Weathering The Breakdown of Rocks
Jan 23, 2016
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