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Where are we???
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Intro to geology
Plate tectonics
Earth’s matter
Minerals
Rocks
& structure
Igneous rocks
Volcanism
Weathering & erosion
Sediments and Sedimentary rocks
Rock forming
Metamorphic rocks
processes
Rock record and Geologic time
Rock deformation
Earthquakes
Evolution of continents
Dynamic
Exploring Earth’s Interior
motions
Earth beneath the ocean, shorelines
Mass wasting
Hydrologic cycle and Groundwater
Streams
External
Deserts & winds
factors,
Glaciers, Ice Ages
resources
Landscapes
Energy and Mineral Resources
Earth’s environment, Global Change,
Human Impacts
Chapter 7: Weathering and erosion
Study questions….
1) Overview
2) Physical weathering
3) Chemical weathering
4) Weathering rates
5) Soil basics
Some MC questions….
Chapter 7: Weathering and erosion
Study Questions:
3.
4.
5.
6.
7.
1. What is the difference between chemical and physical
weathering?
2. What is the difference between weathering and erosion?
3. What are the different types of physical weathering?
4. How does feldspar weather into clay?
5. What are the three main agents of chemical weathering?
6. How does silicate weathering influence climate?
7. What controls rates of weathering (e.g, rock
characteristics,temperature, humidity).
8. What is soil?
Chapter 7: 1) Overview
Ways to move material to lower elevations:
 weathering


rock disintegration (physical breakdown)
rock decomposition (chemical alteration)
 erosion

removal of material by water, wind, ice
mass wasting

transfer of rock & soil downhill
Later chapter
Chapter 7: 2) Physical weathering
Physical forces break rock into smaller & smaller
pieces w/out changing mineral composition
4 ways this happens in nature:
 Frost wedging
 Exfoliation and spheroidal weathering
 Thermal expansion
 Biological activity
Chapter 7: 2) Physical weathering
 Frost wedging
Repeated cycles of freezing & thawing
can break rock into smaller fragments
How?
Water expands ~ 9% upon freezing
Where?
Mountainous regions w/ daily
freeze/thaw cycles
Chapter 7: 2) Physical weathering
 Frost wedging
Chapter 7: 2) Physical weathering
Talus slopes
 Frost wedging
Chapter 7: 2) Physical weathering
 Exfoliation and spheroidal weathering
Expansion of rock from removal/erosion
of overlying rock (“sheeting”)
Why?
Reduction in pressure (since less
overlying weight)
Manifestation:
Slab-like layers/sheets break loose
Chapter 7: 2) Physical weathering
 Exfoliation and
spheroidal weathering
Continued weathering causes
rock slabs to separate & fall
HALF DOME, YOSEMITE NATIONAL PARK (California)
“exfoliation domes”
Chapter 7: 2) Physical weathering
 Exfoliation and
spheroidal weathering
Another “sheeting” example
Chapter 7: 2) Physical weathering
“Devil’s Marbles”
 Exfoliation and spheroidal
weathering
Australia
Chapter 7: 2) Physical weathering
Alternating Heat and Cold
Expansion/contraction of rock
from
heating/cooling
Chapter 7: 2) Physical weathering
Alternating Heat and Cold
Expansion/contraction of rock
from
heating/cooling
Chapter 7: 2) Physical weathering
Alternating Heat and Cold
Expansion/contraction of rock
from
heating/cooling
Chapter 7: 2) Physical weathering
 Biological activity
Weathering of rock from activities of




organisms
plants
burrowing animals
humans
Chapter 7: 2) Physical weathering
Surface area increases due to physical weathering and
more surface becomes available for chemical reactions.
Chapter 7: 3) Chemical weathering
Chemical weathering:
Processes that break rock components and
internal structures of minerals, making new
minerals
Most important agents in chemical weathering:

Water (Dissolution/Hydrolysis)
Carbon dioxide (Hydrolysis)
Oxygen (Oxidation)
Chapter 7: 3) Chemical weathering
 Water (Dissolution)
Dissolving minerals by a
liquid agent (such as water)
How?
Many minerals are water-soluble
example: Halite (salt), Calcite (calcium carbonate)
Pure water is a weak acid
Water molecules break down from H2O into H+ and (OH-)
Chapter 7: 3) Chemical weathering
 Water (Hydrolysis)
Decomposition of granite
Granite (mainly quartz + potassium feldspar)
+
water
 Liberates potassium and silica from feldspar, go into solution
 makes kaolinite - this left over mineral is clay
- has water absorbed (=hydration)
- main inorganic part of soil
 quartz
- very stable, glassy appearance
- carried to sea: beach sand, sand dunes
- cements to form sandstone
See Fig. 7.3
Chapter 7: 3) Chemical weathering
Carbon dioxide (Hydrolysis)
Carbon dioxide combines with water to carbonic acid
CO2 + H2O
H2CO3
A small portion dissociates into hydrogen ions and
bicarbonate ions:
H2CO3
H+ + H CO3 -
 Carbonic acid is a weak + most common natural acid
 Speeds weathering rates of feldspar and calcite compared with
pure water
See Fig. 7.6
Chapter 7: 3) Chemical weathering
See Fig. 7.6
See Fig. 7.6
Chapter 7: 3) Chemical weathering
See Fig. 7.6
Chapter 7: 3) Chemical weathering
 Oxygen (Oxidation)
Chemical reaction that causes loss of electrons
Example
Iron (Fe) + oxygen (O2) (dissolved in water)
oxygen (O) combines with Fe to form iron oxide
= hematite (Fe2O3)
Oxidation decomposes important minerals:
mineral
olivine
pyroxene
hornblende
formula
(Mg,Fe)2SiO4
(Mg,Fe)SiO3
Ca2(Fe,Mg)5Si8O22(OH)2
where originates from
Earth’s mantle
Earth’s mantle
Continental rocks
See Fig. 7.8
Chapter 7: 4) Rates of weathering
Several control factors:

Bedrock characteristics
Some minerals are more stable than others
 Climate
Temperature, moisture (vegetation)
freeze-thaw cycles
 Topography
e.g., gentle or steep slopes
Table 7.15
And 7.2
Chapter 7: 4) Rates of weathering
Differential weathering
Chapter 7: 4) Rates of weathering
Chapter 7: 5) Soil basics
Regolith
Layer of rock and mineral fragments
produced by weathering that cover most of Earth’s land
surface
Soil
“good soil:”
decomposed rock
+
45%
decayed animal/plant life (“humus”)
+
5%
water
+
25%
air
25%
=
Chapter 7: 5) Soil basics
Chapter 7: 5) Soil basics
Important Soil-forming factors
 Parent material
 Climate
 Time
affects fertility, weathering rate
temperature & precipitation
longer time
 Plants/Animals
 Slope
if too steep
thicker soil
organic matter
little/no soil
A few questions…
1. Which of the following processes is not an
example of chemical weathering?
A.
B.
C.
D.
dissolution of calcite
breakdown of feldspar to form kaolinite
splitting of a rock along a fracture
rusting of a nail
A few questions…
2. __________ is a chemical reaction in which an atom
or ion loses electrons.
A.
B.
C.
D.
Carbonation
Exfoliation
Hydration
Oxidation
A few questions…
3. Which of the following statements is true?
A.
B.
C.
D.
High temperature promotes physical weathering.
Frost wedging is a form of chemical weathering.
Chemical weathering is promoted by gentle slopes.
Clay minerals are produced primarily by physical weathering
A few questions…
4. Which of the following will decrease the rate of
chemical weathering of a rock at the Earth’s
surface?
A.
B.
C.
D.
increasing the amount of acid in the rainwater
decreasing the temperature
breaking the rock into smaller pieces
increasing the amount of surrounding soil
A few questions…
5. Which of the following statements is
true?
A. Silicate weathering and volcanism both increase the
amount of carbon dioxide in the atmosphere.
B. Silicate weathering increases and volcanism decreases
the amount of carbon dioxide in the atmosphere.
C. Silicate weathering decreases and volcanism increases
the amount of carbon dioxide in the atmosphere.
D. Silicate weathering and volcanism both decrease the
amount of carbon dioxide in the atmosphere.