Transcript Wearing Away the Land

WEATHERING
CHAPTER 12
Sections 12.1 and 12.2
Weathering
Weathering is the change in the
physical form or chemical composition
of rock materials exposed at the earth’s
surface.
Weathering Outline
 I. Mechanical(Physical) weathering
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A. Ice Wedging
C. Organic Activity
B. Abrasion
D. Exfoliation
 II. Chemical weathering
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A. Hydrolysis
B. Carbonation
C. Oxidation
D. Plant acids
E. Acid precipitation
Weathering Outline
 III. Rate of weathering
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A.
B.
C.
D.
Rock composition
Amount of exposure
Climate
Topography
I. Mechanical weathering
 Mechanical weathering is the process of
breaking down a rock into smaller pieces
without changing the chemical composition
of the rock
A. Ice Wedging
 Ice wedging occurs
when water seeps into
cracks in the rocks and
freezes.
 When water freezes it
increases in volume
about 10%.
 The freezing process
widens the cracks in
the rocks.
ICE WEDGING
B. Abrasion
 Abrasion is the
collision of rocks with
one another resulting
in the breaking or
wearing away of
rocks.
 Agents of abrasion are
gravity, running water,
and wind.
C. Organic Activity
 Organic activity is
weathering caused by
plants or animals.
D. Exfoliation
 As a rock is pushed to
the surface curved
cracks, called joints,
occur parallel to the
surface.
 Exfoliation is the
peeling away of the
sheets of rock
Weathering and exfoliation
II. Chemical weathering
 Chemical weathering, or decomposition,
occurs when chemical reactions take place
between the minerals in the rock and water,
carbon dioxide, oxygen and acids.
A. Hydrolysis
 Hydrolysis is the change of composition of
a mineral when it reacts chemically with
water.
 Leaching occurs when water carries the
dissolved minerals to lower layers of rock
or earth.
B. Carbonation
 Carbonation is when carbonic acid reacts
chemically with some minerals.
 Carbonic acid is formed when carbon
dioxide reacts with water.
 CO2 + H2O --> H2CO3
C. Oxidation
 Oxidation occurs
when oxygen
combines chemically
with an element.
 Rust results from the
oxidation of iron.
D. Plants acids
 Acids produced by
plants can dissolve the
surface of a rock.
E. Acid Precipitation
 Rainwater is slightly acidic and can react
with rocks.
 Acid rain is formed when the oxides of
sulfur and nitrogen react with the water.
Acid rain greatly accelerates the weathering
process.
III. Rates of weathering
A. Rock composition
 The composition of the rock will determine
the rate at which it weathers.
 Quartz is the least affected by weathering.
 Sedimentary rock weather more easily than
igneous or metamorphic rock.
B. Amount of exposure
 The more the surface area of a rock is
exposed, the faster it will weather.
C. Climate
 Rainfall and the freezing and thawing
produced by alternating hot and cold
weather usually have the greatest effect on
the rate of weathering.
 Hot and humid climates produce good
conditions for chemical weathering.
 Weathering takes place slowly in hot and
dry climates as well as cold climates.
D. Topography
 The topography, or height and slope, of the
land influences the rate of weathering.
 Steeper slopes and higher elevations usually
have a greater rate of weathering.
WEATHERING
AND SOIL
CHAPTER 12
Sections 12.3
SOIL
Bedrock is solid, unweathered rock
beneath the earth’s surface.
Humus is the remains of plants and
animals.
Regolith is a layer of weathered rock
fragments on the surface.
Soil is a complex mixture of minerals,
water, gases, and organic material.
SOIL TYPES
BY SIZE
 Clay particles – less than 0.002 mm
– Parent rock is feldspar
 Silt particles
– - between 0.002 to 0.05 mm
 Sand particles
– - between 0.05 and 2 mm
– Parent rock is granite
Factors controlling soil
development
 Climate
– (most important)
 Time
 Type of bedrock
 Local land surface
– (flat vs. mountain side)
 Type of organisms
– (worms)
Soil Formation
 It takes 100-600 years for 1 inch of soil to
form
Soil profile
 The arrangement of the soil horizons
Horizons Layers in the soil
 A horizon - top soil, contains organic
material and small rock fragments
 B horizon - subsoil, contains materials
leeched from top soil, clay and maybe
humus
 C horizon - Infertile weathered rock
 D horizon - bed rock
Soil Ages
 Immature soil
– soil that is poorly developed with few or little
horizons
 Mature soil
– soil that has developed over a longer period of
time/many horizons
Soil Types
 Tropical Soils – oxisols
 Forest Soils- spodosols
 Organic Soils – histosols
 Desert Soils – gridisols
 Tundra Soils- gelisols
Tropical Soils (Oxisols)
 Tropical soils are highly
weathered, reddish or
yellowish soils of humid,
tropical or subtropical regions.
A road cut in Hawaii exposes
an example of this
homogeneous nutrient-poor
soil. In tropical settings,
extensive weathering leaches
nutrients from the soil and
leaves behind iron oxides.
Such soils are extremely low in
fertility and are commonly
found in Hawaii & Puerto
Rico.
Forest Soils (Spodosoils)
 Forest Soils (Spodosoils) Portland,
Oregon
– Form in warm to cool, humid
regions with coniferous cover.
Such soils predominate in the
northeastern United States,
New England, the upper Great
Lakes region, and the Pacific
North West. The light-gray
upper horizon of forest soils
overlies a reddish horizon rich
in aluminum and/or iron.
Subsurface accumulation of
humus and Al & Fe oxides
characterize these acidic soils.
Organic Soils (Histolsols)
 Organic Soils are wetland
soils, dark in color and rich
in decomposed organic
materials. They form in
poorly drained and lowland
environments in the Great
Lakes region and coastal
eastern United States.
These soils play an
important role in
environmental protection
by filtering contaminants
from surface water.
Desert Soils (Aridisols)
 Aridisoils form in Arid
settings. These soils occur
in the desert southwest of
the United States like Utah
and Arizona. These areas,
if not irrigated, are used
mainly for range, wildlife
or recreation. Desert soils
are commonly rich in
calcium carbonate, which
may for impermeable
layers.
Tundra Soils (Gelisols)
 Tundra soils occur
throughout the tundra of
Alaska and in the Artic
areas. A tundra soil profile
consists of a dark layer
rich in organic material
and a mineral rich layer
overlying permafrost. Any
soil, subsoil, or surficial
deposit where temperature
remains below freezing is
considered to be
permafrost.
Assignment
 Reading Assignment
2/24
Section 12.3
 Homework Assignment
2/24
Section 12.3 Worksheet
and Question page 230
 2/24
Section 12.3 Key Terms