Transcript Weathering and Rock Breakdown

Weathering and Rock Breakdown
GLY 2010- Summer 2014 Lecture 9
1
Weathering
• Two types  Physical, also known as mechanical
 Chemical, also known as disintegration
2
Physical Weathering
• Faster when water is present, but
proceeds at slower rates in dry climates
• Many different processes are possible - a
few on shown on the following slides
• Breaks a rock into smaller pieces without
affecting it chemically
• Increases the surface area of rocks
3
Surface Area Increase
4
Weathering by Expansion
5
Boulder Split by Frost Wedging
• Boulder split by the expansion of ice
6
Adsorption
• Adsorption of fluid, usually water, can
increase the volume, and lead to cracking
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Expansive Soils
• A: A sample of an expansive soil with moderate
swell potential
• B: Same soil sample after a small amount of
water - Notice the sample has expanded
considerably
• C: Same sample 48 hours later, after the sample
has had time to shrink to a smaller volume
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Damage Due to Swelling Clays
• Damage that might result from uneven
expansion and contraction of soil
containing swelling clays
9
Building Damage
• Building damaged by expansion and
contraction of clay minerals in the soil
10
Plant Roots
• (Left) Roots grow into cracks in rock,
causing expansion, and enlarging the crack
• (Right) Roots exposed in rock
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Thermal Expansion
• Rocks expand when heated, contract
when cooled
• This process has been simulated in
the laboratory
 Despite many rapid heat/cool cycles,
little effect was observed; likely not a
very important process in nature
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Development of Exfoliation
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Granite Exfoliation
• Enchanted Rock State Natural Area,
south of Llano, Texas
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Stone Mountain, Georgia
Photo: Jon Cook
FAU Junior Field
Camp, March,
2004
• Exfoliation at Stone Mountain, Georgia,
about twenty miles from Atlanta
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Stone Mountain, Georgia
Photo: Dr. Anton
Oleinik, FAU
Junior Field
Camp, March,
2004
• Exfoliation Domes
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Martian Aeolian (Wind) Abrasion
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Chemical Weathering
• Minerals, formed under pressure and/or
thermal conditions different than those at
the surface, may be unstable
• Chemical weathering changes the chemical
composition of minerals that are unstable at
the earth’s surface to minerals which are
stable
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Chemical
Weathering
at Surface
19
Weathering
Rock writing on
chemically
weathered rock
20
Hydration
• Water is added to a mineral, creating a new
mineral
• Anhydrite, CaSO4, may add two water
molecules to create gypsum, CaSO4 2H2O
• Addition of water leads to volume expansion
(70% for the above case), which may lead to
cracking of the rock
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Hydration of Obsidian
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Dissolution
• Ions or ionic groups are removed and
carried away by water
• Ionic bonding is necessary
• Over millions to billions of years,
this process is responsible for
saltiness of the oceans
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Sunland Park
Mall
El Paso, Texas
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Sunland Mall
Marble
Closeup
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Sunland Mall Marble Closeup
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Oxidation
• Addition of
oxygen, often with
a change of
oxidation state
• Rusting is slow
oxidation, while
burning is rapid
oxidation
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Hydrolysis
• Water, H2O, breaks down to yield ions
 H2O  H+ + OH-
• Responsible for the conversion of
feldspars, the most common minerals in
the earth’s crust, to clay, an important
part of soil
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Ion Exchange
• Ions in solution exchange with ions held
on the surface of minerals
 K+ (in solution)  H+ (on clay)
 The reaction may later be reversed,
releasing potassium (K), an important
nutrient for plants
• Important for the retention and later
release of water and nutrients
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Factors Influencing Chemical
Weathering
•
•
•
•
Heat
Water
Time
Stability of minerals
 Minerals weather in the reverse order
of Bowen’s Reaction series
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Biological Influences on
Weathering
• Animals, plants and bacteria can
influence weathering
 1) Burrowing animals increase air and water
to surfaces
 2) Decay of organic matter produces acids
 3) Yellowboy is the product of bacterially
controlled chemical weathering
31
Regolith
• When physical weathering
dominates, rocks are broken into
smaller and smaller pieces called
regolith
• Regolith has few of the properties of
soil - it is just broken rock
32
Soil
• Soil is regolith which has been altered by
weathering, and which may have had
organic matter added to it
• Soil is essential to life as we know it,
since plants need soil to grow, and
animals ultimately depend on plants for
food
33
Parent Rock
• The parent rock is the rock from which
regolith is derived
• Depending on their resistance to physical
and chemical weathering, different parent
rocks may form soil quickly or slowly
• The parent rock also determines how rich
in nutrients the soil is
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Factors Influencing Soil Formation
•
•
•
•
Climate
Vegetation
Topography
Time
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Topography
36
Soil Profile
• Cross-section
of soil layering
• The layers
represent
different
weathering
zones, and are
designated by
letters
• Each layer is
called a
“horizon”
37
O-horizon
• Upper layer, rich in organic matter
 Dead (leaf litter, etc.)
 Living (bacteria, algae, fungi, insects,
worms)
• Poorly developed in most areas of
South Florida
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A-Horizon
• Inorganic mineral matter
• Humus, dark colored decomposed
organic matter
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E-Horizon
• Light-colored mineral particles
• Zone of eluviation and leaching
• Eluviation is the washing out of fine soil
components from the A-horizon by downwardpercolating water
40
B-Horizon
• Enriched by precipitation of minerals
dissolved from O and A layers
• In arid regions, this may include a
caliche layer
 Carbonate minerals deposited in quantity
because of high rate of evaporation
 Brief, heavy rains bring the carbonate
downward, and deposit it in B horizon
 Often impermeable
41
C-Horizon
• Parent material which has been
slightly weathered
• Retains most of its original
appearance
42
Example Soil Profile
• A horizon extends to
bottom of third mark
from top
• B horizon (with
various subunits)
below the third mark
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Lateritic Soil, Brazil
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Tropical Rainforest Soils
• Very poor soils; nutrients are tied up in the
plants
• When a plant dies naturally, its nutrients
support new replacement growth
• If the forest is cleared and the plant material
removed, the nutrients are also removed
45
Soil Taxonomy
• Naming of soils based on their
characteristics
 Physical characteristics - named by
obvious physical characteristics (color,
thick/thin, shrinkage/expansion,
oxidation, etc)
 Texture - description of texture
46
Nutrient Depletion –
Borneo Vs. Java
• The soil on Java is formed from
fresh, nutrient-rich volcanic rock potassium, calcium, and magnesium
rich
• Java has a population density of 460
people per square kilometer
47
Java Pictures
• Jakarta at left
• Cirebon above
• Large cities on Java
48
Borneo
• The soils on Borneo are from the
weathering of intrusive granitic rock,
gabbro intrusions, and andesitic lavas,
and are poor in nutrients
• The population density is 2 people per
square kilometer
49
Borneo Countryside
• A jungle trail and a road leading to a
small village in Borneo
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Importance of Soil to Man
• Soil erosion is the number one cause of
dollar loss by natural events every year
- surpassing hurricanes, earthquakes,
tornadoes, etc
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Losing Ground Video
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