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Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 6
Weathering and Erosion
Weathering and Erosion
Tim Davis/Photo Researchers
Weathering
• Physical and chemical
changes that occur in
sediments and rocks when
they are exposed to the
atmosphere and biosphere.
• Not the same as erosion.
Factors Controlling Rates of Weathering
Chemical weathering
• This process occurs because
minerals formed deep in the
earth’s interior are not stable
under surface conditions.
• Stability is generally the reverse of
Bowen’s reaction series.
• The principle agent of chemical
weathering is water.
Chemical weathering of silicates
• Quartz: very stable
• Feldspars: form clay minerals
• Mafic minerals: decompose to
oxides
Chemical weathering of carbonates
• Easily soluble in water
(especially with some acid
present)
• Ca and Mg taken into solution
Weathering rates of gravestones
Marble
Slate
R. Siever
Fig. 6.1
Etched
and
corroded
feldspar
in the
soil zone
Berner & Holden, 1977
Fig. 6.2
Fig. 6.3a
Fig. 6.3b
Fig. 6.3c
Analogy of weathering: making coffee
fresh grounds + water = coffee + residue
(a solution)
K-feldspar + water = K+ + kaolinite
(a clay mineral)
Weathering and Making coffee
Fig. 6.4
Weathering by solution
•The complete breakup of minerals into
ions in solution
•NaCl (halite) is the best example, but is
geologically unimportant
•calcite (limestone) = CaCO3
•CaCO3 + H2CO3 = Ca2+ + 2HCO3•Mafic silicates dissolve much more
slowly
Mechanical Weathering Changes
the Surface to Volume Ratio
Fig. 6.5
Fig. 6.6
Fig. 6.6
CO2 and the Atmosphere,
Weathering, and the Climate
Fig. 6.7
Fig. 6.8
Weathering Oxides Provide Color
to the Desert Landscape
Betty Crowell
Fig. 6.9
Weathered
Limestone
Ric Ergenbright
Fig. 6.10
Joint-controlled Weathering
Jeff Foott/DRK
Fig. 6.11
Mechanical weathering
Frost — water expands by 9% when it
freezes
Thermal expansion — differential
thermal expansion of minerals creates
stress in rocks
Organic activity — tree roots to microorganisms
Mechanical abrasion — things go bump
Role of Organisms in Weathering
Peter Kresam
Fig. 6.12
Gneiss Boulder Fractured by Frost
Action
Michael Hambrey
Fig. 6.13
Exfoliation Dome in Yosemite
Tony Waltham
Fig. 6.14
Spheroidal
Weathering
Michael Follo
Fig. 6.15
Weathering terms
Bedrock:
Regolith:
Soil:
unaltered rock of any kind
a layer of broken pieces of
rock and slightly altered
rock that overlies the
bedrock
a layer of altered mineral
material usually mixed
with organic material
Weathering, Soil Formation, and Erosion
Fig. 6.16
Weathering, Soil Formation, and Erosion
Fig. 6.16
Change in U.S. soil erosion rates
Soil Profile
Fig. 6.17
Laterite
Fig. 6.18a
Pedalfer
Fig. 6.18b
Pedocal
Fig. 6.18c
Sand
Rex Elliot
Fig. 6.19