Chapters 7 and 8 Slide Lake
Dec 31, 2015
A full, solid block has the least surface area.
The interior is safe from exposure.
A smashed piece has greatest surface area exposed. The interior can now be attacked.
Mechanical WeatheringAKA: physical weathering
Breaks a rock into smaller and smaller piecesOnly size changes occur in this processNo chemical composition change occurs in mechanical weatheringSame chemical composition as the parent rockResults in increased surface area
Types of Mechanical Weathering
• Frost Wedging
• Root Wedging
• Pressure Release
Let’s look at each one
-Repeated freezing and thawing of H2O in rocks
-Can form potholes in cold climate areas-Ice expands by over 9% in volume as it freezes
- Ice at -30oC is harder than granite
Frost wedging
Root Wedging• Plant action-
1.Tiny root hairs seek out small cracks and pits in rock.
2.Once the root hairs find a place they grow and expand.
3.The expansion causes great pressure and cracks the rock.
Pressure Release
1. Rocks formed deep in the Earth are made under high pressure.
2. When the pressure is released the rocks expand & crack.
3. May also be caused by alternate heating and cooling of rocks by weather conditions.
Chemical Weathering
• The breakdown of rocks by chemical means
• Depends on the stability of minerals
More Stable Less Stable
•Form at low Temps. • Form at high Temps.
• Quartz, K-Feldspar • Olivine
Two Agents of Chemical Weathering
• Oxygen causes oxidization (Rust)
• Carbon dioxide forms a weak acid
(carbonic acid), which helps dissolve
rock material.
Chemical Weathering of Statues, Bath, UK
The Result of Chemical Weathering
• New minerals are formed
• Dissolved particles are released
• The amount of chemical weathering is determined by the composition of the Parent rock
• Temperature increases increase chemical reactions
How Granite WeathersChemical weathering – the alteration of the internal
structure of minerals by chemical reactions
K Feldspar + carbonic acid + H2O Clay mineral + K bicarbonate + Silica
Differential WeatheringRocks weather at different rates because of different
mineral makeup and exposure to the elements.
More resistant rock protrudes as ridges and pinnacles.
Bryce National Park, Utah