Weathering and
Soils
Weathering
and Soils
Marble
Graniteheadstones
headstone in
Southern Vermont
Weathering and Soils


Mechanical Weathering- the physical
breakdown of rocks
Chemical Weathering- reaction of rocks
with water, acids, oxygen, etc.
Mechanical
Weathering
Mechanical Weathering
Tectonic
Jointing
Mechanical Weathering
Exfoliation
Mechanical Weathering
Exfoliation
Mechanical Weathering
Exfoliation
Chemical
Weathering
Solution:
H2O + CO2 =
H2CO3 ( H+ + HCO3- )
in air (acid rain) acid
Limestone dissolves readily
gas
CaCO3 + 2 H+ (aq) =
Ca+2 + CO2 + H2O
Chemical Weathering
Solution: Karst Topography
Chemical Weathering
Solution: Karst Topography
Chemical Weathering
Karst Sinkhole- Florida
Chemical Weathering
Karst Sinkhole- Florida
Chemical Weathering
Tower Karst - China
Chemical Weathering
Oxidation:
Fe2SiO4 + ½ O2 = Fe2O3 + SiO2
Chemical Weathering
Chemical Weathering
Differential Weathering
Chemical Weathering
Differential Weathering
Weathering can preferentially
dissolve out mafics & increase
the sialic component in
sediments to yield “granite”
Chemical Weathering
What happens when granite is weathered?
The biotite and hornblende will undergo hydrolysis to form
clay, and oxidation to form iron oxides.
The feldspars will undergo
hydrolysis to form kaolinite
(clay) and release Na+ and
K+ ions into solution
The quartz (and muscovite, if
present) will remain as residual
minerals because they are very
resistant to weathering.
Zone of Leaching
Idealized soil profile from a humid climate at middle latitudes
Horizon Development and
Climate
Soil textural classes
Topsoil ecosystem
in a temperate
climate
Effect of climate on soil development
Chemical Weathering
Transported Soils
Wind-blown Glacial
Loess Deposits
Wind-blown Glacial
Loess Deposits
The Principal Soil Types
Effect of climate on soil types
Temperature
hot
laterites hot
pedocals
pedalfers
tundra soils
cold
undra soils low
50 cm/yr
Precipitation
cool
high
Principal soil types in the USA
Effect of climate on soil types
A typical pedocal with
white caliche (CaCO3)
deposits in a shallow B
horizon
Effect of climate on soils
Making
laterite
bricks
Tundra
soil
Laterite
Temple at
Angkor
Wat
Cambodia
(700 yrs
old)
A Newer
Classification
of Soil Types
Pf
Pc
Pf
Pf, Pc
L
Pf, L
Rates of soil formation:
Central USA:
~ 1 cm/100 years in very porous
and permeable rock
Humid N. Carolina: ~ 1 cm/5 years
“
Granite may require over 1000 years for a cm
Both Ends of the Erosion Problem
Serious soil erosion and gully
formation related to diversion of
water runoff over a few months
in central California
Sediment pollution following
heavy rains in North Carolina
Soil Degradation







Erosion
Deforestation
Agricultural depletion
Contamination - Pollution
 Salinization
 Pesticides
 Chemical additions and spills
Urbanization
Desertification
Overgrazing
Soil Erosion
Raindrop impact: millions
in a single shower hit at ~
30 mph
Soil Erosion
Rill-wash and gullying in loose
and poorly-consolidated soils
Soil Erosion
Woods
Grass
Corn
Fallow
Soil Erosion for Various Uses
Soil Lost (tons/km2/yr)
% runoff
~0
0.12
10
6.5
18087
42
17050
49
Soil Erosion
Number of dust storms during the month of March, 1936
Effects of logging in Madagasgar
Soil Erosion
Figure 3.12: (a) a natural forested slope
(b) the same slope following a clearcut
Soil Erosion
Figure 3.12: (c) the same slope after conversion to farmland
(b) the same slope following urbanization
Soil Erosion
1968 motorcycles
first come to Deer
Creek near
Bakersfield, CA
1972 after only 4
years of ORV use
little vegetation or
topsoil remains
Soil Erosion
ORV users increased
from 21 to 38 million
between 1982 and 1992
ATVs rip through a marsh in
West Virginia
Great Grizzly Hike, Montana
Soil Erosion
1985 photo of bulldozer and jeep tracks in
Alaskan tundra made in 1946
Soil Erosion
Salt accumulation due to irrigation in
an arid climate: San Fernando Valley
Worldwide Distribution of Salt-affected Soils
Chemical Contamination of Soils
Grain production
levels off while
population
continues to grow
Combating Soil Degradation
Strip-cropping
and
Contour plowing
Combating Soil Degradation
Rice terraces in
Asia
Combating Soil Degradation
No-till
Farming