Download weathering, soil and mass movements

WEATHERING AND SOIL
FORMATION
Earth Science
Michele Holler
MECHANICAL WEATHERING
Physical forces break
rock into smaller and
smaller pieces without
changing the rock’s
composition
 Increases the surface
area that weathering
can act on
 3 Types
 Frost wedging
 Unloading
 Biological activity
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FROST WEDGING
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Water enters the cracks in
the rocks and expands when
it freezes, which enlarges
the crack
After many freeze-thaw
cycles the rock breaks into
pieces
Occurs in mountainous
regions
At the base of steep rocky
cliffs, sections of rock will
form a pile at the bottom,
this is called talus
UNLOADING
When large masses of
igneous rock become
exposed due to uplift
and erosion, which
reduces the pressure on
the rock
 Slabs of rock separate
like layers on an onion,
called exfoliation
 Forms large domeshaped formations
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BIOLOGICAL ACTIVITY
Activity of organisms,
like plants, burrowing
animals and humans
 Plants grow into the
crack of the rocks,
breaking them apart
 Burrowing animals
bring rocks up to the
surface
 Humans accelerate
mechanical weathering
through deforestation,
etc
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CHEMICAL WEATHERING
Transformation of rock into one or more new
compounds
 Most important agent is water, which absorbs
gases from the environment and makes acids
 With carbon dioxide= carbonic acid
 With sulfur and nitrogen oxides, makes acid
precipitation
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WEATHERING OF GRANITE AND
SILICATE MINERALS
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Carbonic acid turns the
feldspar into clay, quartz
stays untouched
Quartz is picked up by the
water and is deposited near
the shore to form beaches
When silicates undergo
weathering, most of the
elements they contain are
dissolved in water, oxygen
forms iron oxide and the
rest becomes clay
SPHEROIDAL WEATHERING
Changes the physical
shape and composition
of the rock
 Weathers the corners
and edges rapidly
causing them to take
on the shape of a
sphere
 Allows water to
penetrate more forming
clay minerals, etc
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RATE OF WEATHERING
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Affected by two factors
Rock characteristics:
the mineral composition
and does the rock have
cracks
Climate: the amount of
temperature and
moisture in an area.
Warm and moistchemical
Cold and moistmechanical
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Arid regions, chemical
weathering is slow
DIFFERENTIAL WEATHERING
Different parts of the rock weather at different
rates
 Causes pinnacles to form when the most
resistant rock is left standing
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CHARACTERISTICS OF SOIL
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Weathering produces a
layer of rock and mineral
fragments called regolith
Soil is the part of the
regolith that supports
plant growth
Soil has four major
components, mineral
matter, organic matter
(humus), air and water
Amount of organic matter
varies due to location
SOIL TEXTURE
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Refers to the proportion
of particle sizes
Based on the percentages
of sand, clay and silt
Sandy soils dry out too
quickly
Clay rich soils drain very
slowly
Loam soils are best for
plant growth, can retain
water and nutrients
better
SOIL TEXTURE TRIANGLE
SOIL FORMATION
Based on five factors:
 Parent material
 Time
 Climate
 Organisms
 Slope
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PARENT MATERIAL
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Source of mineral matter in
soil
Soil that forms on bedrock
and is similar in make-up is
called residual soil
Soil that forms on
unconsolidated deposits and
was moved from another
location by gravity, wind,
water or ice is called
transported soil (develops
more rapidly)
Affects the rate of
weathering and rate of soil
formation
TIME
The longer a soil forms,
the thicker it becomes
 The parent material
largely determines how
long it takes a soil to
form
 Climate overrides the
affects that parent
material has
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CLIMATE
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Includes temperature and
precipitation
Has the greatest effect on soil
formation
Hot and wet climates have a
thick layer of chemically
weathered soil
Cold dry climate has a thin
layer of soil
Similar soils can be produced
in different climates from
different parent material
Dissimilar soils can be
produced from the same parent
material in different climates
ORGANISMS
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Soils can be named from the
type of natural vegetation
Plants are the main source
of organic matter
Microorganisms decompose
dead plants and animals
convert nitrogen gas to
nitrates that the plant can
use
Burrowing animals pull
organic matter into the soil
SLOPE
Steep slopes have a lot
of erosion and very
little plant life, soils
are about nonexistent
 Flat land has a lot of
plants and drains
poorly, soils are dark
and thick
 South facing slope are
warmer and drier
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SOIL PROFILE
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Soil varies in composition,
texture structure and color,
these variations are called
horizons
A horizon- topsoil; contains
a lot of organic matter
B Horizon- subsoil; contains
fine clay particles and a
compact impenetrable layer
called hardpan
C Horizon- partially
weathered parent material
SOIL TYPES
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Pedalfer – forms in
temperate areas. Found in
forest on the eastern half of
the US. Has iron oxides and
aluminum rich clays
Pedocal – forms in the drier
western portions of the US.
Have abundant calcite, or
calcium carbonate
Laterite – hot wet tropical
areas. A lot of iron oxides
and aluminum oxides.
Hardens when it dries, so
used in pottery
SOIL EROSION
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Important part of the rock
cycle
Water erodes three ways:
sheets of water = sheet
erosion ; forms tiny streams
called rills; and then
trenches called gullies
Human activities have
greatly accelerated the rate
of erosion by removing
vegetation
Excessive erosion also
causes sediment build up in
streams and behind dams
Controlled by crop rotation,
contour plowing and
planting windbreaks