EXTERNAL FORCES PART 1

IN THIS CHAPTER YOU WILL STUDY: SOME KEY DEFINITIONS DIFFERENT TYPES OF WEATHERING (I)PHYSICAL (II) CHEMICAL (III) BIOLOGICAL

SOME KEY DEFINITIONS DENUNDATION THE WEARING AWAY OF THE LAND BY WEATHERING AND EROSION. IT INCLUDES ALL NATURAL AGENCIES LIKE THE SUN, RAIN, FROST, WIND, RIVERS, SEA, ICE, TEMPERATURE CHANGE AND EVEN THE ACTION OF PLANTS AND ANIMALS.

WEATHERING THE WEARING AWAY (DISINTEGRATION AND DECOMPOSITION) OF ROCKS BY THE EFFECTS OF THE WEATHER AND THE ATMOSPHERE. EROSION THE WEARING AWAY OF THE EARTH’S SURFACE BY WATER, ICE AND WIND.

THE DIFFERENCE BETWEEN WEATHERING AND EROSION THE KEY DIFFERENCE HERE IS MOVEMENT IN THE CASE OF WEATHERING, NO MOVEMENT IS INVOLVED. HOWEVER, IN EROSION, THE AGENT BREAKING UP THE ROCK ALSO MOVES THE DEBRIS ALONG WITH IT.

MASS WASTING ALSO KNOWN AS MASS MOVEMENT; FOLLOWS WEATHERING. IT IS THE DOWNHILL MOVEMENT OF ROCK DEBRIS AND SOIL DUE TO THE FORCE OF GRAVITY. WATER ACTS AS A LUBRICANT, MAKING THE DEBRIS MORE SLIPPERY AND EASY TO MOVE.

TYPES OF WEATHERING

PHYSICAL WEATHERING THE BREAKING UP OF ROCK THAT DOES NOT CAUSE THE CHEMICAL COMPOSITION OF THE ROCK TO CHANGE.

TYPES OF PHYSICAL WEATEHRING FROST SHATTERING FOR THIS TYPE OF WEATHERING TO OCCUR THE TEMPERATURE MUST FLUCTUATE (CHANGE/VARY). THERE MUST ALSO BE BARE ROCK EXPOSED TO THE SURFACE, WITH LITTLE VEGETATION TO PROTECT THE ROCK FROM THE WEATHER CONDITIONS.

IF IT RAINS DURING THE DAY, OR THERE IS MOISTURE FROM DEW OR MELTING ICE OR FROST, WATER CAN TRICKLE INTO CRACKS IN THE ROCK. IF AT TNIGHT THE TEMPERSATURE DROPS BELOW O 0 C, THE WATER IN THE CRACKS FREEZES, AND THE ICE TAKES UP MORE SPACE, AND SO EXERTS PRESSURE ON THE ROCK. REPEATED FREEZE-THAW WEATHERING SLOWLY WIDENS THE CRACKS IN THE ROCK UNTIL PIECES OF ROCK BREAK OFF.

FROST SHATTERING

EXFOLIATION OR ONION- PEELING ROCKS EXPAND WHEN HEATED AND CONTRACT WHEN COOLED. IN DIURNAL TEMPERATURES (WHERE THERE IS CONSTANT HEATING AN COOLING) SUCH AS IN THE DESERT, THIS TYPE OF WEATHERING TAKES PLACE. THE OUTSIDE LAYER OF THE ROCK EXPANDS AND CONTRACTS REGULARLY. THIS WEAKENS IT AND EVENTUALLY IT PEELS OFF. WHEN THIS LAYER FALLS OFF, THE OTHER LAYER IS ATTACKED, AND IT PEELS OFF LIKE THE SKIN OF AN ONION.

EXFOLIATION

THERMAL EXPANSION

GRANULAR DISINTEGRATION ROCKS ARE MADE UP OF DIFFERENT YYPES OF MINERALS AND THESE MINERALS EXPAND AND CONTRACT AT DIFFERENT RATES, AND THIS CAUSES THE ROCK TO DISINTEGRATE.

PRESSURE RELEASE ROCKS DEEP UNDERGROUND HAVE THE WEIGHT OF OTHER ROCKS PRESSING DOWN ON THEM. OVER TIME THE UPPER ROCKS ARE ERODED AWAY, SO THE PRESSURE IS REDUCED AND THE STRESSES ARE LESS. THE ROCK EXPANDS AND CAUSES THE CRACKS TO OPEN UP, ALMOST AS IF THEY ARE BREATHING A SIGH OF RELIEF!

PRESSSURE RELEASE

SALT CRYSTAL GROWTH THE WATER FROM SLAT WATER EVAPORATES AND LEAVES THE SALT CRYSTALS BEHIND. WHEN THEY ARE HEATED, THEY EXPAND, CAUSING STRESS IN THE ROCK AND CRACKS FORM. THIS CAUSES THE ROCKS TO BREAK UP OVER TIME. THE WATER FROM SLAT WATER EVAPORATES AND LEAVES THE SALT CRYSTALS BEHIND. WHEN THEY ARE HEATED, THEY EXPAND, CAUSING STRESS IN THE ROCK AND CRACKS FORM. THIS CAUSES THE ROCKS TO BREAK UP OVER TIME.

SALT CRYSTAL GROWTH

CHEMICAL WEATHERING CHEMICAL WEATHERING IS THE CHEMICAL DECOMPOSITION OF ROCKS AS THEIR MINERALS REACT WITH RAINWATER, GASES AND MINERALS. CHEMICAL WEATHERING ALTERS THE MINERALS WHICH MAKE UP THE ROCK, IT MAY PRODUCE A CHANGE IN COLOUR AND TEXTURE.

PROCESSES OF CHEMICAL WEATHERING CARBOBNATION & SOLUTION OXIDATION AND REDUCTION HYDRATIONHYDROLYSIS

CARBONATION & SOLUTION IS THE MOST COMMON PROCESS CARBON DIOXIDE IS ABSORBED BY BOTH RAIN WATER AND GROUND WATER, AND WHEN DISSOLVED IN WATER FORMS A WEAK ACID SOLUTION (CARBONIC ACID). THE WEAK ACID SOLUTION SOAKS THROUGH THE ROCKS REMOVING THE SOLUBLE PARTICLES AND LEAVING THE INSOLUBLE PARTICLES BEHIND IN THE CARIBBEAN CARBONATION AND SOLUTION ARE MOST COMMON IN LIMESTONE AREAS

OXIDATION & REDUCTION OXIDATION OS THE REACTION OF MINERALS WITH OXYGEN IN THE AIR AND WATER, TO FORM MINERAL OXIDES. THE MAIN MINERAL INVOLVE IS IRON. SINCE IRON IS FOUND IN MOST ROCKS, THIS PROCESS IS COMMON. THE IRON-OXIDE CRUST CRUMBLES EASILY AND WEAKENS THE ROCK.

OXIDATION IS VERY COMMON IN HOT WET AREAS SUCH AS THE CARIBBEAN, AND ROCKS THAT ARE AFFECTED OFTEN HAVE A YELLOW, BROWN OR REDDISH COLOUR, FORM THE “RUSTING” OF THE IRON COMPOUNDS.

HYRDRATION IN WET AREAS, MINERALS OFTEN ABSORB WATER AND SWELL. THE ROCK MINERALS ARE ALTERED BY THE PHYSICAL STRESSES, AND THIS MAY CASUE THE ROCK TO BREAK UP INTO FINER PARTICLES.

IN CONTRAST WITH MECHANICAL WEATHERING, CHEMICAL WEATHERING TENDS TO BE MORE ACTIVE INAREAS WHICH HAVE A WARM, HUMID CLIMATE, WHILE MECHANICAL WEATHERING TENDS TO TAKE PLACE IN MORE DRY AREAS.

HYDROLYSIS HYDROLYSIS IS THE BREAKING DOWN OF ROCK MINERALS BY HYDROGEN IONS FOUND IN WATER. THE MINERALS COMBINE WITH WATER AND BREAK DOWN CHEMICALLY. MINERALS SUCH AS CALCIUM, SODIUM AND POTASSIUM ARE REPLACED BY THE HYDROGEN IONS. THE ROCKS IN WHICH THESE MINERALS WERE FOUND BECOME WEAKER SINCE THEY ACT AS BINDING PARTICLES.

FOR EXAMPLE, FELDSPARS, BY THE PROCESS OF HYDROLYSIS, ARE DECOMPOSED INTO FINE CLAYS. (AS SEEN IN THE NEXT SLIDE)

BIOLOGICAL WEATHERING THE BREAKING DOWN OF ROCKS BY PLANTS AND ANIMALS

PHYSICAL BIOLOGICAL WEATHERING CERTAIN ANIMALS BURROW INTO THE GROUND (RABBITS AND EARTHWORMS) AND CAUSE THE ROCKS TO BECOME WEAKER OR LOOSER. SOMETIMES THE ROCKS AND THE PLANTS GET INTO CRACKS AND THE PLANT GROWS. THE CRACKS BECOME LARGER AND WIDER, EVENTUALLY CAUSING THE ROCK TO BREAK UP.

CHEMICAL BIOLOGICAL WEATHERING WHEN VEGETATION DECAYS, HUMUS IS FORMED. WATER DRAINING THROUGH THE HUMUS BECOMES ACIDIC, RESULTING IN THE WEATHERING OF ROCK BELOW.

QUESTIONS

ON THE WORLD MAP PROVIDED NAME ONE AREA WHERE WEATHERING BY TEMERATURE CHANGE WILL TAKE PLACE AND ONE AREA WHERE FREEZE-THAW WEATHERING WILL TAKE PLACE

EXPLAIN 2 WAYS IN WHICH WEATHERING IS AFFECTED BY CLIMATE EXPLAIN WHY A ROCK WITH MANY JOINTS OR CRACKS WILL BE AFFECTED BY (A) FROST SHATTERING (B) CHEMICAL WEATHERING (C) BIOTIC WEATHERING