Chapter Three: Weathering, Soil, and Mass Wasting
Chapter Three Learning Objectives:
Describe the processes of weathering, erosion, and mass wasting.
Explain the difference between mechanical and chemical weathering.
Discuss soil composition, texture, structure, formation, and classification
List and describe the various types of mass wasting.
Chapter Three Outline:
Earth's external processes include
Mass wasting- the transfer of rock material downslope under the influence of gravity
Erosion-the incorporation and transportation of material by a mobile agent, usually water, wind, or ice
Two kinds of weatheringMechanical weathering
Breaking of rocks into smaller pieces
Four processes
Frost wedging
Unloading
Thermal expansion
Biological activity
Chemical weathering
Alters the internal structures of minerals by moving or adding elements
Most important agent is water
Oxygen dissolved in water oxidizes materials
Carbon dioxide dissolved in water forms carbonic acid and alters the material
Weathering of graniteWeathering of K-feldspar produces
Clay minerals
Potassium bicarbonate
Silica in solution
Weathering of silicate minerals produces
Soluble sodium, calcium, potassium and magnesium products
Insoluble iron oxides
Clay minerals
Rates of weatheringRate of Weathering
continuedOther important factors are
Mineral makeup
Marble (calcite) readily dissolves in weakly acidic solutions
Silicate minerals weather in the same order as their order of crystallization
Climate
Temperature and moisture are the most crucial factors
Chemical weathering is most effective in areas of warm temperatures and abundant
moisture SoilSoil texture
and structure
Texture
Refers to the proportions of different particle sizes
Sand (large size)
Silt
Clay (small size)
Loam is best suited for plant life
Structure
Soil particles clump together to give a soil its structure
Four basic soil structures
Platy
Prismatic
Blocky
Spheroidal
Controls of soil formation
Parent material
Residual soil-parent material is the bedrock
Transported soil-parent material has been carried from elsewhere and deposited
Time
Important in all geologic processes
Amount of time to evolve varies for different soils
Climate
Plants and animals
Organisms influence the soil's physical and chemical properties
Furnish organic matter to soil
Slope
Steep slope-often poor soils
Optimum is a flat-to-undulating upland surface
Soil Profile
Soil forming processes operate from the surface downward
Horizons - zones of layers of soil
Horizons in temperate regions
O - organic matter
A - organic and mineral matter
E - little organic matter
B - zone of accumulation
C - partially altered parent material
O and A together called topsoil
O, A, E, and B together called solum, or "true soil"
Soil Types
Hundreds of soil types worldwide
Three very generic types
Pedalfer
Accumulation of iron oxides and Al - rich clays in the B horizon
Best developed under forest vegetation
Pedocal
Accumulate calcium carbonate
Associated with drier grasslands
Laterite
Hot, wet, tropical climates
Intense chemical weathering
Soil Erosion
Recycling of Earth materials
Natural rates of erosion depend on
Soil characteristics
Climate
Slope
Type of vegetation
Weathering creates ore deposits
Process called secondary enrichmentConcentrates metals into economical deposits
Two ways of enrichment
Removing undesired material from the decomposing rock, leaving the desired elements behind
Desired elements are carried to lower zones and deposited
Examples
Bauxite, the principal ore of aluminum
Many copper and silver deposits
Mass Wasting
The downslope movement of rock, regolith, and soil under the direct influence of gravity
Gravity is
the controlling force
Important triggering factors are
Saturation of the material with water
Destroys particle cohesion
Water adds weight
Oversteepening of slopes
Stable slope angle is different for various materials
Oversteepened slopes are unstable
Removal of anchoring vegetation
Ground vibrations from earthquakes
Types of
mass wasting
Generally each type is defined by
The material involved
Debris
Mud
Earth
Rock
The movement of the
material
Fall ( free-fall of pieces)
Slide (material moves along a surface)
Flow ( materials moves as a viscous fluid)
The velocity of the movement
Fast
Slow
Forms of mass wasting
Slump
Rapid
Movement along a curved surface
Along oversteepened slopes
Rockslide
Rapid
Blocks of bedrock move down a slope
Mudflow
Rapid
Flow of debris with water
Often confined to channels
Serious problem in dry areas with heavy rains
Mudflows on the slopes of volcanoes are called lahars
Forms of Mass Wasting
Earthflow
Rapid
On hillsides in humid regions
Water saturates the soil
Creep - the slow movements of soil and regolith downhill
Solifluction
Slow
In areas underlain by permafrost
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