Weathering

The Breakdown of Rocks

What is Weathering?• The physical breakdown (disintegration) and chemical

alteration (decomposition) of rocks

• 2 types– Mechanical weathering– Chemical Weathering

Yellowstone, 2004

Weathering

• Mechanical Weathering

– Physical disintegration of rock

– Results in smaller fragments

• Chemical Weathering

− Internal structure of a mineral is altered

− A new mineral is formed

Weathering Examples

• Mechanical weathering • Chemical weathering

Mechanical Weathering• Rocks are broken into smaller pieces without

changing the rock’s mineral composition

• 3 types– Frost wedging– Unloading– Biological activity

Mechanical – Frost Wedging

• Breaking of rock from freezing and thawing

Mammoth, Twin Lakes (2006)

• Repeated expansion and contraction break rock bodies into angular pieces (talus)

Mechanical - Unloading• Erosion removes overlying rock• Decreases pressure• Underlying rocks rise and expand

• Rocks break off in layers (exfoliation)

Mechanical – Biological Activity

• Organisms break rock into fragments

− Plants

− Humans

− Animals

Chemical Weathering• Chemical transformation of rock into one or more

new compounds

• 3 Types– Oxidation – Hydration – Carbonation

Feldspar Kaolinite

Chemical - Oxidation• Oxygen reacts with minerals and forms iron

oxide (rust)

− Results in rocks with a reddish or yellowish coloring

Bryce National Park

Chemical - Hydration• Water interacts with minerals and forms clays

− Granite composed of silicate minerals (feldspar, hornblende and biotite)

− Silicate minerals weather into clays (kaolinite)

Chemical - Carbonation• Carbon dioxide combines with water and forms

a weak acid (carbonic acid)• Dissolves limestone

Shells (CaCO3)Limestone CavesCaves Stalactites

Rates of Weathering

• Factors controlling weathering rates

− Rock Characteristics

− Climate

− Differential Weathering

Rock Characteristics• Composition

– Certain rocks are more resistant to weathering• Particle size

– Small pieces generally weather faster than larger pieces• Joints & Fractures

– Allows water to penetrate between cracks

MarbleGranite

Climate

• Warm, moist areas tend to weather minerals faster

Differential Weathering• Certain rock types are more resistant to

weathering

− Volcanic neck and dikes are more resistant

Which will Weather Faster?

Boulder vs. Pebbles

Which Area Will a Faster Rate of Weathering?

Mammoth vs. Maui

What Type of Weathering is Displayed?

Oxidation & Differential Weathering