Geology Lecture Outline

Weathering & Erosion – (Ch 6 and 15)


Lecture Content

Definitions and Classification                                  

Mechanical Weathering      

Chemical weathering                  

Soil Development, Degradation, and Conservation

Weathering-related Mineral Resources

Erosion - Definition/ Agents of Erosion

Mass Wasting - Styles and Mechanisms

Erosion and Landform Development


I. Introduction

A.   The Connection between Magmatism, Weathering/Erosion and Plate

      Tectonics in the Rock Cycle  -- The Endless Crustal Cycle of

         Building It Up and Tearing It Down.


B.   The Great Importance of Weathering and Erosion in Shaping the Earth’s

   Surface and Soils, and Its Influence upon the Climate and Biosphere.


II. Defining and Classifying Weathering and Erosion

A.    What is a Weathering?  --  The physical breakdown and chemical

         alteration (decomposition) of rocks and minerals at or near Earth’s



v  Two Types of Weathering Processes

·       Mechanical

·       Chemical


B.   What is Erosion? – The Removal and Transport of Weathered Material

                    from the Site of Origin


v  Several Types of Erosional Agents

·               Simple Gravity – Mass Wasting

·               Running Water

·               Glaciers

·               Wind

·               Marine Waves and Currents


III. Mechanical Weathering

A.What is Mechanical Weathering? – The physical breakdown of Earth

       material into ever-smaller pieces that retain the chemical composition of

         the parent material.


B. Types of Mechanical Weathering

·        Frost Action (wedging)

·        Pressure Release (jointing, exfoliation)

·        Heating/Cooling (expansion/contraction)

·        Salt Crystal Growth (wedging)

·        Organisms (wedging and grinding)


C.   Conditions Favoring Mechanical Weathering

·       Rugged Terrain

·       Inherently Weak Rock

·       Very Cold and/or Hot Climates

·       Shorelines

·       Wet Climates

·       Heavy Forestation


D.    Good Example Localities of Mechanical Weathering

·       Yosemite

·       San Diego Coastline and Backcountry

·       Find some others



IV. Chemical Weathering

A.    What is Chemical Weathering?Those processes by

 which rock materials are decomposed by chemical alteration of

 the parent material.


B.    Agents of Chemical Weathering

·       Water

·       Atmospheric gases (mainly oxygen)

·       Acids

·       Organisms


C. Types of Chemical Weathering

·                                Solution (simple dissolving of mineral ions)

·                                Oxidation (mineral “rusting”)

·                                Hydrolysis (water-ion/mineral-ion exchange)

·                                Acid Solution (dissolving minerals with an acid)

Also called “Carbonation”


D. Factors that Affect the Rate of Chemical Weathering

·        Particle Size

·        Mineral/Rock Structure (cleavage, joints, fractures)

·        Parent Rock Mineralogy

Ø     The Variability of Silicate Mineral Chemical Stability at

         or near the Earth’s Surface

ü    Olivine - Very Unstable at surface conditions

ü      Pyroxene

ü      Amphibole

ü      Ca-Plagioclase

ü      Biotite 

ü      Na-Plagioclase

ü      K-Feldspar

ü      Muscovite

ü      Quartz

ü    Clay -- Very Stable at Surface Conditions


Ø    Carbonate Minerals – Very Susceptible to Acid


·       Climate – A Biggie!

Ø     Hot/Wet = Highest Rate

Ø     Cold/Wet

Ø     Hot/Dry

Ø     Cold/Dry = Slowest Rate


·        Biological Activity


E.    Chemical Weathering Products

·        Clays – (from weathered silicates)

·        Dissolved Ions – (from all rock types)

·        Acids – (from all rock types)


F.   Good Example Localities of Chemical Weathering

·       Amazon and Congo Rainforest Basins

·       Florida

·       Find some other places


V. Soil Development

A.   What is a Soil? The uppermost portion of the surface

 regolith that consists of weathered rock, air, water, and

 organic material, and is supportive of vegetation.

      A soil typically consists of:

·        Several layers called horizons (O, A, B, and C)

·        45% Weathered rock (sand, silt, clay)

·        55% Humus, Air and/or Water

·        Humus is decayed organic mater


B.   Soil Profiles

·       Horizon O = Very thin top layer (mainly organic debris)

·       Horizon A = “Topsoil”, “Zone of Leaching” – the organic-

rich, productive layer of the soil profile.

·       Horizon B = “Zone of Accumulation” – Organic-poor, and

not very productive.

·       Horizon C = Partially altered bedrock/regolith


C.   Classification of Major Soil Types

·       Pedalfers =  Al-rich Clays +Organic-rich – Develops in

  temperate wet climates (ex: Eastern US and Canada)

·       Pedocals = Ca/Alkali-rich + Mod/low Organics - Develops

   In warm arid/semiarid regions (ex: Southwest US)

·       Laterites = Extremely leached - Al/Fe-rich, Silica-poor; All

 soluble ions leached out; red color; very deep soil profiles;

 Forms in the tropics (ex: Amazon)


D.  Factors That Affect Soil Development

1.   Climate – The most crucial factor, which includes the

consideration of mean temperature, precipitation, and organic


2. Parent Material

Ø    Native (in place) versus  Transported (exotic)

Ø    Granite versus Basalt versus Limestone

3. Terrain – Relief and Slope

Ø    Steep/Rugged versus Flat/Gentle

Ø    North-facing versus South-Facing


        E. Factors That Affect Soil Development

·        Physical and Chemical Activities of Organisms

Ø    “Bioturbation” – (boring, burrowing, ingesting)

Ø    Acid-producing bio-processes

·        Time – Soil needs lots of time to development


VI. Soil Wasting and Degradation


A.   Loss and Deterioration of Soil is a Serious Worldwide

        Concern to Humans

·       Current studies estimate close to 20% of world’s

  productive soils have been degradated.

·       Soil is considered a non-renewable resource

·       Most of recent soil degradation is attributed to human



B.  Major Factors that Contribute to Soil Degradation

·        Primary Factors

Ø    Improper Farming Practices

Ø    Overgrazing

Ø    Deforestation

·        Secondary Factors

Ø    Wind

Ø    Running Water


C.   Soil Conservation Practices

·        Contour Plowing

·        Terracing

·        Windbreaks

·        Strip-cropping

·        Crop-rotation

·        Reduced grazing

·        Open Space Preserves


VII. Weathering-Related Ore Deposits

·       Bauxite  (Primary aluminum ore) - Aluminum oxide/

hydroxide deposits = Super Extreme leaching of soil 


VIII. Erosion and Mass Wasting

A.    What is Erosion? – The Removal and Transport of

  Weathered Material from the Site of Origin


v  Several Types of Erosional Agents

·                                                                       Simple Gravity = Mass Wasting

·                                                                       Running Water

·                                                                       Glaciers

·                                                                       Wind

·                                                                      Marine Waves and Currents


B.What is Mass Wasting? – The Downslope Removal of

Material from the Site of Origin

v  Several Types of Mass Wasting –

v  Classified According to Type of Movement

v  Rate of Movement Varies Greatly

·   Rockfalls  - Very fast

·   Landslides, Rockslides and Slumps  Fast to slow

·   Earthflows, mudflows, Debris flows - Fast

·   Creep – Slow to Very slow


C.Factors That Influence Mass Wasting Events

1) Slope (material) Shear Strength

§        Geology (composition and structure)

§        Water Content

2) Slope Angle

3) Weathering and Climate/Weather Conditions

4) Vegetation

5) Overloading

6) Temporary Disturbing Forces – “The Last Straw”


D. Mass Wasting Triggering Mechanisms

1) Heavy rainstorm

2) Earthquakes

3) Eruptions

4) Explosions

5) Strong Vibrations from a loud sound

6) Slope undercutting


        E. Characteristics of the Different Types of Mass Wasting

1) Rockfalls

·        Falling (freely) Loose Rock

·        Associated with Eroding Cliffs/Bluffs


2) Landslides, Rockslides and Slumps

·        Solid (cohesive) Masses Move over a Glide

Surface (planar or curved surface)

·   Characteristic “Scarps” “Hummocks & “Toes”

·        Can be Very Extensive

·        Rate of Movement can vary greatly


3) Earthflows, mudflows, Debris flows

·        Behaves like a Fluid

·        Rate of Movement Varies, but can be very fast.

·        Flows follow Low areas (drainage channels)

·        Flows can be Very Extensive

·        Closely Associated with Water saturation

4) Earth and Soil Creep

·       A Very Slow form of Flow

·       The Most Widespread form of Mass Wasting

·       Difficult to Recognize and Control       


F. Mass Wasting Hazard Management

                     1) Importance of Identifying Unstable Areas

·       Geology/Risk Assessment Studies

2) Create a Slope-stability Map

3) Methods to Reduce or Eliminate Slope Instability

·       “Cut and Fill”

·       “Benching” or “Terracing”

·       “Rock Bolting”

·       “Retaining Walls and Cribbing”


G.   Local Examples of Recent Mass Wasting

1) San Diego’s Coastline

·       La Jolla Shores

·       Black’s Beach

·       North County


2) San Diego Backcountry


                        3) Name some other notable regions


IX.   Erosion and Landforms

A.  Mountains, plateaus, peneplains, plains, coastlines,

 canyons, valleys, hills, mesas, buttes, rivers, lakes,

 and caves.

B. The unique characteristics of each landform type.


D.Illustrations of Major Landforms.



X.   Vocabulary Terms

1.  Study the Vocabulary list at the end of Chapters 5 and 14.