Download Chemical Weathering - Bakersfield College

Mass Wasting
Erosion
Soils
Weathering
Weathering:
the physical and chemical
breakdown of rock – exposed to
wind, water, ice, and living
organisms
Regolith:
loose or broken rock layers
resulting from weathering
Regolith covers most of the earth.
Soil:
the upper-most layer of
regolith that supports life
Do “things” last for ever ? Consider :
new concrete
a 1955 Chevrolet
Rocks can be broken down or chemically altered on the
earth’s surface.
Mechanical Weathering:
Rocks are physically broken down by various
surface processes into smaller parts without
changing physical properties.
Chemical Weathering:
Rocks are altered from one form to a
completely new form with a different
set of physical properties.
I
weathering
Discuss with a friend:
1. Describe the difference between
mechanical and chemical weathering.
2. Give two examples of MW and CW you
have observed.
I will get an A on my exams and quizzes
Mechanical Weathering – the breaking down of rocks
How do we expose “more surface” area for chemical
weathering to do its job ?
Let’s show some surface
Rock
Less surface area
More surface area
Increased mechanical weathering accelerates
chemical weathering.
Types of Mechanical Weathering Processes
Frost Wedging: repeated cycles of freezing and thawing
Water
rock
rock
Water Freezes (expands)
pieces break off
(watch for falling rock)
Dominantly takes place in freezing / thawing areas
Unloading – reduction of pressure from
overlying rocks -- take the pressure off me.
overlying rocks
granite pluton
granite pluton
eroded and uplifted
Frost Wedging – rocks are broken into
plates and split apart.
Exfoliation Dome – granite is
“sheeting” from loss of pressure due
to unloading processes.
Types of Mechanical Weathering Processes
Biological Activity – breaking down rocks through
• animal burrowing
• humans
• plant roots
Human intervention
Animal burrowing
Plant root weathering
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mechanical weathering
Discuss with a friend:
1. Why is MW important?
2. Briefly describe how frost wedging,
unloading and biological activity
contributes to mechanical weathering
I will get an A on my exams and quizzes
Chemical Weathering
Rocks are chemically altered producing a new
compound
Why would a perfectly good rock want to change
it’s appearance?
STABILITY! Rocks require stability and achieve
equilibrium with it’s surroundings
•Most rocks are not chemically stable at the
earth’s surface
•Minerals (compositions) change to become stable
at surface conditions
What common agent can begin the process of chemical alteration?????
•The Universal Solvent – WATER (H2O)
How would we speed up the action of water? Rain + ?????
What in the atmosphere is added to water to create a weak acid?
H2O (rain) + CO2 (atmosphere) = H2CO3 (carbonic Acid)
Let’s weather some granite with the natural acid H2CO3
Granite – the most abundant continental rock
weathering the potassium feldspar (orthoclase)
orthoclase
carbonic acid
water
2KAlSi2O8 + 2(H+ + HCO3) + H2O
kaolinite clay
Al2Si2O5(OH)4 + 2K + 2HCO3 + 4SiO2
Granite has been
chemically weathered
and broken down into
individual grains.
in solution
Reduced to Clay
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Chemical weathering
Discuss with a friend:
1. What is the universal solvent?
2. How is carbonic acid naturally
Produced?
3. How would you describe the “last” or
most reduced material ----- the end
product of chemical weathering?
I will get an A on my exams and quizzes.
Soil –
product of weathering containing solids,
liquids, and gas.
What’s in your soil?
CLAY
Loam – dominant soil fraction
Humus – partially decayed
organic material
CO2/Air – fills the soil voids
Water – wets the soil
SAND
and carries high
levels of dissolved
substances
SILT
Soil Profiles:
• horizons: a layer of soil with
distinctive physical and chemical
properties.
• sequence of soil horizons from
the surface to the underlying
bedrock
O- Organic matter (humus)
A- Dark layer mixed with mineral
and organic matter
E- light-colored layer produced
from removal of soluble material
“leaching”
B- accumulation of clay
C- weathered parent rock
O
A
E
B
C
Factors that influence soil formation:
• Parent Material:
• residual regolith – soil forms emplace or “insitu”
Soil forms and
weathers emplace
from parent
material.
• transported regolith – regolith that is moved by rivers,
wind, and ice
transported river
transported
deposits
regolith on Mars
Factors that influence soil formation:
• Climate – soil profiles differ around the world
which is directly influenced by climate
arid soils vs. tropical soils
• Living organisms – a HUGE influence on soils
Plants make organic matter.
(humus) – animals mix the soil
and recycle air and water.
• Topography – the contour or “slope” of the land
• Time – the longer the time the more mature a
a soil becomes --- 1000’s of years
Classification of soils
Soil Taxonomy – Soil classification is based on physical/chemical properties
of a soil profile, influenced by climate, living organisms,
topography, and time. --- How many different soils are there?
Six categories of soil classification:
order
sub-order
great groups
12-orders
sub groups
family
series
Broadest
Specific
19,000 -series
12-basic soil orders:
Names of soils are derived from Latin/Greek.
Latin = solum – soil
Alfisols (High-Nutrient Soil)
Andisols ((Volcanic Soil)
Aridisols (Desert Soils)
Entisols (New Soils)
Gelisols (Permafrost Soils)
Histosols (Organic Soils)
Inceptisols (Young Soils)
Mollisols (Prairie Soils)
Oxisols (Tropical Forest Soils)
Spodosols (Conifer Forest Soils)
Ultisols (Low-Nutrient Soil)
Verisols (Swelling Clay Soil)
Distribution of Earth’s soils
What soil type (order) characterizes the
San Joaquin Valley?
Alfisols
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Sedimentary Rocks
Discuss with a friend:
1. Draw a “typical” soil profile and
identify the common horizons.
2. Identify at least 3 factors that
influence soil formation.
3. Describe the difference between
regolith and soil.
I will get an A on my exams and quizzes.
Sedimentary Rocks
• Ig/met rocks represent 90-95% of the Earth’s crust.
• Sediment and sedimentary rock make up 75% of the Earth’s surface
• sediment covers the ocean floor --- except for mid-ocean ridges areas
• sedimentary rock concentrated at the surface interacting with the
atmosphere, hydrosphere, lithosphere, and biosphere
Sediment/Sedimentary Rock
Chapter-6
Sediment
Compaction
Cementation
Weathering
Transportation
Sedimentary Rock Cycle
Deposition
Lithification
Weathering
Transportation
Deposit Sediment
Sediments are separated into 3-broad categories:
• Detrital (clastic) sediments:
inorganic grains or mineral fragments ranging in
size from boulders to clay particles (flour size)
• Chemical sediment:
formed by the precipitation of minerals dissolved
in lakes, rivers, or seawater environments
• Biogenic (bioclastic) sediment:
sediment composed of animal and plant remains or
material precipitated by biological processes
To be a sedimentary rock, it must be lithified.
Lithification: processes by which sediment is transformed
into sedimentary rock
Sediment can be lithified in three common processes:
Compaction: Overlying weight of the sediment “squeezes”
and compresses pore spaces, which pushes particles together.
A
B
A. loose, unconsolidated sediment with
abundant pore space
B. compacted, compressed sediment with
reduced pore space
Cementation: Pore water is expelled from voids, and rising
water carries iron, calcium carbonate, and silica which
precipitates as geological glue (cement) holding the grains.
Grains are cemented together by three
types of cements:
•iron cement
•calcium carbonate cement
•silica cement
Lithification processes
Recrystallization: Overlying pressure causes less stable
minerals to convert to more stable minerals, producing new
substances that cement pre-existing grains.
Overlying pressure
Grain boundaries convert
to more stable mineral
compositions, “cementing”
grains together.
Detrital (Clastic) Sediment:
Classification based on
grain size
lithified
gravel sediment ranging
from pea-sized to larger
conglomerate
>2mm
lithified
sand-sized particles
(fine to coarse sandpaper)
sandstone
lithified
finer particles – size of table salt
finest sedimentary
particles (flour-size)
lithified
1/16 mm
siltstone
1/256 mm
shale
Chemical Sediments and Sedimentary Rocks
Chemical Sediment: sediment formed by the precipitation
of minerals dissolved in a lake, river, or seawater
Ca+2
+2
SO2-2 Ca
SO2-2
Ca+2
Ca+2
SO2-2 Ca
Ca+2
SO2-2
+2
.
All surface water and groundwater
contain dissolved ions, creating
a venue for precipitation of
chemical sediment.
Ca+2
Precipitation of chemical rocks takes place in two ways:
• Plants and animals alter the chemical balance
of the water body (lake, ocean).
Increasing amounts of calcium carbonate
cause precipitation of limestone.
• evaporation of chemically saturated water
bodies
Evaporation of chemically saturated water creates
a chemical sedimentary rock (evaporite).
Na+ Cl- Na+ Cl
Cl
+
+
Na
Na ClEvaporation
NaCl
a lake containing sodium (Na+)
and chlorine (Cl-)--free ions
Lake water is evaporated,
concentrating sodium and
chloride ions (saturation).
Sodium and chlorine combine
to form halite (table salt).
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The Great Salt Lake, Utah
(evaporite halite)
As lake becomes saturated,
density of water increases.
Utah
Chemical – sedimentary rocks that have been
precipitated or are the result of evaporation
Gypsum – evaporite CaSO4
Halite – evaporite
NaCl
Limestone-precipitated
calcium carbonate
Verities of
Chert-SiO2
Biogenic Sediments and Biogenic Rocks:
Biogenic sediment is composed of organic remains of plants and
animals (bioclastic rocks).
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• shell, bones, teeth, plant fragments, wood, roots
Common Biogenic Rocks:
• Limestone
• most abundant biogenic sedimentary rock
• composed of calcium carbonate CaCO3(calcite, dolostone,
aragonite)
• lithified shells, skeletal material
• Chert
• composed of silica SiO2
• precipitated silica shells (made by sea animals) that protect
microscopic sea animals – Animals die, and silica shells
sink, creating layers of chert beds on the ocean floor.
“When the animal dies, it loses the chert off its back.”
Biogenic (bio-clastic) rocks –
result from animal and plant secretions -- The term
“clastic” indicates that these rocks contain fossils or
parts of plants, shells, compacted plant material
Animal parts
Cemented shell fragments
Fossil Limestone
Microscopic fossils
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Coquina
Chalk
White Cliffs of Dover, England
• made of microscopic hard parts of organisms
• soft, porous material
Common Biogenic Rocks
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Peat to Coal
• accumulated remains of terrestrial plants
• with time and pressure ---- peat
• continued pressure and lithification ------ coal
Peat
lithification
metamorphism
Anthracite coal
Bituminous coal
Inorganic
(Chemical)
Bio-clastic
(Biochemical)
Detrital sedimentary rocks the
most common sedimentary type
Sedimentary Rock
Classification Chart
Inorganic
(Chemical)
Sedimentary Rock Classification Summary Tree
Sedimentary Rocks are divided into 3-classification categories:
Clastic sediments
Chemical sediments
Bio-clastic (chemical)
sediments
formed by precipitation
composed of inorganic
grains
classification based on
distribution of grain sizes
Conglomerate
from
plants and evaporation
animals
processes
composed of
“animal parts”
Classification based on
lack of grains and chemical
composition
Gypsum (CaSO4)
Sandstone
Siltstone
Shale
Halite (NaCl)
Limestone (CaCO3)
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Chemical weathering
Discuss with a friend:
1. Briefly describe the textural differences
between the 3 categories of sedimentary
rock classification.
I will get an A on my exams and quizzes.
What do sedimentary rocks tell the earth
scientist?
These rock “layers” were
deposited one layer at a
time and lithified.
strata
The banded appearance
is known as bedding or
groups of layers called
strata.
The boundary between
each stratum is called a
bedding surface.
By observing the sedimentary rock type (clastic, chemical,
biogenic) and depositional patterns, an earth scientist can
decipher the geologic history – like reading pages in a book.
Sedimentary Environments:
• accumulation of sediments characterized by physical and biological
conditions
• textural and compositional characteristics of sediment are “clues”
that dictate the type of sedimentary environment.
• Continental Sedimentary Environment
• dominated by stream erosion/deposition
• wind erosion producing dune landscapes (eolian environment)
• characterized by playa deposits (evaporites)
• glacial deposits – unsorted particles (clay to boulder size)
• Marine Sedimentary Environment
•Shallow marine (200m to surface)
• boarders the worlds continents (continental shelf's)
• sediment type dependent on water depth, climate
• receives large quantities of continental sediment
• warm climates – carbonate mud's dominant
• cold climates – higher in silicate clasitc material
• Deep marine
• includes all the deep ocean floor
• predominately fine particles “settling” on the ocean floor
• Transitional Sedimentary Environment
• shoreline represents the transition zone between continental and
marine environments
• beach, spits, barrier islands, delta type environments
Transitional
Continental
Continental
Continental
Continental
Continental
Transitional
Deep
Marine
Shallow
Marine