Mechanical versus chemical weathering Mechanical versus chemical
WEATHERING Changes that occur in sediments and rocks near Earth’s surface Two main types: Chemical and Physical (Mechanical)
The breakdown of rock into smaller pieces or sediments. What is weathering? The breakdown of rock into smaller pieces or sediments. Two Types of Weathering: Mechanical – disintegration (smashing a rock with a hammer) Chemical – decomposition of rocks due to chemical reactions (minerals become chemically altered, ex. Calcite reacting with acid)
Mechanical Weathering Smashing a rock with a hammer Rock is not altered, it is broken down to smaller pieces
Frost action Pushing rock apart by alternate freezing and thawing of rock
exfoliation cracking of rocks as they expand due to removal of overburden pressure
Exfoliation
Thermal expansion cracking of rocks from alternate expansion and contraction due to daily changes in temperature (mostly found in the desert)
Wetting and drying constant swelling and shrinking causes the rock to crack
abrasion wind, waves, rain, glaciers, and so on abrade rocks’ surfaces, wearing them down
Organic activity expansion of cracks by growth of roots and the breakup by burrowing processes
Mollusks live in small holes they scrape in the rocks
chemical Weathering Calcite reacting with acid Minerals making up the rock are chemically altered
Plant acids lichens secrete acids that dissolve rocks
oxidation oxygen combines with mineral to break down and soften the original mineral
hydrolysis hydroxide (oh) combines with minerals to break down and soften the original mineral, ex. Feldspar turns to clay
Sulfuric acid minerals are dissolved by reaction with acid, ex. Acid rain
Effects of acid rain on cleopatra’s needle (c. 1543–1292 BC)
carbonation carbonic acids dissolve rocks and form caves underground
Rates of weathering Chemical weathering most active in warm and moist climates where water is abundant Mechanical processes are active everywhere and can be sped up by interacting with chemical weathering