Weathering and Soils
Earth’s external processes
Weathering – the physical breakdown (disintegration) and chemical
alteration (decomposition) of rock at or near Earth’s surface
 Mass wasting – the transfer of rock and soil downslope under the
influence of gravity
 Erosion – the physical removal of material by mobile agents such as
water, wind, ice, or gravity

Weathering
 Two
types of weathering
weathering – breaking of rocks into smaller pieces
Four types of mechanical weathering
Mechanical
wedging – alternate freezing and thawing of water in fractures
and cracks promotes the disintegration of rocks
 Frost
Frost wedging
Mechanical
Weathering continued
– exfoliation of igneous and metamorphic rocks at the
Earth’s surface due to a reduction in confining pressure
 Thermal expansion – alternate expansion and contraction due to
heating and cooling, also alternate wet and dry cycles with dew
 Biological activity – disintegration resulting from plants and animals
 Unloading
Exfoliation of igneous rocks
Exfoliation on Half Dome
Biologic Activity
Agents of Erosion
 Chemical Weathering
Breaks
down rock components and the internal structures of
minerals
Most important agent involved in chemical weathering is water
(responsible for transport of ions and molecules involved in
chemical processes)
 Major
processes of chemical weathering
Dissolution
 Aided
by small amounts of acid in the water
 Soluble
ions are retained in the underground water supply
Oxidation
 Any
chemical reaction in which a compound or radical loses electrons
in decomposing ferromagnesian minerals
 Important
 Major
processes of chemical weathering
Hydrolysis
 The
reaction of any substance with water
 Hydrogen ion attacks and replaces other positive ions
Oxidation and Hydrolysis
 Alterations caused by chemical weathering
Decomposition
of unstable minerals
Generation or retention of materials that are stable
Physical changes such as the rounding of corners or edges
Joint-controlled weathering in igneous rocks
 Rates
of weathering
Advanced
mechanical weathering aids chemical weathering by
increasing the surface area
 Others
factors affecting weathering
Rock
characteristics
 Rocks
containing calcite (marble and limestone) readily dissolve in
weakly acidic solutions
Increase in surface area by mechanical weathering
 Others factors affecting weathering
Rock
characteristics continued
 Silicate
minerals weather in the same order as their order of
crystallization
Climate
 Temperature
 Chemical
and moisture are the most crucial factors
weathering is most effective in areas of warm, moist climates
Order of Weathering Same as order of Formation
 Differential weathering
Masses
of rock do not weather uniformly due to regional and
local factors
Results in many unusual and spectacular rock formations and
landforms
Differential Weathering
Soil
 Soil
is a combination of mineral and organic mater, water,
and air
That
portion of the regolith (rock and mineral fragments
produced by weathering) that supports the growth of plants
Typical components in a soil that yield good plant growth
Organic Material
 Factors
controlling soil formation
Parent
material
soil – parent material is the underlying bedrock
 Transported soil – forms in place on parent material that has been
carried from elsewhere and deposited
 Residual
 Factors
controlling soil formation
Time
 Important
in all geologic processes
 Amount of time for soil formation varies for different soils depending
on geologic and climatic conditions
Climate
 Most
 Key
 Factors
influential control of soil formation
factors are temperature and precipitation
controlling soil formation
Plants
and animals
 Organisms
 Also
influence the soil’s physical and chemical properties
furnish organic matter to the soil
Slope
 Steep
slopes often have poorly developed soils
terrain is a flat-to-undulating upland surface
 Optimum
Variations in soil development due to topography
Not Much Soil Development in Mountains
 The soil profile
Soil
forming processes operate from the surface downward
Vertical differences are called horizons – zones or layers of soil
 The
soil profile
horizon – organic matter
A horizon – organic and mineral matter
O
 High
biological activity
 Together the O and A horizons make up the topsoil
E
horizon – little organic matter
 Zone
 The
of eluviation (fines) and leaching (solubles)
soil profile
horizon – zone of accumulation (can be hardpan)
C horizon – partially altered parent material
B
 The
O, A, E, and B horizons together are called the solum, or
“true soil”
An idealized soil profile
A soil profile showing different horizons
Shallow Soil Profile
A Less-Than Idealized Profile
 Soil
types
The
characteristics of each soil type primarily depend on the
prevailing climatic conditions
 Three
very generic soil types
Pedalfer
 Accumulation
 Three
of iron oxides and Al-rich clays in the B horizon
very generic soil types
Pedalfer
 Best
continued
developed under forest vegetation
Pedocal
 High
accumulations of calcium carbonate
with dry grasslands and brush vegetation
 Associated
 Three
very generic soil types
Laterite
 Hot
and wet tropical climates
chemical weathering
 Intense
Bauxite
 Soil
erosion
Recycling
of Earth materials
Natural rates of soil erosion depend on
 Soil
characteristics
 Climate
 Slope
 Type of vegetation
In
many regions the rate of soil erosion is significantly greater
than the rate of soil formation
Sedimentation and chemical pollution
 Related
to excessive soil erosion
 Occasionally soil particles are contaminated with pesticides
Aeolian Soil Erosion
Mass Wasting: The Work of Gravity
Mass Wasting and
landform development
 Mass
wasting refers to the downslope movement of rock,
regolith, and soil under the direct influence of gravity
 Role of mass wasting
Geologic
process that often follows weathering
Combined effects if mass wasting and running water produce
stream valleys
 For
mass wasting to occur, there must be a slope angle
Most
rapid events occur in areas of rugged, geologically young
mountains
As a landscape ages, less dramatic downslope movements occur
Controls and triggers of
mass wasting
 Gravity
is the controlling force
 Important triggers include
Saturation
of the material with water
 Diminishes
particle cohesion
 Water adds weight
Oversteepening
 Stable
 Important
of slopes
slope angle (angle of repose) is different for various materials
triggers include
Oversteepening
 Oversteepened
Removal
of slopes
slopes are unstable
of anchoring vegetation
Ground vibrations from earthquakes
 May
cause expensive property damage
 Can cause liquefaction – water saturated surface materials behave as
fluid-like masses that flow
 Landslides
without triggers
Slope
materials weaken over time
Random events that are unpredictable
Classification of mass
wasting events
 Generally
Type
each event is classified by
of material involved
 Debris
 Mud
 Earth
 Rock
 Generally
Type
each event is classified by
of motion
 Fall
(free-falling pieces)
 Slide (material moves along a surface as a coherent mass)
 Flow (material moves as a chaotic mixture)
The
velocity of the movement
 Fast
 Slow
Forms of mass wasting
 Slump
Movement
of a mass of rock or uncon-solidated material as a
unit along a curved surface
Occurs along oversteepened slopes
A slump with an earthflow at the base
La Conchita Slump
Local Slumping
 Rockslide
Blocks
of bedrock slide down a slope
Generally very fast and destructive
 Debris
flow (mudflow)
Consists
of soil and regolith with a large amount of water
Often
confined to channels
Slumps Due to Earthquake
Rockslide
 Debris
flow
Serious
hazard in dry areas with heavy rains
Debris flows composed mostly of volcanic materials on the
flanks of volcanoes are called lahars
A lahar from the 1980 Mt. St. Helens eruption
 Earthflow
Form
on hillsides in humid regions
Water saturates the soil
Commonly involve materials rich in clay and silt
An earthflow on a newly formed slope
 Slow
movements
Creep
 Gradual
movement of soil and regolith downhill
by the alternate expansion and contraction of the surface
material
 Aided
Creep
Some visible effects of creep
Evidence of Creep
 Slow
movements
Solifluction
 Promoted
by a dense clay hardpan or impermeable bedrock layer
 Common in regions underlain by permafrost
 Can occur on gentle slopes
Permafrost regions in the Northern Hemisphere
Ground subsidence in Alaska due to permafrost
Key Terms Chapter 7
Weathering
Regolith
Soil
Mechanical and chemical weathering
Joint
Dissolution
Clay
Sand
Humus
Soil horizon (O, A, E, B, C), soil profile
Erosion
Bed load
Suspended load
Saltation
Glacier
Mass wasting
Slope failure
Flow
Creep