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WEATHERING AND SOIL
I.
Weathering, Erosion, and Transportation
A.
1. The group of
processes that change the physical and chemical
character of rock at or near Earth’s surface.
2. Rocks exposed at Earth’s surface are constantly being altered by
3. Weathering breaks down rocks that are either
.
B.
1. This is the
of rock particles by an agent
such as gravity, wind, running water (streams and glaciers), or wave action.
2. Most eroded rock particles are at least partially
3. Rock can be eroded before it has weathered at all.
C.
1. After a rock fragment is picked up (
), it is transported.
2. The same agents that erode rocks also transport the rocks.
3.
II.
processes continue during transportation.
Weathering and Earth Systems
A. ________________________________________________
1. Chemical Weathering requires ____________________ and _______________________
2. Mechanical Weathering requires _____________________ evaporated from the hydrosphere
and redistributed by the hydrologic cycle. Water in the liquid and solid forms are important in
the weathering of rocks and minerals.
3. The composition of Earth’s atmosphere is affected by ________________ weathering.
a.
Affects the carbon cycle and the formation of limestone.
b.
This has effects on:
•
Climate
•
Plate Tectonics
ESC 101: Professor Vorwald
Weathering & Soil
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B.
________________________________
1. Plants can physically break apart rocks.
2. After death, plants and animals decompose to mostly __________ and _______________.
a) Soil water becomes slightly acidic.
b) Organic matter is an important constituent of ___________________.
A.
Also called
B.
Includes several processes that break rock into smaller pieces.
C.
There is little or no chemical weathering.
D.
Types of Mechanical Weathering
III.
1.
: This is the mechanical effect of
freezing water on rocks.
a)
: The expansion of freezing water pries rock
apart by getting into a system of rocks called joints.
b)
: Lifts rock a soil vertically. Ice forms under
buried rock fragments in soil because being a better conductor of heat, the rocks are colder
than the soil. The expanding ice pushes cobbles and boulders out of the ground.
2.
: The breaking of rocks by the growth
and expansion of plant roots in rock pores and cracks.
3.
: Various minerals in rocks expand and
contract at different rates and directions when heated and cooled. This results in rocks
weakening and breaking. Extreme changes (such as in a forest fire) can cause this.
4.
: The grinding away of rock by friction
and impact during transportation.
5.
: A reduction of pressure on a body of
rock can cause it to crack as it expands.
a)
The removal of a great mass of rock above a batholith (
granite to expand upward.
b)
: cracks that develop parallel to the outer
surface of the rock . Gravity may loosen the rock between such joints in concentric slabs.
This processes is called exfoliation. Exfoliation domes are large, rounded landforms
developed in massive rock, such as granite, by exfoliation.
6.
7.
) allows the
: Expose underlying rock to weathering
agents.
: Salt precipitated from evaporating water
in small pore spaces in rock helps disintegrate rocks in desert areas.
ESC 101: Professor Vorwald
Weathering & Soil
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Also called rock decomposition.
IV.
Transforms rocks and minerals exposed to water and air into new chemical products. Minerals change
gradually at Earth’s surface until that reach an equilibrium with the surrounding conditions.
Example: A mineral formed in an intrusive, water deficient magma may eventually be exposed at
Earth’s surface where it will react with water forming a new and weaker minerals.
A.
1.
:
a)
The chemical combination of oxygen, in the atmosphere or dissolved in water, with one
mineral to form a completely different mineral.
b)
forms in the rusting of an iron
nail. Iron oxide formed this way is a weathering product of numerous minerals
containing iron (
) such as olivine, pyroxene,
amphibole and biotite mica, hornblende, biotite mica.
Oxidation Examples:
4Fe
+
iron
3O2
oxygen
v
2Fe2O3
AND 2Fe2SiO4 + 4H2O + O2
iron oxide
olivine
water
v
oxygen
2Fe2O3
hematite
+
2H4SiO4
silcic acid
2. __________________________________
a.
b.
c.
The chemical union of water with a mineral.
The mineral absorbs water and the new product is a new mineral.
A good example is the conversion of hematite to limonite.
2Fe2O3 + 3H20
d.
B.
2Fe2O3 . 3H20
Some geologists consider it a physical type of weathering because the new mineral is the
same composition as the original.
The Role of Acids
1. _____________________________________________________
a.
b.
c.
d.
e.
The process is referred to as _____________________________
An exchange reaction involving minerals and water.
Free hydrogen (H+) and hydroxide (OH)- ions in water are able to replace mineral ions and
drive them into solution. As a result, the mineral's atomic structure is changed into a new
form.
It is a process whereby silicate minerals like potassium feldspar are weathered and a clay
mineral is formed.
Feldspar is common mineral in many igneous, sedimentary, and metamorphic rocks.
ESC 101: Professor Vorwald
Weathering & Soil
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f.
The following reaction occurs when the most common feldspar, a sodium plagioclase
reacts with carbonic acid (formed by carbon dioxide dissolving in rain water and
groundwater)
2NaAl3Si3O8 + 2H2CO3 + 9H2O
Na-Plagioclase
Carbonic
Acid
v
2Na + 2HCO3 + 4H4SiO4 + Al2Si2O5(OH)4
Water
Dissolved components
Clay Minerals
g. The sodium eventually accumulates in the oceans as dissolved
h. The weathering process is the same for all feldspars (K-feldspar called orthoclase) and Cafeldspar (plagioclase). The ions resulting (Ca and bicarbonate) are common in rivers and
groundwater.
2.
a.
Carbonic acid dissolves ____________ which is the main mineral in the rocks
_____________________________ and __________________.
b.
CaCO3 +
calcite
c.
A type of ______________________________________________ .
H2CO3
v
carbonic acid
Ca
+
2HCO3
calcium bicarbonate
3. Chemical Weathering By Living Organisms
a.
b.
c.
_____________________ are a fungus and algae in a symbiotic relationship
Acids are secreted which react with the minerals in the rock on which they’re found.
Produces a habitat for mosses and eventually higher plants.
4. Strong Acids on Earth’s Surface
a. Some occur naturally but are rare.
b. Sulfuric acid and hydrofluoric acid are emitted during many volcanic eruptions.
i. They cause intense chemical weathering of rocks in addition to killing treees.
ii. Bubbling mud of Yellowstone’s National Park’s mudpots and some hot springs.
V. Factors Affecting the Rate of Weathering
A.
1. The more exposed the rocks are to the atmosphere, the faster the rate of weathering.
2. a particle breaks up into smaller pieces, its volume remains the same but its surface area
increases.
B.
1.
2.
weathering is the term for varying rates of weathering in
an area where some rocks are more resistant to weathering than others.
Generally, igneous and metamorphic rocks are more resistant than sedimentary rocks.
1.
2.
Chemical weathering is most intense where there’s is abundant liquid
In moist cold climates ___________________________________ is dominant.
C.
ESC 101: Professor Vorwald
Weathering & Soil
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VI. WEATHERING PRODUCTS
A.
MINERALS
Quartz and clay minerals commonly are left after complete chemical weathering of a rock
B.
SOLUBLE IONS and SILICA
These byproducts of chemical weathering remain in solution and may eventually find their way
into a stream and then into the ocean.
C.
SOIL
1. This is the layer of weathered, unconsolidated material on top of bedrock. A true soil contains
2.
This is the loose surface sediment and soil is the
upper part of the regolith.
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Weathering & Soil
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: A soil of approximately equal amounts of sand, silt, and clay along with a
significant amount of organic matter.
a. Loamy soils are often well-drained and are ofen very fertile.
b. The clay-sized particles usually consist of clay minerals
i. Help hold water and plant nutrients in a soil.
ii. Holds water loosely enough so that most of it is available for uptake by plant roots.
iii. Plant nutrients (Ca+ + and K +) often from weathering of feldspar also are held loosely on
surfaces of clay minerals.
c. Quartz grains help keep soil loose and aerated, permitting good water drainage.
3.
Soil Texture Diagram
Sample A
#
65% sand
#
20 % silt
#
15 % clay
Sample B
#
33% of each of sand, silt and clay
Sample C
#
40% sand
#
42% silt
#
18% clay
4.
__________________________: Basically the same thing as loam and is the upper part of the soil.
It is more fertile than the underlying subsoil.
5.
Soil _______________________________
As soils mature, distinct layers appear in them. These layers are called soil horizons. The rate
of soil formation is controlled by rainfall, temperature, and the type of bedrock. Most soils
take a long time to form.
Immature Soils
ESC 101: Professor Vorwald
Weathering & Soil
Mature Soil
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a)
—
—
b)
Horizon
—
—
c)
Dark-colored soil layer
Contains humus (topsoil) and usually dark in color from accumulated organic matter.
Horizon or Zone of
—
—
—
—
d)
Characterized by the downward movement of water
_________________: Removal of fine-grained soil components (clay) by downward
movement of water.
Dissolved chemicals are carried to lower portions of the soil profile.
•
In a humid climate iron oxides and dissolved calcite are leached downward along
with clays.
This horizon may appear pale and sandy.
Horizon or Zone of
—
—
—
e)
Zone of accumulation of material leached from the A horizon.
Is often clayey and stained from iron oxides.
Calcite may also build up in this horizon.
Horizon
—
—
—
6.
Horizon
Uppermost layer
Entirely consists of undecomposed and highly decomposed organic material such as fallen
leaves and needs along with ground vegetation.
Incompletely weathered parent material below the B horizon
Contains fragments of weathered bedrock
Transitional between unweathered bedrock and the developing soil above.
Residual Soils and Transported Soils
a)
—
—
A soil that develops from weathering of the rock directly beneath it.
The soil will have a similar appearance as the bedrock in terms of mineral content and
color.
—
—
A soil which did not form from the rock.
It was brought in from another region, usually as sediment deposited by running
water, wind (loess), or glacial ice.
b)
ESC 101: Professor Vorwald
Weathering & Soil
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7.
Soils and Climate
a)
—
—
—
Soils formed in wet climates, as in the eastern United States.
High content in aluminum and iron oxides (Al and Fe)
Also called forest soils
—
—
—
Arid climates
Western U.S.
Thin with little leaching and upward movement of soil water (caused by evaporation
and capillary action.
Salts (usually calcium salts such a calcite) are deposited inhibiting plant growth.
b)
—
c)
—
—
—
Hard layer of Earth material that is difficult to dig or drill.
In wet climates it forms from clay minerals, silica, and iron compounds accumulating
in the B horizon.
In arid climates hardpan called caliche forms from the cementing of soil by calcium
carbonate and other salts precipitated by evaporating water.
d)
—
—
—
—
8.
Tropical soils that are highly leached (high rainfall and temperatures).
Usually red (iron and aluminum oxides).
the mineral bauxite, a principle ore of aluminum forms from weathering of al-rich
igneous rocks.
Laterites are unproductive soils, even in a lush tropical jungle.
Summary of Factors Affecting Soil Formation
a) ________________________________
— Source of the weathered mineral matter that makes up most of a soil.
— Granite Parent Rock:
- Results in a sandy soil containing quartz and feldspar which may weather to clay.
- The variety of grain sizes results in a soil with good drainage and water-retention
properties.
— Basalt Parent Rock:
- Fine grained feldspars may weather directly to clay minerals.
- No coarse grained minerals and no quartz so it may never be sandy.
- Will not drain well.
- May be quite fertile.
b) _____________________
— Steep slopes:
- Soil thin or not present
- Because there is sparse vegetation roots are not present to hold weathering rock in place.
ESC 101: Professor Vorwald
Weathering & Soil
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—
Gentle Slopes: Optimal topography for soil formation
- Good drainage
- Minimal erosion
- Healthy vegetation cover
c) _______________________:
— Contribute to the formation of humus
— Burrowing animals bring soil to the surface and mix the organic an mineral components of
the soil
d) _______________________
—
The most influential factor affecting soil thickness and character.
e) ______________________
— Affects thickness and character of the soil.
— Young residual soils will retain the character of the parent rock.
VII Weathering, the Carbon Cycle, and Global Climate
A.
The Carbon Cycle
1. Carbon dioxides dissolves in water to form carbonic acid in the atmosphere.
2. Carbonic acid reacts with sediment and rocks during chemical weathering.
a. Calcium ions and bicarbonate ions (HCO3-) are released.
b. These ions are carried by rivers into the sea.
3. The precipitation of CaCO3 (calcite) mineral in the oceans forms layers of limestone rock.
4. Deep burial of limestone leads to metamorphism.
a. Reacts with silica and calcite
b. Forms calcium silicate minerals and carbon dioxide.
5. CO2 remains trapped in Earth’s interior until it is released during volcanic eruptions.
B.
The Greenhouse Effect of the Atmosphere
1. The atmosphere is relatively transparent to short-wave incoming solar radiation.
2. Earth’s surface is heated and radiates long-wave infrared radiation.
3. CO2, water vapor, and methane in the atmosphere absorb some of the terrestrial long-wave.
radiation.
a. While there is relatively little CO2 in Earth’s atmosphere, it’s enough to keep most of the
surface above freezing, but not too hot to support life.
(1) The early Earth probably had much more CO2 and was probably more like Venus is
today.
(2) It’s estimated that a quantity of CO2 that is about 65,000 times the mass of CO2 in the
modern atmosphere is buried in Earth’s crust and upper mantle.
•
Some was used to make organic molecules during photosynthesis and is now
trapped as buried organic matter and fossil fuels in sedimentary rocks.
•
A majority was converted to bicarbonate during chemical weathering that was
incorporated into carbonate minerals in layers of limestone.
b. Variations in atmospheric CO2 are believed to have affected Earth’s temperature over
time.
ESC 101: Professor Vorwald
Weathering & Soil
Page 9
C.
The Uplift Weathering Hypothesis
1. Asserts that chemical weathering is the driver of climate change.
2. Considers availability of fresh rock and mineral surfaces to be weathered.
3. Uplift and Weathering
a.
Tectonics results in the uplifting of Earth’s crust and the formation of mountains at many
plate boundaries.
b.
In regions of uplift exposure of freshly fragmented rock is enhanced
c.
Factors increasing weathering rates in uplifting regions.
d. Active Tectonic Uplift Cools Climate
(1)
(2)
Uplift accelerates chemical weathering
Draws CO2 out of the atmosphere and cools climate
e. Slower uplift results in Greenhouse Conditions
(1)
(2)
Less Chemical Weathering
More CO2 in the atmosphere
ESC 101: Professor Vorwald
Weathering & Soil
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