Download Weathering and Soils Weathering and Soils

Weathering and Soils
Weathering
and
Soils
Weathering and Soils
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Weathering
Weathering is the breakingbreaking-down of
rock and sediments over time.
-Most rocks, like granite, form under earth’s surface.
-The rocks uplift and eventually make their way to earth’s surface.
-Conditions on the surface are much different from inside earth.
-Rocks are not used to these conditions, become unstable, and break down.
There are two major types of weathering:
Mechanical Weathering
Chemical Weathering:
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Mechanical Weathering
Mechanical Weathering:
- Physically breaks rock into smaller pieces.
- Rock actually disintegrates, but does not change in composition.
composition.
- Example: like a jackhammer smashing up rock or concrete.
The major types of mechanical weathering are:
- Frost wedging
- Exfoliation/Unloading
- Organic Activity
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Mechanical Weathering: Frost Wedging
- Water gets into fractures in rock.
- When the water freezes, it expands and breaks the rock apart.
- Most often in regions of constant “freeze and thaw”
thaw” cycles.
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Mechanical Weathering: Exfoliation
1. Rock weathers and erodes away.
2. There is less weight and pressure pushing down on rock.
3. The rock expands upward under the reduced pressure and breaks
into sheets or slabs.
Pressure
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Mechanical Weathering: Organic Activity
1. Plant roots grow into rocks and split it apart.
2. Burrowing animals loosen sediments.
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Mechanical Weathering
Area= L X W
Surface Area: (Area) X (# of sides.)
- Once a rock weathers, its surface area increases.
- Increase in surface area exposed allows for more weathering.
THUS: Weathering speeds up over time.
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Chemical Weathering
Chemical Weathering:
- Breaking down (dissolving) of minerals in rocks.
- Occurs when rock reacts to air, water, and other chemicals.
The major types of chemical weathering are:
- Oxidation
- Acids Reactions
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Chemical Weathering: Oxidation
1. When rocks containing iron are exposed to air and water.
2. The rocks oxidize or “rust”
rust” producing a red color.
Example: a rusty nail or a rusty bike chain
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Chemical Weathering: Acid Reactions
1. When water reacts with carbon dioxide it forms carbonic acid.
2. The acid reacts with rocks that contain calcite (limestone).
3. Rocks like limestone will dissolve from the acid.
Example: When we used Hydrochloric Acid in lab to test for calcite.
Carbonic Acid Chemical Reaction with Limestone:
H2O
Water
+
CO2
Carbon Dioxide
CaCO3 + H2CO3
Calcite
Carbonic Acid
H2CO3
Carbonic Acid
Ca2+ + 2HCO3Calcium Ion
Bicarbonate Ion
Dissolved Limestone
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Factors affecting weathering:
Two major factors that affect weathering:
1. Mineral Composition of Rock
2. Climate of a region.
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Factors affecting weathering:
Mineral Composition of Rock
- Some minerals are less stable (dissolve faster) than others.
Quartz (sand) is one of the
most stable minerals.
Which is why it is so
common on earth’s
surface.
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Factors affecting weathering:
Climate of a Region.
- The warmer and more wet a region is, the more weathering occurs.
occurs.
- Climate affects both mechanical and chemical weathering.
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From weathering to soils: Soil Formation
Once rock weathers, a layer of rock and mineral fragments is produced.
produced.
- This layer of sediment is called Regolith.
- Regolith eventually turns into soil over time.
HOW DOES REGOLITH TURN INTO SOIL?
-The
The rock and mineral fragments continue to break down.
-Organic
Organic matter from plants & animals is added to the regolith.
-The
further.
ther.
The decaying organic matter helps to break down the regolith fur
-Eventually
Eventually the rock and mineral fragments turn into fertile soils.
soils.
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Factors affecting Soil Formation
There are 5 major factors that affect soil formation.
- Climate
Rate of weathering changes with climate: warmer and wet = more weathering
- Slope of the Land
Erosion removes soil-forming materials. Steep land has poor soils.
- Types of rocks weathering
Some rocks weather easier than others: granite vs. limestone.
- Types of vegetation in the region
Vegetation adds organic matter to soils which helps in soil formation.
- Amount of time rock has been weathered.
The more time that goes by, the more weathering = more rich fertile soils.
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Soil Composition
Composition of Soils: A mixture of:
1.
2.
3.
4.
Rock & mineral fragments
Organic matter
Air
Water.
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Soil Profile
- A soil profile is a cutcut-away look at layers of soil built up over time.
- Each layer in a profile is called a horizon.
horizon.
- There are 3 major soil horizons each with different characteristics.
characteristics.
•Layer known as “top soil.”
•Littered with organic material.
•Humus: decayed organic material.
•Dark and fertile; nutrient rich.
o
A- H
o
B- H
rizon
rizon
n
orizo
C- H
•Less organic material. Less fertile
•Accumulation of mineral matter that
was leached from A horizon.
•Contains the “parent rock.”
•Rock that has produced above soil.
•Little weathering has taken place
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Soil Types
• Soils rich in Al- and Fe-oxides and
hydroxide
• Generally with a thin A-Horizon over a
thicker B-Horizon
• Typical of soil in areas of moderate-to-high
rainfall such as the eastern US and Canada
as well as Europe
•Generally good agricultural soils
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Soil Types
• Soils rich in Ca (typically as
CaCO3)
• Thin A- and B-horizons commonly
with a crust of soil cemented by
CaCO3 (caliche)
•Typical of soils developed in warm,
dry areas (e.g. southwest US)
•Generally poorer soils (little organic
matter)
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Soil Types
• Deep, red soils stripped of all silicates
leaving mostly Fe- and Al-oxides and
hydroxides
• Very thin soil organic-rich humus layer
due to constant recycling to the surface
plants
• Typical of equatorial rain forests (e.g.
Brazil)
• Results in poor soils subject to rapid
damage
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