Download Soil Erosion

Chapter 6: Weathering and Soils
風化與土壤
Key Questions

How does rock change as it weathers
physically?

How does rock change as it weathers
chemically?

What factors influence the intensity of
weathering?
bauxite
caliche
carbonic acid
dehydrate
dissolution
exfoliation
frost wedging
goethite
hematite
hydration
hydrolysis
joints
鋁土礦
鈣質層
碳酸
脫水
溶解
頁狀剝落作用
冰劈(凍裂)作用
針鐵礦
赤鐵礦
水合作用
水解作用
節理
kaolinite
高嶺土
laterite
紅土
leaching
淋溶作用
paleosol
古土壤
regolith
風化表土
secondary enrichment
次生富集
soil horizon
土壤層
soil profile
土壤剖面
spall
剝落
spheroidal weathering
球狀風化
weathering rind
風化皮
Weathering
Mechanical breakdown and chemical alteration
of rocks or sediments when exposed to
air, moisture and organic matter.

Integration of physical and chemical processes.

A part of the rock cycle

A major process to the formation of soil.
Physical Weathering
Mechanical breakdown as a result of
following processes
 Plate tectonic movement.
 Loading and unloading of glaciers.
 Erosion.
 Heating and cooling
 Wedging by ice or plants.
 Crystal growth.
 Activity of organisms.
Development of Joints

Joints occur as a widespread set or sets of parallel
fractures.

Rocks break at weak spots when they are twisted,
squeezed, or stretched by tectonic forces.

Removal of the weight of overlying rocks releases
stress on the buried rock and causes joints to open
slightly, thereby allowing water, air, and microscopic life
to enter.

When dikes, sills, lava flows, and welded tuffs cool they
contract and form columnar joints (joints that split
igneous rocks into long prisms or columns).
Crystal Growth

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Water moving slowly through fractured rocks
contains ions, which may precipitate out of
solution to form salts.
The force exerted by salt crystals growing
can be very large and can result in the
rupture or disaggregation of rocks.
The effects can often be seen in deserts.
Frost Wedging

Wherever temperatures fluctuate about the freezing
point, pore water periodically freezes and thaws.

As water freezes to form ice, its volume increases
by about 9 percent, forcing rocks apart.

Frost wedging probably the most effective at
temperatures of -5o to -15oC.

Frost wedging is responsible for most of the rock
debris seen on high mountain
Daily Heating and Cooling
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Surface temperatures as high as 80oC have been
measured on exposed desert rocks.
Daily temperature variations of more than 40o
have been recorded on rock surfaces,
Despite a number of careful tests, no one has yet
demonstrated that daily heating and cooling
cycles have noticeable physical effects on rocks.
Chemical Weathering

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Chemical reactions transform rocks and
minerals into new chemical combinations.
Four different chemical pathways :
–
–
–
–
Dissolution.
Hydrolysis.
Leaching.
Oxidation.
Dissolution

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Chemicals in rocks are dissolved in water.
Halite (NaCI) is a mineral that can be
removed completely from a rock by
dissolution.
Calcite (CaCO3), if carbonic acid is present,
dissolves rapidly in rainwater
Hydrolysis

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Any reaction involving water that leads to the
decomposition of a compound is a hydrolysis
reaction.
Hydrogen ions produced by the ionization of
carbonic acid most commonly cause hydrolysis. For
instance, hydrogen ions decompose potassium
feldspar and create kaolinite.
Hydrolysis is one of the chief processes involved in
the chemical breakdown of common rocks.
Leaching


Leaching is the continued removal, by water
solution, of soluble matter from bedrock or
regolith.
Soluble substances leached from rocks
during weathering are present in all surface
and ground water. Sometimes their
concentrations are high enough to give the
water a distinctive taste.
Oxidation of Iron

Oxidation involves the removal of electrons from an
atom, increasing the oxidation number of an element.

An iron atom that has given up two electrons forms a
ferrous ion (Fe2+).

In the presence of oxygen, a ferrous ion is oxidized, by
giving up a third electron, to a ferric ion (Fe3+).

The incorporation of water in a mineral structure is
called hydration.

The hydrolysis, oxidation and hydration of ferrous iron
compounds will form ferric hydroxide (Fe(OH)3).
Oxidation of Iron (2)

The ferric hydroxide may dehydrate, meaning it will
lose some water, to form goethite (FeO.OH).

Goethite may dehydrate still further to form hematite
(Fe2O3).

The colors of ferric hydroxide, goethite, and hematite,
ranging from yellowish through brownish red to brick
red, can provide clues to the degree or intensity of
weathering.
Combined Reactions
Chemical weathering often involves more
than one reaction pathway.
 Dissolution plays a part in virtually all
chemical weathering processes.
 The effects of dissolution, hydrolysis, and
leaching of carbonate rocks are widely
seen in the landscapes underlain by
carbonate rocks.

Effects of Chemical Weathering on
Common Minerals and Rocks



Granite or basalt decomposes by the
combined effects of dissolution, hydrolysis,
and oxidation to form clay minerals, goethite
and soluble ions.
Limestone is attacked by dissolution and
hydrolysis, leaving behind only the nearly
insoluble clay minerals and quartz.
Gold, platinum, and diamond persist during
weathering.
Effects of Chemical Weathering on Basalt and Granite
Basalt
Granite
Surface Area



The effectiveness of chemical weathering
increases as the surface area exposed to
weathering increases.
Surface area increases simply from the
subdivision of large blocks into smaller
blocks.
Chemical weathering therefore leads to a
dramatic increase in the surface area.
Factors Influencing Weathering (1)

Mineralogy
– The resistance of a silicate mineral to
weathering is a function of three principal
things:
 The
chemical composition of the mineral.
 The extend to which the silicate tetrahedra in
the mineral are polymerized.
 The acidity of the waters with which the
mineral reacts.
Factors Influencing Weathering (2)

Ferric oxides and hydroxides. - Most stable
Aluminum oxides and hydroxides.
Quartz.
Clay minerals.
Muscovite.
Potassium feldspar.
Biotite.
Sodium feldspar (albite-rich plagioclase).
Amphibole. Pyroxene.
Calcium feldspar (anorthite-rich plagioclase).
Olivine.

Calcite.
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- Least stable
Factors Influencing Weathering (3)

Rock type and structure.
– Differential weathering: differences in the
composition and structure of adjacent rock
units can lead to contrasting rates of
weathering.
Examples: hillcrest, cape
Factors Influencing Weathering (4)

Slope angle.
–
–
On a steep slope, solid products of weathering
move quickly away, continually exposing fresh
bedrock to renewed attack.
On gentle slopes, weathering products are not
easily washed away and in places may
accumulate to depths of 50 m or more.
Factors Influencing Weathering (5)

Climate
–
Moisture and heat promote chemical reactions.


Weathering is more intense and generally extends to
greater depths in a warm, moist climate than in a cold,
dry one.
In moist tropical lands, like Central America and
Southeast Asia, obvious effects of chemical weathering
can be seen at depths of 100 m or more.
Factors Influencing Weathering (6)
–
–
Rocks such as limestone and marble are highly
susceptible to chemical weathering in a moist
climate and commonly form low, gentle
landscapes.
In a dry climate, however, the same rocks form
bold cliffs because, with scant rainfall and only
patchy vegetation, little carbonic acid is present to
dissolve carbonate minerals.
Factors Influencing Weathering (7)

Burrowing animals.
–
Large and small burrowing animals bring partly
decayed rock particles to the land surface.

Although burrowing animals do not break down rock
directly, the amount of disaggregated rock they move
over many millions of years must be enormous.
Factors Influencing Weathering (8)

Time.
–
–
–
Hundred to thousands of years are required for a
hard igneous or plutonic rock to decompose.
Weathering processes are speeded up by
increasing temperature and available water, and
by decreasing particle size.
The rate of weathering tends to decrease with
time as the weathering profile, or a weathering
rind, thickens.
Key Questions

How are weathering and soil formation
related?

How have human activities accelerated
the rate of soil erosion?

What evidence can geologists use to
infer a relationship between weathering
and plate tectonics?
Soil: Origin And Classification

Soils support the plants that are the basic
source of our nourishment.

Soils store organic matter, thereby influencing
how much carbon is cycled in the atmosphere
as carbon dioxide.

Soils are derived from (1) physical and
chemical weathering processes, (2) decay
of dead plants and animals.
Soil Profiles
–
As a soil develops from the surface downward, a succession
of horizontal weathered zones, called soil horizons, forms.
–
The uppermost horizon may be a surface accumulation of
organic matter (O horizon).
–
The A horizon is the mixture of dark humus (decomposed
residue of plant and animal tissues) with mineral matter.
–
The B horizon is enriched in clay and/or brownish or reddish
iron and aluminum hydroxides.
–
The C horizon is the deepest horizon and consists of rock in
various stages of weathering.
Soil Types
Different soils result from the influence of six formative
factors:
 Climate.
 Vegetation cover.
 Soil organisms.
 Composition of the parent material.
 Topography.
 Time.
Engineering Properties of Soils
Plasticity
Soil strength
Friction
Sensitivity
Compressibility
Erodibility
Permeability
Ease of excavation
Shrink-swell
potential
Mineral Deposits Formed by Weathering
Limonite and Hematite: Iron-rich Laterite

Formed by leaching of other minerals from ironrich sedimentary rocks under warm and rainy
tropical climate.

Limonite (FeO.OH) and hematite (Fe2O3), the
least soluble minerals, form an iron-rich crust of
laterite during chemical weathering.

Secondarily enriched deposit of limonite and
hematite contains high concentration of iron.
Mineral Deposits Formed by
Weathering Bauxite: aluminous laterite

Formed by leaching of clay minerals.

Silica is removed in solution and a residue of
Gibbsite (Al(OH)3), 三水鋁石, remains.

Gibbsitic bauxites are the major mineral
deposits of aluminum.
Rate of Soil Formation (1)

In southern Alaska retreating glaciers
leave unweathered parent material.
–
–
Despite the cold climate, within a few
years an A horizon develops on the newly
exposed and revegetated landscape.
As the plant cover becomes denser,
carbonic and organic acids acidify the soil
and leaching becomes more effective.
Rate of Soil Formation (2)
–
–
After about 50 years, a B horizon appears
and the combined thickness of the A and B
horizons reaches about 10 cm.
Over the next 150 years, a mature forest
develops on the landscape and the O
Horizon continues to thicken (but the A
and B horizons do not increase in
thickness).
Soil Erosion
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It may take a very long time to produce a
well-developed soil but destruction of soil
may occur rapidly.
Rates of erosion are determined by:
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–
–
–
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Topography.
Lithology
Climate.
Vegetation cover.
Human activity.
Soil Erosion Due to Human
Activity
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In many third world nations, population growth has forced
farmers onto lands of steep slopes or semiarid regions
that foster rapid soil erosion.
Planting of profitable row crops that often leave the land
vulnerable to increased rates of erosion.
Deforestation has accelerated rates of surface runoff and
destabilization of soils due to loss of anchoring roots.
When the O and A horizons are eroded away, the fertility
and the water-holding capacity of the soil decrease.
Soil Erosion Crisis
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Farmers in the United State are now losing about 5
tons of soil for every ton of grain they produce.
In India the soil erosion rate is estimated to be more
than twice as high.
The world’s most productive soils are being depleted
at the rate of 7% each decade.
Increased farming in the region above a dam in
Pakistan has reduced the dam life expectancy from
100 to 75 years.
Control of Soil Erosion
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Reduce areas of bare soil to a minimum.
Reduce overgrazing.
If row crops such as corn, tobacco, and
cotton must be planted on a slope,
alternating strips of grass or similar plants
resist soil erosion;
Crop rotation.
Erosion on Slopes
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On a gentle, 1 percent slope, an average of 3
tons of soils are lost per hectare each year.
A 5 percent slope loses 87 tons per hectare.
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At this rate a 15 cm thickness of topsoil would disappear
in about 20 years,
On a 15 percent slope, 221 tons per hectare
per year are lost.
Terracing can reduce the loss of soil on
farmed slopes.
Global Weathering Rates and High
Mountains (1)

These following three rivers deliver about 20
percent of the water and dissolved matter
entering the oceans:
–
The Yangtze River, which drains the Tibetan
Plateau of China.
–
The Amazon River, which drains the northern
Andes in South America.
–
The Ganges-Brahmaputra river system which
drains the Himalayas.
Global Weathering Rates and High
Mountains (2)

High mountains forces moisture-bearing winds to rise,
resulting in large amounts of precipitation, high rates of
runoff and erosion.

Evidence from ocean sediments points to an increase
of dissolved matter reaching the oceans in the past 5
million years.

Sediments shed from the rising Himalayas coarsen
from silts deposited about 5 million years ago to
gravels about 1 million years old.
–
Rivers draining the mountains gained increasing energy.
–
Mountain slopes or stream channels became steeper.
Homework: 解釋名詞
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Exfoliation
Laterite
A horizon
Weathering rind
Paleosol
*請於下週二上課時交給李在平助教或121室