WEATHERING AND EROSION
The Rock Cycle-
The rock cycle shows how one type of rocky material
gets transformed into another in response to changing conditions.
Weathering, Erosion, and Transportation
Rocks exposed at Earth’s surface are constantly changed
by the atmosphere, hydrosphere, and biosphere.
Weathering is the group of physical and chemical
processes that change the character of rocks at or near Earth’s surface.
Weathering involves the breakdown of rock and the formation of new minerals.
the physical removal and transportation of rock particles from the
source by water, wind, or ice.
There are two types of
-The physical disintegration of rock into smaller pieces.
-Facilitated by the action of ice,
water, and wind.
-Chemical decomposition of rock
from exposure to atmospheric gases (oxygen, water vapor and carbon dioxide) or
liquid water (which is often acidic).
rate of chemical weathering
increases as surface area of rock increases.
Weathering and the Earth Systems
Within our solar system, Earth seems to be the only
planet with surface rocks actively being weathered by wind, water, and living
Gases like O2, CO2,
and H2O are critical to the chemical weathering of rock. These
gases react with pre-existing minerals to produce new minerals that are in equilibrium with surface
ex. CO2 + H2O
is essential to life in that it removes carbon dioxide from the
atmosphere, reducing green-house effect.
also causes the mechanical breakdown of rock via a sandblasting effect.
Hydrosphere- Oxygen dissolves in water and
then can oxidize iron in rocks:
ex. FeO + O2
Biosphere- Decaying organic matter in soils produces acidic moisture
Types of Mechanical Weathering
Running water/glacial ice- Mechanically loosens and
abrades mineral and rock particles.
Thermal cycling- Large temperature changes fracture
rocks via repeated expansion and contraction.
The freezing and
expanding of water causes frost wedging and frost heaving.
The removal of overlying rock from
erosion allows expansion and fractures (sheet joints and exfoliation).
Plant growth- Growing roots widen fractures (root
Lead to accelerated mechanical weathering.
Chemical Weathering of Minerals
Feldspars (the most common minerals in
Earth's crust)- Slightly acidic rainwater (H2CO3)
attacks feldspar, producing clay minerals (hydrated sheet silicates).
The K+, Na+,
and Ca2+ ions are released into the water.
Minerals highest on Bowen’s
reaction series are most susceptible to weathering. Therefore, olivine and
pyroxene weather quickly to produce clays and iron oxides.
the least susceptible to
chemical weathering. It can be transported all the way to the oceans.
Soil is a layer of weathered, unconsolidated material
on top of bedrock. Common soil constituents include:
Clay minerals (from
Residual soil develops from weathering of
underlying rock in situ
Transported soil- Soil brought in from elsewhere
else. Examples are floodplain deposits (from
river-transported sediments) and wind-transported deposits called loess.
Soil composition is determined by
the parent rock
composition. The soil composition evolves with time and chemical weathering.
O horizon - uppermost layer;
A horizon - dark layer rich in
humus, organic acids
E horizon - zone of leaching;
fine-grained components removed by percolating water
B horizon - zone of accumulation;
clays and iron oxides leached down from above
C horizon - partially weathered
- Increases with time
- Typically greater in wetter climates
- Greater in areas with shallow slopes vs. steep slopes
Soils and Climate- Soil thickness and composition
are greatly affected by climate. Warm, wet climate conditions maximize
-More chemical weathering and thicker soils
-Soils in moderately wet climates tend to have significant
-Less chemical weathering and thinner soils
-Subsurface evaporation leads to build-up of salts
-Caliche- Calcite-rich accumulation zones may form,
cementing soil together into a hardpan called caliche.
-Desert varish- Wind-transported clays desert heat → dark
varnish (manganese plays an important role; Mn levels are 50 times greater in varnish than
the average crust)..
Extremely wet climates (e.g., tropical rainforest):
-Highly leached and unproductive soils (laterites and
-Most nutrients come from thick O/A horizons.