Chapter 6,Weathering and Soils
Introduction,Weathering—
The Breakdown of Rock
? At the Earth’s surface,rocks are exposed
to the effects of weathering,the chemical
alteration and mechanical breakdown of
rock,when exposed to air,moisture,and
organic matter.
? Weathering is an integral part of the rock
cycle,
? Weathering converts rock to regolith.
Weathering profile
Physical Weathering
? Rocks break at weak spots when they are
twisted,squeezed,or stretched by tectonic
forces.
? Such forces form joints.
? Rocks adjust to removal of overlying rock by
expanding upward.
? 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.
Joints
? Joints occur as a widespread set or sets of
parallel fractures.
? 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).
Frost Wedging
? Wherever temperatures fluctuate about the
freezing point,water in the ground periodically
freezes and thaws.
? As water freezes to form ice,its volume
increases by about 9 percent.
? This process leads to a very effective type of
physical weathering known as frost wedging.
? Frost wedging probably the most effective at
temperatures of -5o to -15oC.
Frost Wedging
Daily Heating and Cooling
? 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.
Spalling and Wedging by Roots
? Fire can be very effective in disrupting
rocks.
? Because rock is a relatively poor conductor
of heat,only a thin outer shell expands and
breaks away as a spall.
? When plants grow they extend their roots
into the cracks in rock,where their
growth can force the rock apart.
Chemical Weathering Pathways
? In chemical weathering,chemical reactions
transform rocks and minerals into new
chemical combinations.
? There are four different chemical pathways by
which chemical weathering proceeds:
? Dissolution.
? Hydrolysis.
? Leaching.
? Oxidation.
Dissolution
? The easiest reaction pathway to
comprehend is dissolution; this means
that chemicals in rocks are dissolved in
water,
? Halite (NaCI) is a mineral that can be
removed completely from a rock by
dissolution.
Hydrolysis
? Any reaction involving water that leads
to the decomposition of a compound is a
hydrolysis reaction.
? Potassium feldspar,for instance,decomposes
in the clay mineral 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 an distinctive taste.
Oxidation
? Oxidation is a process by which an ion
loses an electron.
Common chemical weathering reactions
Chemical Weathering of Iron (1)
? A common example is the oxidation of iron.
? An iron atom that has given up two electrons
forms a ferrous ion (Fe2+).
? When a ferrous ion is oxidized further by
giving up a third electron,the result is a ferric
ion (Fe3+).
? The incorporation of water in a mineral
structure is called hydration.
? The ferric hydroxide will soon dehydrate,
meaning it will lose some water,in which case it
will form goethite (FeO.OH).
Chemical Weathering of Iron (2)
? Goethite may dehydrate still further to form
hematite (Fe2O3).
? The intensity of the colors of ferric hydroxide,
goethite,and hematite,ranging from yellowish
through brownish red to brick red,can provide
clues to how much time has elapsed since
weathering began,and the degree or intensity
of weathering,
Combined Reactions (1)
? Almost all instances of chemical
weathering involve more than one
reaction pathway.
? Dissolution plays a part in virtually all
chemical weathering processes and is
usually accompanied by hydrolysis and
leaching.
Combined Reactions (2)
? Calcite,if carbonic acid is present,
dissolves rapidly in rainwater.
? The effects of these processes are widely
seen in the distinctive landscapes—
including caves,caverns,and sink-
holes— underlain by carbonate rocks,
Effects of Chemical Weathering on
Common Minerals and Rocks (1)
? When a granite decomposes,it does so by
the combined effects of dissolution,
hydrolysis,and oxidation.
? Feldspar,mica,and ferromagnesian
minerals weather to clay minerals and
soluble Na1+,K1+,and Mg2+ ions.
? The quartz grains,being relatively inactive
chemically,remain essentially unaltered.
Effects of Chemical Weathering on
Common Minerals and Rocks (2)
? When basalt weathers,the plagioclase
feldspar and ferromagnesian minerals it
contains form clay minerals and soluble
ions (Na1+,Ca2+,and Mg2+),
? The iron from ferromagnesian minerals,
together with iron from magnetite,forms
goethite.
Effects of Chemical Weathering on
Common Minerals and Rocks (3)
? When limestone,the most common
sedimentary rock that contains calcium
carbonate,is attacked by dissolution and
hydrolysis,it is readily dissolved,leaving
behind only the nearly insoluble impurities
(chiefly clay and quartz) that are always
present in small amounts in the rock.
? Minerals such as gold,platinum,and diamond
persist during weathering,
Exfoliation and Spheroidal
Weathering
? During weathering,concentric shells of rock
may spall from the outside of an outcrop or a
boulder,a process known as exfoliation.
? Exfoliation is caused by differential stresses
within a rock that result mainly from chemical
weathering.
? Spheroidal weathering produces,by such
progressive decomposition,rounded boulders,
Spheroidal weathering
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.
Weathering of
Rock Cubes
Factors Influencing Weathering (1)
? The resistance of a silicate mineral to
weathering is a function of three principal
things:
1,The chemical composition of the mineral.
2,The extend to which the silicate tetrahedra in
the mineral are polymerized.
3,The acidity of the waters with which the
mineral reacts.
Factors Influencing Weathering (2)
? Most stable chemical compositions:
? Ferric oxides and hydroxides.
? Aluminum oxides and hydroxides.
? Quartz.
? Clay minerals.
? Muscovite.
? Potassium feldspar.
? Biotite.
? Sodium feldspar (albite-rich plagioclase).
? Amphibole.
Factors Influencing Weathering (3)
? Least stable chemical compositions:
? Pyroxene.
? Calcium feldspar (anorthite-rich plagioclase).
? Olivine.
? Calcite.
? Rock type and structure.
? Differences in the composition and structure of
adjacent rock units can lead to contrasting rates of
weathering and to landscapes that reflect such
differential weathering.
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.
? Therefore,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.
Climate and
Weathering
? 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 (6)
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.
Weathering Rates
? Soils are one of the most important natural
resources
? Soils support the plants that are the basic
source of our nourishment and provide food for
domesticated animals.
? Soils store organic matter,thereby influencing
how much carbon is cycled in the atmosphere
as carbon dioxide,and they also trap pollutants.
Soil,Origin And Classification
Origin of Soils
? Soils are produced by:
? The physical and chemical breakdown of
solid rock by weathering processes.
? The organic matter derived from the
decay of dead plants and animals.
Soil Profiles (1)
? As a soil develops from the surface downward,
an identifiable succession of approximately
horizontal weathered zones,called soil horizons,
forms.
? The uppermost horizon may be a surface
accumulation of organic matter (O horizon).
? An A horizon may either underlie an O horizon
or lie directly beneath the surface.
? The A horizon is dark because of the presence
of humus (decomposed residue of plant and animal
tissues,which is mixed with mineral matter).
Soil Profile
? The B horizon is enriched in clay and/or iron
and aluminum hydroxides produced by the
weathering of minerals within the horizon
? The C horizon is the deepest horizon and
consists of rock in various stages of weathering.
Soil Profiles (2)
Soil profile developed
under different climate
and vegetation conditions
Soil Types
? Different soils result from the influence of six
formative factors:
1,Climate.
2,Vegetation cover.
3,Soil organisms.
4,Composition of the parent material.
5,Topography.
6,Time.
Polar soils
? Polar soils generally are dry and lack well-
developed horizons.
? They are classified as entisols 新成土,
? In wetter high-latitude environments,mat-like
tundra vegetation overlies perennially frozen
ground,they form water-logged soils that are
rich in organic matter,called histosols 有机土,
? On well-drained sites soils develop recognizable
A and B horizons called inceptisols 始成土,
Temperate Latitude Soils
? Temperate-latitude soils:
? Alfisols淋溶土,characteristic of deciduous 落叶的
woodlands,have a clay-rich B horizon beneath.
? Acidic spodosols 灰土 develop in cool,moist
evergreen forests.
? Grasslands and prairies typically develop
mollisols软土 having thick,dark-colored,
organic-rich A horizons.
? Soils formed in moist subtropical climates,
commonly displaying a strongly weathered B
horizon,are called ultisols 老成土,
Desert Soils
? In dry climates,where lack of moisture reduces
leaching,carbonates accumulate in the profile
during the development of aridisols 旱成土,
? Over extensive arid regions of the southwestern
United States,carbonates have in this way built
up a solid,almost impervious layer of whitish
calcium carbonate known as caliche 生硝,
Distribution of Soil Types
Caliche in soil profile,semiarid climate
Tropical Soils
? In tropical climates soils are oxidized in oxisols
氧化土,
? Vertisols 转化土 contain a high proportion of clay.
? In tropical regions where the climate is very
wet and warm,the product of deep weathering
is called laterite 红土,
? The resulting stone-like material,called
lateritic crust or ironstone,can be used as
construction material.
Temple Wall of Laterite
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.
Evolution of a soil profile
Rate of Soil Formation (2)
? After about 50 years,a B horizon
appears and the combined thickness of
the A and B horizons reachesabout 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).
Paleosols
? Buried soils which have become part of the
stratigraphic record are called paleosols古土壤,
Paleosols
Australian Laterite,they are not forming today,
but tens of millions years ago
Soil Erosion
? 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,
? Topography.
? Climate.
? Vegetation cover.
? Human activity.
Soil Erosion Due to Human Activity (1)
? With world population greater than 6 billion,
global agricultural production increases
dramatically.
? In many third world nations,population
growth has forced farmers onto lands that
foster rapid soil erosion:
? Steep slopes.
? Semiarid regions where plowed land is prone to
severe wind erosion.
Soil Erosion Due to Human Activity (2)
? Economic pressures have led farmers to
shift from ecologically favorable land-use
practices to the planting of profitable row
crops that often leave the land vulnerable
to increased rates of erosion.
? Deforestation has led to accelerated rates
of surface runoff and destabilization of
soils due to loss of anchoring roots,
Soil Erosion Due to Human Activity (3)
? Soils in the humid tropics,when stripped
of their natural vegetation cover and
cultivated,quickly lose their fertility.
? When the O and A horizons are eroded
away,the fertility and the water-holding
capacity of the soil decrease.
Soil Erosion Due to Human Activity (4)
? Farmers in the United State are now losing
about 5 tons of soil for every ton of grain they
produce.
? Soil loss in Russia is at least as rapid.
? In India the soil erosion rate is estimated to be
more than twice as high.
? Increased farming in the region above a dam in
Pakistan has reduced the dam life expectancy
from 100 to 75 years.
? Increased soil erosion has produced more silt
that is filling the lake formed by the dam.
Control of Soil Erosion
? 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
? 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.
? 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)
? Although a multitude of rivers enter the sea,
the three largest contributors of dissolved
substances are:
1,The Yangtze River,which drains the high
Tibetan Plateau of China.
2,The Amazon River,which drains the northern
Andes in South America.
3,The Ganges-Brahmaputra river system which
drains the Himalayas.
Global Weathering Rates and High
Mountains (2)
? These three rivers deliver about 20 percent of
the water and dissolved matter entering the
oceans.
? The amount of dissolved matter in rivers is
greatest in areas of sedimentary rocks
(Himalayas,Andes,and Alps).
Global Weathering Rates and High
Mountains (3)
? Because high mountains forces moisture-
bearing winds to rise,they generally receive
large amounts of precipitation,resulting in:
? High rates of river runoff.
? High erosion rates.
? Evidence from ocean sediments points to an
increase in the amount of dissolved matter
reaching the oceans in the past 5 million years.
Global Weathering Rates and High
Mountains (4)
? 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 became steeper.
? Stream channels become steeper.
Introduction,Weathering—
The Breakdown of Rock
? At the Earth’s surface,rocks are exposed
to the effects of weathering,the chemical
alteration and mechanical breakdown of
rock,when exposed to air,moisture,and
organic matter.
? Weathering is an integral part of the rock
cycle,
? Weathering converts rock to regolith.
Weathering profile
Physical Weathering
? Rocks break at weak spots when they are
twisted,squeezed,or stretched by tectonic
forces.
? Such forces form joints.
? Rocks adjust to removal of overlying rock by
expanding upward.
? 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.
Joints
? Joints occur as a widespread set or sets of
parallel fractures.
? 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).
Frost Wedging
? Wherever temperatures fluctuate about the
freezing point,water in the ground periodically
freezes and thaws.
? As water freezes to form ice,its volume
increases by about 9 percent.
? This process leads to a very effective type of
physical weathering known as frost wedging.
? Frost wedging probably the most effective at
temperatures of -5o to -15oC.
Frost Wedging
Daily Heating and Cooling
? 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.
Spalling and Wedging by Roots
? Fire can be very effective in disrupting
rocks.
? Because rock is a relatively poor conductor
of heat,only a thin outer shell expands and
breaks away as a spall.
? When plants grow they extend their roots
into the cracks in rock,where their
growth can force the rock apart.
Chemical Weathering Pathways
? In chemical weathering,chemical reactions
transform rocks and minerals into new
chemical combinations.
? There are four different chemical pathways by
which chemical weathering proceeds:
? Dissolution.
? Hydrolysis.
? Leaching.
? Oxidation.
Dissolution
? The easiest reaction pathway to
comprehend is dissolution; this means
that chemicals in rocks are dissolved in
water,
? Halite (NaCI) is a mineral that can be
removed completely from a rock by
dissolution.
Hydrolysis
? Any reaction involving water that leads
to the decomposition of a compound is a
hydrolysis reaction.
? Potassium feldspar,for instance,decomposes
in the clay mineral 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 an distinctive taste.
Oxidation
? Oxidation is a process by which an ion
loses an electron.
Common chemical weathering reactions
Chemical Weathering of Iron (1)
? A common example is the oxidation of iron.
? An iron atom that has given up two electrons
forms a ferrous ion (Fe2+).
? When a ferrous ion is oxidized further by
giving up a third electron,the result is a ferric
ion (Fe3+).
? The incorporation of water in a mineral
structure is called hydration.
? The ferric hydroxide will soon dehydrate,
meaning it will lose some water,in which case it
will form goethite (FeO.OH).
Chemical Weathering of Iron (2)
? Goethite may dehydrate still further to form
hematite (Fe2O3).
? The intensity of the colors of ferric hydroxide,
goethite,and hematite,ranging from yellowish
through brownish red to brick red,can provide
clues to how much time has elapsed since
weathering began,and the degree or intensity
of weathering,
Combined Reactions (1)
? Almost all instances of chemical
weathering involve more than one
reaction pathway.
? Dissolution plays a part in virtually all
chemical weathering processes and is
usually accompanied by hydrolysis and
leaching.
Combined Reactions (2)
? Calcite,if carbonic acid is present,
dissolves rapidly in rainwater.
? The effects of these processes are widely
seen in the distinctive landscapes—
including caves,caverns,and sink-
holes— underlain by carbonate rocks,
Effects of Chemical Weathering on
Common Minerals and Rocks (1)
? When a granite decomposes,it does so by
the combined effects of dissolution,
hydrolysis,and oxidation.
? Feldspar,mica,and ferromagnesian
minerals weather to clay minerals and
soluble Na1+,K1+,and Mg2+ ions.
? The quartz grains,being relatively inactive
chemically,remain essentially unaltered.
Effects of Chemical Weathering on
Common Minerals and Rocks (2)
? When basalt weathers,the plagioclase
feldspar and ferromagnesian minerals it
contains form clay minerals and soluble
ions (Na1+,Ca2+,and Mg2+),
? The iron from ferromagnesian minerals,
together with iron from magnetite,forms
goethite.
Effects of Chemical Weathering on
Common Minerals and Rocks (3)
? When limestone,the most common
sedimentary rock that contains calcium
carbonate,is attacked by dissolution and
hydrolysis,it is readily dissolved,leaving
behind only the nearly insoluble impurities
(chiefly clay and quartz) that are always
present in small amounts in the rock.
? Minerals such as gold,platinum,and diamond
persist during weathering,
Exfoliation and Spheroidal
Weathering
? During weathering,concentric shells of rock
may spall from the outside of an outcrop or a
boulder,a process known as exfoliation.
? Exfoliation is caused by differential stresses
within a rock that result mainly from chemical
weathering.
? Spheroidal weathering produces,by such
progressive decomposition,rounded boulders,
Spheroidal weathering
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.
Weathering of
Rock Cubes
Factors Influencing Weathering (1)
? The resistance of a silicate mineral to
weathering is a function of three principal
things:
1,The chemical composition of the mineral.
2,The extend to which the silicate tetrahedra in
the mineral are polymerized.
3,The acidity of the waters with which the
mineral reacts.
Factors Influencing Weathering (2)
? Most stable chemical compositions:
? Ferric oxides and hydroxides.
? Aluminum oxides and hydroxides.
? Quartz.
? Clay minerals.
? Muscovite.
? Potassium feldspar.
? Biotite.
? Sodium feldspar (albite-rich plagioclase).
? Amphibole.
Factors Influencing Weathering (3)
? Least stable chemical compositions:
? Pyroxene.
? Calcium feldspar (anorthite-rich plagioclase).
? Olivine.
? Calcite.
? Rock type and structure.
? Differences in the composition and structure of
adjacent rock units can lead to contrasting rates of
weathering and to landscapes that reflect such
differential weathering.
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.
? Therefore,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.
Climate and
Weathering
? 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 (6)
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.
Weathering Rates
? Soils are one of the most important natural
resources
? Soils support the plants that are the basic
source of our nourishment and provide food for
domesticated animals.
? Soils store organic matter,thereby influencing
how much carbon is cycled in the atmosphere
as carbon dioxide,and they also trap pollutants.
Soil,Origin And Classification
Origin of Soils
? Soils are produced by:
? The physical and chemical breakdown of
solid rock by weathering processes.
? The organic matter derived from the
decay of dead plants and animals.
Soil Profiles (1)
? As a soil develops from the surface downward,
an identifiable succession of approximately
horizontal weathered zones,called soil horizons,
forms.
? The uppermost horizon may be a surface
accumulation of organic matter (O horizon).
? An A horizon may either underlie an O horizon
or lie directly beneath the surface.
? The A horizon is dark because of the presence
of humus (decomposed residue of plant and animal
tissues,which is mixed with mineral matter).
Soil Profile
? The B horizon is enriched in clay and/or iron
and aluminum hydroxides produced by the
weathering of minerals within the horizon
? The C horizon is the deepest horizon and
consists of rock in various stages of weathering.
Soil Profiles (2)
Soil profile developed
under different climate
and vegetation conditions
Soil Types
? Different soils result from the influence of six
formative factors:
1,Climate.
2,Vegetation cover.
3,Soil organisms.
4,Composition of the parent material.
5,Topography.
6,Time.
Polar soils
? Polar soils generally are dry and lack well-
developed horizons.
? They are classified as entisols 新成土,
? In wetter high-latitude environments,mat-like
tundra vegetation overlies perennially frozen
ground,they form water-logged soils that are
rich in organic matter,called histosols 有机土,
? On well-drained sites soils develop recognizable
A and B horizons called inceptisols 始成土,
Temperate Latitude Soils
? Temperate-latitude soils:
? Alfisols淋溶土,characteristic of deciduous 落叶的
woodlands,have a clay-rich B horizon beneath.
? Acidic spodosols 灰土 develop in cool,moist
evergreen forests.
? Grasslands and prairies typically develop
mollisols软土 having thick,dark-colored,
organic-rich A horizons.
? Soils formed in moist subtropical climates,
commonly displaying a strongly weathered B
horizon,are called ultisols 老成土,
Desert Soils
? In dry climates,where lack of moisture reduces
leaching,carbonates accumulate in the profile
during the development of aridisols 旱成土,
? Over extensive arid regions of the southwestern
United States,carbonates have in this way built
up a solid,almost impervious layer of whitish
calcium carbonate known as caliche 生硝,
Distribution of Soil Types
Caliche in soil profile,semiarid climate
Tropical Soils
? In tropical climates soils are oxidized in oxisols
氧化土,
? Vertisols 转化土 contain a high proportion of clay.
? In tropical regions where the climate is very
wet and warm,the product of deep weathering
is called laterite 红土,
? The resulting stone-like material,called
lateritic crust or ironstone,can be used as
construction material.
Temple Wall of Laterite
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.
Evolution of a soil profile
Rate of Soil Formation (2)
? After about 50 years,a B horizon
appears and the combined thickness of
the A and B horizons reachesabout 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).
Paleosols
? Buried soils which have become part of the
stratigraphic record are called paleosols古土壤,
Paleosols
Australian Laterite,they are not forming today,
but tens of millions years ago
Soil Erosion
? 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,
? Topography.
? Climate.
? Vegetation cover.
? Human activity.
Soil Erosion Due to Human Activity (1)
? With world population greater than 6 billion,
global agricultural production increases
dramatically.
? In many third world nations,population
growth has forced farmers onto lands that
foster rapid soil erosion:
? Steep slopes.
? Semiarid regions where plowed land is prone to
severe wind erosion.
Soil Erosion Due to Human Activity (2)
? Economic pressures have led farmers to
shift from ecologically favorable land-use
practices to the planting of profitable row
crops that often leave the land vulnerable
to increased rates of erosion.
? Deforestation has led to accelerated rates
of surface runoff and destabilization of
soils due to loss of anchoring roots,
Soil Erosion Due to Human Activity (3)
? Soils in the humid tropics,when stripped
of their natural vegetation cover and
cultivated,quickly lose their fertility.
? When the O and A horizons are eroded
away,the fertility and the water-holding
capacity of the soil decrease.
Soil Erosion Due to Human Activity (4)
? Farmers in the United State are now losing
about 5 tons of soil for every ton of grain they
produce.
? Soil loss in Russia is at least as rapid.
? In India the soil erosion rate is estimated to be
more than twice as high.
? Increased farming in the region above a dam in
Pakistan has reduced the dam life expectancy
from 100 to 75 years.
? Increased soil erosion has produced more silt
that is filling the lake formed by the dam.
Control of Soil Erosion
? 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
? 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.
? 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)
? Although a multitude of rivers enter the sea,
the three largest contributors of dissolved
substances are:
1,The Yangtze River,which drains the high
Tibetan Plateau of China.
2,The Amazon River,which drains the northern
Andes in South America.
3,The Ganges-Brahmaputra river system which
drains the Himalayas.
Global Weathering Rates and High
Mountains (2)
? These three rivers deliver about 20 percent of
the water and dissolved matter entering the
oceans.
? The amount of dissolved matter in rivers is
greatest in areas of sedimentary rocks
(Himalayas,Andes,and Alps).
Global Weathering Rates and High
Mountains (3)
? Because high mountains forces moisture-
bearing winds to rise,they generally receive
large amounts of precipitation,resulting in:
? High rates of river runoff.
? High erosion rates.
? Evidence from ocean sediments points to an
increase in the amount of dissolved matter
reaching the oceans in the past 5 million years.
Global Weathering Rates and High
Mountains (4)
? 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 became steeper.
? Stream channels become steeper.
