Chapter 12
WEATHERING
Define Weathering: The break up of rock due to exposure to processes
that occur at Earth’s surface.
2 Types of weathering:
1. MECHANICAL: Breaks into smaller pieces
Agents of MECHANICAL weathering include:
Ice and Water: Contractions and Expansions, Freezing and Thawing
Frost or Ice Wedging.
How does it happen? Water freezes and expands making a hole bigger,
contraction occurs with thawing. A cycle of thawing and freezing causes
rocks to weather.
And where is it most common? Location and Climate? Places where there
are temperature extreme temperature changes (very hot/very cold, icing and
thawing).
Wetting and Drying.
How does it happen? Swelling when wet, shrinking when dry
What rock does it affect? CLAY
ABRASION: Wearing away of rock by grinding action
Agents of Abrasion include: Water, wind, and ice ie: moving water such as
rivers and streams
PLANTS AND ANIMALS: Growth of Plants and Animals
How does it happen? Roots growing in cracks cause widening of cracks.
Animals dig holes allowing air and water into the soil and opening rocks.
UPWARD EXPANSION: Exfoliation, causing outer layers to wear away.
What is it and how does it happen? Rocks formed deep under ground
(example: granite) are exposed and lifted up while the rocks above it are
worn away.
Is there a particular rock type that it affects more than another? GRANITE
2. CHEMICAL WEATHERING:
Agents of Chemical Weathering Include:
Water- Hydrolysis: chemical weathering by reaction of water with other
substances.
What minerals react? Feldspar, Hornblende, and Augite
ACID IN WATER: water’s chemical effect on minerals is increased by the
presence of acid in water.
What minerals react and what do they become? Calcium  Calcite
What happens to calcite and why is it of MAJOR importance? When acid
mixes with calcite, it wears (erodes, wears away) the calcite down and as a
result sinkholes and caves may be formed.
ACID RAIN: Rain water that contains unusually high levels of acid
Acids found in acid rain include: Hydrochloric acid (HCL), Sulfuric Acid
What damage does it do? Erodes, wears away the rock.
OXYGEN:
Oxidation: chemical reaction of oxygen with other substances.
What minerals react and what do they become? Anything that contains
IRON and results in RUST.
RATE OF WEATHERING
3 Factors that affect weathering:
SURFACE AREA: which type of weathering is influenced more by surface
area? CHEMICAL WEATHERING
COMPOSTION OF ROCK:
Which minerals resist chemical weathering?
QUARTZ
Which minerals respond to chemical weathering? CALCITE
What type of weathering has a greater impact on sandstones and other quartz
bearing rocks? MECHANICAL
What does shale become as a result of weathering? CLAY
Marble and Limestone are resistant to MECHANICAL weathering.
Marble and Limestone are affected by CHEMICAL weathering.
CLIMATE:
What climate is best for Mechanical Weathering?
Climate with a lot of weather/seasonal changes (freezing/thawing, wet/dry)
What climate is best for Chemical Weathering?
Rainy, warm, tropical