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10 Weathering
Earth’s Dynamic Systems
Eric H Christiansen
Brigham Young University
The Nature of Weathering
 Earth's atmosphere and hydrosphere
tend to weather rocks at surface:
 (1) by mechanical disintegration, in
which rocks are broken by physical
forces,
 (2) by chemical
decomposition
Why do rocks weather (break down)?
 Most rocks and minerals are formed at
high temperatures and pressures
 They are in a state of equilibrium at the T and
P of formation
 At the Earth’s surface, rocks and minerals are
mechanically and chemically unstable
 Secondary minerals and structures form that
are in equilibrium at the T, P, and fluid
composition common at the Earth’s surface
Weathering
 Chemical weathering
 The decomposition of rocks and minerals as chemical
reactions alter them into new minerals stable at the
Earth’s surface
 Physical weathering
 The disintegration or disaggregation of rocks by
physically breaking them apart
Physical Weathering
 Physical breakage of rocks into smaller
pieces
 Ice Wedging
 Sheeting
 Biologic
 Thermal fracturing
 No change in chemical composition
Ice Wedging
 Freeze - Thaw cycles are
effective at breaking apart
rocks
 Water expands when it
freezes

Volume increases by 9%
 The stress of expansion
breaks the rock
 Ice melts and the water
percolates deeper into the
newly expanded cracks
 Angular blocks accumulate
at base of cliff
Ice Wedging
This occurs where daily freeze thaw takes place,
at least seasonally—not where permanently frozen.
Ice Wedging
Talus Cones
Mount Timpanogos
Ice
Wedging
Talus Cones
Sheeting
 Release of confining pressure on rocks
formed deep within the Earth
 Development of fractures and joints
caused by expansion
 Rocks break along fractures and joints
Fig. 10.4. Sheeting in granite
Sheeting
Half Dome, Yosemite Park
Sheeting (Geologic Exfoliation)
Other Forms of
Physical Weathering
 Heat
 Heat causes rocks (most
solids) to expand
 Rocks are poor conductors
of heat
 Outer layer of rock that
expands breaks off (spall)
 Crystal Growth
 Minerals precipitate along
fractures
 Similar to ice wedging
Other Forms of Physical Weathering
 Root Growth
 Roots may exert enormous forces in growing
 Root tips pressures may exceed 10,000 kg per square
meter
 Seeds gather in cracks in rock and germinate
 Growing plant and roots slowly wedge rock apart
Tree Roots, etc.
Chemical Weathering
 Minerals are destroyed or altered by
chemical reactions
 Dissolution
 Acid Hydrolysis
 Oxidation
Dissolution
 Some minerals are soluble in water
 e.g., Halite - NaCl
 Minerals dissolve into constituent ions
 Ions removed with water by leaching
 Solubility of compound controls
leachability
Hydrolysis
 Completely dissolves
Dissolution
 H+ attacks minerals by replacing other ions in the
mineral structure
 Promotes dissolution
 Calcite hydrolysis by carbonic acid solution
CaCO3 + H2CO3 = Ca+2 + 2HCO3-
Dissolution and Secondary Minerals
 CO2 mixes with water to produce
carbonic acid, H2CO3
 Decaying organic matter produces acid
 Anthropogenic sources of acid
 Acid rain
 Dissolves and precipitates new minerals
Acid Hydrolysis
Hydrogen ions replace other ions in a
mineral.
Feldspar is the most abundant mineral in the crust. What is the
most abundant sedimentary rock type?
Acid Hydrolysis
Plagioclase feldspar
Acid Hydrolysis
 New “secondary” minerals may be created by this
process
 H+ ion replaces the K+ ion in the feldspar structure
 K+ ion goes into the water solution
 Kaolinite, a clay mineral, formed
2KAlSi3O8 + 2H2CO3 + 9H2O =
2K+ + 2HCO3- + 4H4SiO4 + Al2Si2O5(OH)4
Solution and Secondary Minerals
Oxidation
 Valence state increases
 Often associated with free O2
in the environment
 Iron is usually found as the
Fe+2 ion in silicate minerals
 Exposed to the atmosphere it
will oxidize to the Fe+3 ion
 Change in valence state
disrupts crystal structure
 Oxidation works in
combination with hydrolysis
and dissolution
Oxidation of olivine
2FeSiO4 + 4H2O + O2 = 2 Fe2O3 + 2H4SiO4
Trends in Chemical Weathering
 Alkali and alkaline earth elements removed
into solution (Where?)
 Al, Si, and Fe are enriched in secondary
minerals
 For example, Fe is stored in stable (insoluble)
ferric oxides (rust)
 Secondary minerals have water and oxidized
iron
 Increases in T and moisture increase
weathering rates
Resistance to Weathering: Relative stability of minerals
Mechanical and Chemical
Weathering Linked
 Principle: Mechanical weathering enhances chemical
weathering by producing more surface area. Chemical
weathering proceeds at the surface of a mineral.
Differential Weathering
 Caused by variations in weathering rate
 Occurs over a broad range of scales
 Unusual weathering features

Arches, spindles
Differential Weathering
Differential Weathering
Differential Weathering
Differential Weathering
Products of Weathering
 Dissolved ions in solution
 Broken up rock material

Secondary minerals

Spheroidal rock forms

Mechanical weathering--Fractures and joints
 Regolith: A blanket of broken up rock, clay, and soil that
covers the bedrock.

Soil—all soil is regolith, but not all regolith is soil!
The Result of Weathering?
Geometry of
Weathered Rocks
 Fractures in rock form
 Generally form in
groups
 Parallel joints
 Intersecting joints
 Cut large blocks
into smaller blocks
 Then weathering eats
away at margins
making spheroids
Products of Weathering
Bedrock
Regolith
Soil: A special type of regolith
 Principle: Soils are more weathered at the top.
Dissolved matter from the top of a soil column travels
downward, and is deposited in the lower portions.
Fig. 10.14
Soil
Horizons
Common
soil profile
 A Horizon
A dark gray to black horizon, rich in organic matter.
 Leaching carries dissolved ions and fine particles

 B Horizon
Accumulates leached material from above
 Enriched in clay minerals and oxides

 C Horizon

Parent rock in various stages of weathering
Climate & Weathering
 Climatic conditions
strongly influence
weathering reactions
 Amount of rainfall

Most reactions need
water
 Average temperature

Increase of 10oC
doubles reaction rate
10 Weathering
 Physical weathering is the mechanical
fragmentation of rocks from stress acting
on them. Ice wedging may be the most
important type.
 Chemical weathering involves chemical
reactions with minerals that
progressively decompose the solid rock.
The major types of chemical weathering
are dissolution, hydration, and
oxidation.
 Joints and fractures facilitate weathering
because they permit water and gases in
the atmosphere to attack a rock body at
considerable depth. They also greatly
increase the surface area on which
chemical reactions can occur.
 The major products of weathering are
spheroidal rock forms, a blanket of
regolith, and dissolved ions.
 Soil is the upper part of the regolith--a
mixture of clay minerals, weathered rock
particles, and organic matter.
 Climate greatly influences the type and
rate of weathering. The major
controlling climatic factors are
precipitation and temperature.
10 Weathering:
Expected Learning Outcomes
 Recognize bedrock and regolith
from observations in the field or
from photographs of the landscape.
 Explain the mechanics of ice
wedging and the factors that control
its functioning.
 Show by simple diagrams how joints
influence weathering and why there
is a universal tendency toward
spheroidal weathering.
 Explain how climate controls types
of weathering and resulting
weathered products.
 Explain hydrolysis, dissolution, and
oxidation of rock materials, and list
the products of these reactions.
 List the major products of
weathering