weathering the breakdown of rocks. what is weathering? the physical breakdown (disintegration) and...

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