. THE MOST IMPORTANT DENUDATION PROCESS.

 

RATES OF SOLUTION ARE RELATED TO CLIMATE ESPECIALLY TO THE AMOUNT OF RAINFALL.

 

E.G. LOS ALAMOS, N.M. (PPT 25-40mm/YR) REMOVAL BY SOLUTION  <1m3/km2/yr.

 

SOMERSET ISLAND, N.W.T. (PPT 130mm/YR) 2m3/km2/yr.

 

CENTRAL FRANCE   (PPT 1500-2000mm/YR) 24m3/km2/yr.

 

INDONESIA (3000mm/YR) >83m3/km2/yr.

 

OTHER CHEMICAL WEATHERING PROCESSES

 

OXIDATION/REDUCTION

 

OXIDATION IS THE REACTION OF A MINERAL WITH OYGEN TO FORM OXIDES OR HYDROXIDES, E.E. RUST (FERRIC IRON).

REDUCTION IS THE OPPOSITE PROCESS. OXYGEN IS TAKEN OUT OF THE MINERAL IN ANAEROBIC CONDITIONS (FERROUS IRON).

 

HYDRATION/HYDROLYSIS

 

HYDRATION IS THE ADDITION OF WATER TO A MINERAL, E.G. IRON + WATER = IRON HYDROXIDE. CAUSES AN INCREASE IN CRYSTAL SIZE & LEADS TO THE DISINTEGRATION OF A ROCK. THIS PROCESS IS INTIMATELY LINKED WITH

 

Hydrolysis-Is the chemical reaction between the MINERAL & WATER (HYDROGEN OR HYDROXYL IONS IN THE WATER) & LEADS TO CHEMICAL CHANGES.   

            Chelation (complexing)

            HERE organic acids from leaf leachate OR LICHEN

            INTERACT WITH MINERALS & CHANGE THEM. 

 

           HYDROTHERMAL ALTERATION 

THE PASSAGE OF SUPER HOT GEOTHERMAL WATER (ASSOCIATED WITH MAGMA CHAMBERS OR METAMORPHISM) THROUGH A ROCKS LADEN WITH CHEMICALS CAUSES MINERAL CHANGES, E.G. IN GRANITES WHERE FELSPARS ARE ALTERED TO KAOLINITE (CHINA CLAY)

The Weathering zone

THE DEPTH & TEXTURE OF THE WEATHERED MATERIAL (SAPROLITE) ABOVE THE FRESH ROCK. THE JUNCTION OF THE TWO IS THE WEATHERING FRONT. JOINT PATTERN LEADS TO DIFFERENTIAL WEATHERING & UNDULATIONS IN THE WEATHERING FRONT.. UNWEATHERED RESIDUALS OCCUR AS OUTCROPS – TORS, INSELBERGS, BORNHARDTS, KOPJIES).

SHALLOW WEATHERING ZONES

(<<<1M) ARE CHARACTERISTIC OF ARID & POLAR ZONES & GLACIATED PARTS OF THE TEMPERATE ZONE. ALSO ON RESIDUALS IN WET TROPICAL AREAS. AT MOST A FEW MM TO A FEW CM OF WEATHERING. e>W (EROSION>WEATHERING). SANDY MATERIAL IS FORMED WITH WEATHERABLE MINERALS.

DEEP WEATHERING ZONES

(>>>1M). OCCUR IN HUMID TROPICS & UNGLACIATED TEMPERATE AREAS.  WEATHERING ZONES >50M, >120M, >270M. WITH RESIDUALS & CORESTONES. w>e (WEATHERING>EROSION) . CLAYEY MATERIAL WITHOUT WEATHERABLE MINERALS IS PRODUCED.. THIS COVER HAS BEEN REMOVED BY GLACIATION IN MANY PARTS OF THE TEMPERATE ZONE – SOURCE OF TILL MATERIAL?).

PHYSICAL WEATHERING   

LEADS TO PHYSICAL DISINTEGRATION OF ROCKS WITHOUT CHEMICAL CHANGES TAKING PLACE.

PHYSICAL & CHEMICAL PROCESSES USUALLY OCCUR TOGETHER & IN MANY CASES IT IS DIFFICULT TO KNOW WHICH IS THE MORE IMPORTANT.

BY FRAGMENTING ROCK PHYSICAL WEATHERING INCREASES THE TOTAL SURFACE AREA THAT CAN BE ATTACKED BY CHEMICAL PROCESSES.

PHYSICAL WEATHERING PROCESSES USED TO BE CONSIDERED MORE IMPORTANT IN ARID, COLD OR HOT ENVIRONMENTS WHERE WATER WAS LACKING BECAUSE OF LOW PPT. OR IT WAS LOCKED UP IN ICE & SNOW. NOW MOST PHYSICAL WEATHERING IS COONSIDERED TO BE A MANIFESTATION OF CHEMICAL PROCESSES EVEN WHERE WATER IS SCARCE.

THE TYPE OF DEBRIS FORMED BY PHYSICAL PROCESSES IS DETERMINED BY:

1)    THE LITHOLOGY OF THE ROCK

2)    THE JOINT STRUCTURE OF THE ROCK

PHYSICAL WEATHERING IS INITIATED BY CRACK FORMATION IN A ROCK.. THIS TAKES PLACE IN 3 WAYS:

1)    DIFFERENTIAL EXPANSION OF THE ROCK MINERALS (DILITATION, UNLOADING, EXFOLIATION).

2)    CHEMICAL DECOMPOSITION ALONG JOINTS, BEDDING PLANES OR CONTACTS BETWEEN ROCK TYPES.

3)    THE EXPANSION OF FREEZING WATER (CONGELIFRACTION OR FROST WEATHERING) OR SALT CRYSTALS IN THE ROCK

(HALOCLASTY)

OTHER PROCESSES ARE FAIRLY INSIGNIFICANT: THERMAL EXPANSION & CONTRACTION, SHRINKING & SWELLING OF MINERALS, PHYSICAL ABRASION, WEDGING BY PLANT ROOTS, BURROWING BY ANIMALS ETC.