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Weathering and Soils
Chapter 7
Weathering Vocabulary
• Exfoliation: outer layers of rock are
stripped away like an onion
• Hydrolysis: reaction of water with
substance
• Frost wedging: repeated thawing and
freezing of water in the cracks of rocks
• Oxidation: reaction of oxygen with a
substance
Earth’s surface processes
First a definition:
• Weathering – Physical breakdown and
chemical alteration of rock at Earth’s
surface
Types of weathering
• Chemical: breakdown of material using
changes in chemical composition
• Physical: breakdown of material without
chemical change.
Physical or Mechanical
Weathering
• Physical weathering –Physical weathering is
the disintegration of earth material without
undergoing a chemical change.
Frost wedging – freezing and thawing of water
in cracks disintegrates rocks
Exfoliation– exfoliation of igneous rocks at
Earth’s surface due to erosion of
“overburden” and reduction in pressure
Biological activity – disintegration resulting
from plants and animals
- root wedging, animal burrows
Frost
Wedging
Evidence of
Frost
Wedging in
Wheeler
Park,
Nevada
Source: Tom Bean/DRK Photo
Rockfall caused by frost wedging
Salt Wedging
Mechanical Exfoliation in
Yosemite National Park
Source: Phil Degginger/Earth Scenes
Mechanical
Exfoliation
of granite
Tree roots
growing in rock
fractures,
plus animal
burrows, expose
deep rocks to
water
Source: Runk/Schoenberger/Grant Heilman
Physical weathering increases
the rate of chemical weathering
by increasing surface area.
Joint-controlled weathering
in igneous rocks
Joints in sandstone - Canyonlands Natl Park, Utah
Chemical Weathering
• Breaks down rock and minerals
• Important agent in chemical
weathering is water
• Water dissolves and transports ions
and molecules
• The dissolved ions later bond & form
the cements in sedimentary rocks
Chemical Weathering
• Major processes of chemical weathering
• Dissolution
– Soluble ions contained in underground water
• Oxidation
– Chemical reaction where compound loses electrons
– Important in breaking down mafic minerals
– Rust colored weathering of Basalt
• Hydrolysis H+ or OH- ions
– Reaction of any substance with water
– Water’s ions replace different ions in mineral
– Feldspars, most abundant crust minerals,
become fine clay particles.
Easily transported.
Dissolution
Weathered and
Unweathered
Limestone
Boulders
Source: Ramesh Venkatakrishnan
Angular Boulder
Decomposes and
Rounds
Source: Paul McKelvey/Tony Stone Images
Hydrolysis
Cleopatra’s
Needle,
(Egypt)
Granite in a
Dry Climate
Source: New York Public Library, Locan
History and Genealogy Division
Hydrolysis
Cleopatra’s
Needle,
(Central
Park, NYC)
Granite in a
Wet Climate
Source: Runk/Schoenberger/Grant Heilman
Oxidation of Basalt
Rust (Iron Oxide) forms
Weathering
• Rates of weathering
• Mechanical weathering aids chemical
weathering by increasing surface area
• Others factors affecting weathering
• Rock characteristics e.g. minerals
– Marble and limestone easily dissolve in weak
acidic solutions -Dissolution
Soils
Our food grows in soils. Also very important in recognizing past climates
Soil
• Soil - combination of mineral and organic
matter, water, and air
• It is that portion of the regolith (weathered
rock and mineral) that supports the growth
of plants
Components in soil
that support plant growth
Soil
• Factors controlling soil formation
• Parent material
–parent material is the underlying
bedrock - composition affects soil
types
Soil
• Factors controlling soil formation
• Time
– Soils get better developed (Thicker, with
greater differences between layers)
with more time
• Climate
– Biggest control on soil formation
–Key factors are temperature and
precipitation
Soil
• Factors controlling soil formation
• Plants and animals
– Organisms influence soil properties
– Also furnish organic matter to the soil
(especially plants)
• Slope
– Steep slopes have poorly developed soils
(due to faster erosion and downslope
transport
– Flatter terrain accumulates soil faster
Variations in soil
development
due to topography
Note location of agriculture
Soil Profile
• The soil profile
• Soil forming processes operate from
the surface downward
• Vertical differences are called
horizons – zones or layers of soil
Soil Horizons and Profiles
Soil Horizons
• Layers in Soil
• Not Deposited, but Zones of
Chemical Action
Soil Profile
• Suite of Horizons at a Given
Locality
An idealized
soil profile
with horizons
Soil
• The soil profile
• O horizon – organic matter
• A horizon – organic and mineral matter
– High biological activity (animals live here)
– Together the O and A horizons make up topsoil
• B horizon – zone of accumulation
• C horizon – partly altered parent material
Mnemonic: Only Active Educators Become Champions
Soil Profiles
A mature soil profile with well
developed A, B, and C
horizons takes hundreds to
thousands of years to develop.
Horizon “A”
Horizon “B”
Horizon “C”
Image: NRCS
Soils-3-6
Remember the different horizons
O
A
Organic
Animal Activity
Soluble Minerals
E
B
C
Exited
Soluble Minerals
Back
Crushed Rock
Animal Activities in “A” horizon
Source: Runk/Schoenberger/Grant Heilman
• Soil types
• The characteristics of each soil type primarily depend on the prevailing
climatic conditions
• Three very generic soil types
• Pedalfer Moist All Year
• Accumulation of iron oxides and Al-rich clays in the B
horizon. Brown B horizon
– Best developed under forest landscapes
• Pedocal - High Evaporation
– White calcium carbonate (caliche) in B horizon
– Associated with dry grasslands and dry brush vegetation
• Laterite e.g. Monsoonal Climate
–
–
–
–
Alternating Hot dry and Cool Wet tropical climates
Intense chemical weathering
Red Iron oxide – horizons not distinct
Very thick soils but thin useful topsoil-Trees are Buttressed
pedalfer
Evergreen forests
pedocal
Shortgrass
laterite
tropics
Pedocal with Caliche in the B horizon
O
A
E
Organic
Activity
Leached
B
Accumulation
C
Crushed Rock
Caliche
Source: Teaching Collection/University of Washington, Department of Geological Science
Laterite in Sarawak, Borneo
Source: Fletcher & Baylis/Photo Researchers, Inc.
Earth’s surface processes
• Erosion – the physical removal of
material by mobile agents like water,
wind, ice, or gravity
Soil Erosion
• Soil erosion
• Recycling of Earth materials
• Natural rates of soil erosion depend on
–Soil characteristics
–Climate
–Slope
–Type of vegetation
Erosion
Headed for the Sea
Source: Ramesh Venkatakrishnan
Soil
• Soil erosion
• In many regions the rate of
soil erosion is significantly
greater than the rate of soil
formation
• Farmers now level fields
with lasers to slow loss of
topsoil
• Terraces
Limits of Soil Development
Balance Between:
• Downward Lowering of Ground Surface
• Downward Migration of Soil Horizons
If erosion rapid or soil evolution slow, soils
may never mature beyond a certain point.
Extremely ancient soils may have lost
everything movable
Soil Developed on a Lava Flow
End of Chapter 7
Source: Stanley Chernicoff/Patrick Spencer