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Weathering Notes
 WEATHERING: the process by which rocks are BROKEN down by the action of
PHYSICAL or CHEMICAL processes.
2 Types of Weathering
1. MECHANICAL weathering: rocks are broken down into SMALLER pieces without being
chemically changed. This is caused by several things.
 Examples:
 ICE:
 FROST action – repeated freezing and thawing
 ICE Wedging: frost action where WATER seeps into cracks during warm weather,
then freezes, EXPANDS, and breaks rock during cold weather.
 ABRASION: other rocks or sand rub and WEAR away other rocks
 WIND - abrasion caused by the wind BLOWING sediments
 WATER – abrasion caused by waves moving sediments
 GRAVITY: abrasion caused by rocks falling on other rocks
 PLANTS: ROOTS get into cracks, grow, expand, and break rocks
 ANIMALS: animals burrow & cause abrasion by BURROWING (digging)
 Wetting and Drying - causes clay minerals to expand and contract, and salts may
dissolve and re precipitate.
 Sheet Joining - surface sheets of material fracture and exfoliate because of pressure
release.
 Exfoliation - The peeling of rounded thin layers from the rocks surface.
 The rocks are essentially torn apart by physical force, rather than by chemical
breakdown.
2. CHEMICAL weathering: rocks broken down into smaller pieces and are BROKEN DOWN by
a CHEMICAL reaction.
 WATER: (Hydrolysis) can chemically change rocks over millions of years
 ACID RAIN: caused by PRECIPITATION (or pollution)
 ACIDS IN LIVING THINGS: ex: LICHENS produce acids that break down rock.
 AIR- OXIDATION: OXYGEN in air reacts with iron in certain rocks to make RUST
(oxide).
 Carbon Dioxide mixes with rain water to form Carbonic Acid. It dissolves rock and
caves form underground.
 Sulfuric Acid is pollution from factories that dissolves in rainwater to weather
metals and rocks.
RATES OF WEATHERING
1. How fast does weathering happen?
 This depends on many things such as CLIMATE, elevation, and composition (what the
rock is made of).
 DIFFERENTIAL WEATHERING: when SOFTER rocks wear away from the weather &
leave the HARDER and more weather-resistant rocks behind (ex: Devil’s Tower – the
softer part of the volcano is gone, but the harder rock is left behind.)
 THE SHAPE OF THE ROCK: the MORE rock that is exposed, the MORE it will weather
away.
 CLIMATE:
 Chemical weathering happens FASTER in warmer, HUMID climates
 Mechanical weathering happens faster in COOLER climates where there is a lot of
FREEZING & thawing
 ELEVATION:
 HIGHER rocks weather away faster because they are exposed to ICE, WIND & RAIN
more
 The steeper the SLOPE of the mountain, the faster the SEDIMENTS will flow downhill
and the faster the ROCKS will weather away