Chapters 7 and 8
Slide Lake
Weathering – the breakdown of
rock at or near the earth’s surface
15.02.b1
Types of Weathering
• Physical – Mechanical weathering
• Chemical – Chemical weathering
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 Weathering
AKA: physical weathering
Breaks a rock into smaller and smaller
pieces
Only size changes occur in this process
No chemical composition change occurs
in mechanical weathering
Same chemical composition as the
parent rock
Results in increased surface area
Types of Mechanical Weathering
• Frost Wedging
• Root Wedging
• Pressure Release
Let’s look at each one
Frost wedging
-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
Root Wedging
• Plant action1. 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
•Form at low Temps.
• Quartz, K-Feldspar
Less Stable
• Form at high Temps.
• 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 Weathers
Chemical weathering – the alteration of the internal
structure of minerals by chemical reactions
K Feldspar + carbonic acid + H2O  Clay mineral + K bicarbonate + Silica
Rates of Weathering
Factors
influencing
weathering:
-Mineral
makeup
-Climate
Granite versus Marble
Differential Weathering
Rocks 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
Where is the rock more resistant to
weathering?
Where is the rock least resistant to
weathering?