Transcript McConnell_1e_PPT_Ch09

Chapter 9: Weathering and Soils
1.
2.
3.
4.
The Dirt on Weathering
Physical Weathering
Chemical Weathering
Biological Weathering
and Decay
5. Weathering Rates
6. Soils: An Introduction
7. Soil Erosion and
and Conservation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Learning Objectives
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Students will explain concepts related to weathering and soils.
Students will distinguish among physical, chemical, and biological
weathering.
Students will analyze the environmental factors that contribute to
physical weathering.
Students will describe the processes and products of hydrolysis,
dissolution, and oxidation.
Students will compare biological weathering processes to those of
physical and chemical weathering.
Students will describe the factors responsible for different
weathering rates.
Students will identify the key factors that contribute to the
formation of different soil orders.
Students will compare and contrast soil erosion due to water and
wind.
Students will list methods to combat soil erosion.
The Dirt on Weathering
“For water continually dropping will wear hard rocks hollow”
-Plutarch, Greek historian
Were these volcanic
rocks originally
formed with large
holes? How did
they develop the
appearance of
Swiss cheese?
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Weathering is the physical, chemical, and biological breakdown of rocks
and minerals.
Five World Heritage Sites
11th Century temple complex
Very few ancient structures are
preserved in higher latitudes with
colder climates. Why might that be?
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Rocks exposed at Earth’s surface were originally formed under
different temperature/pressure conditions, and weathering
converts their minerals into new minerals that are stable at
Earth’s surface.
13th century cliff dwellings
The world heritage sites are those
deemed to be “of outstanding value to
humanity.”
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Weathering modifies the geological and cultural landscape
around us.
2400 year old Parthenon
The weathering of the Parthenon
adds character to the structure, but
is accelerated by air pollution.
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Why do some rocks show more deterioration than others?
15th century Inca Village
Machu Picchu sits high in the
Andes Mountains. Is it particularly
susceptible to weathering? Why?
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Why is weathering faster in some places than others?
12th century mosque
What can be done to stop or slow
down the natural processes that
cause weathering of important
structures?
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Q: How many components
of the Earth system are
contained in soil?
Soil (or dirt) is
composed of:
A: All four:
• Regolith (rock and
mineral fragments
Geosphere
• Water
Hydrosphere
• Air
Atmosphere
• Organic material
Biosphere
The Good Earth/Chapter 9: Weathering and Soils
The Dirt on Weathering
Soil forms slowly by weathering, and is often
eroded faster than it is replaced.
• loss of soil is a threat to agriculture
• Influenced by human activity and natural factors
A thin soil layer over
bedrock, Giant’s
Causeway, N. Ireland.
On average, soil is 45%
mineral fragments, 25%
water, 25% air, and 5%
organic material.
The Good Earth/Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Physical Weathering
The Good Earth/Chapter 9: Weathering and Soils
Physical Weathering
Disintegration of rocks and minerals
into smaller pieces
• Unloading
− Erosion strips away overlying material
− Decrease in overlying pressure (load) causes
underlying rock to expand upward
− Leads to pressure release cracks in the exposed rock
• Wedging
− Water enters cracks in surface materials (rocks,
roads)
− Temperature drop causes water to freeze, expand,
and force the cracks to expand.
− Process repeats when ice melts, water finds new
cracks, freezes again and expands the cracks
− Example: potholes
The Good Earth/Chapter 9: Weathering and Soils
Physical Weathering Checkpoint 9.1
Examine the following photos. Both images show granite outcrops in the
Sierra Nevada Mountains, Ca. Which outcrop contains pressure release
cracks? Explain your choice.
A. Outcrop a
B. Outcrop b
The Good Earth/Chapter 9: Weathering and Soils
Physical Weathering
• Wedging can be caused by
other substances
− Growth of salt crystals in
small rock openings
Honeycomb weathering – a result of
salt crystal growth on surface of rock,
expanding the size of the hollows.
Physical weathering
breaks rocks into
smaller pieces,
increasing the surface
area over which
weathering can occur.
The Good Earth/Chapter 9: Weathering and Soils
Physical Weathering Conceptest
Outcrops of granite are examined in California at similar elevations in
the interior of the state more than 100 km (63 miles) from the Pacific
Ocean, and in outcrops along the coast. The granites have identical
compositions and textures. On the basis of the following information,
which granite outcrop would weather most rapidly?
a) Outcrop A; located at a coast, contains fractures spaced 1 meter
apart
b) Outcrop B; located at a coast, does not have fractures
c) Outcrop C; located in an interior, contains fractures spaced 1 meter
apart
d) Outcrop D; located in an interior, does not have fractures
The Good Earth, Chapter 9: Weathering and Soils
Physical Weathering Checkpoint 9.4
Imagine that you have been appointed to a team of researchers
charged with determining which of the five World Heritage Sites in
Figure 9.1 is at greatest risk from physical weathering. Identify at least
three general questions you will ask as you begin to gather data for
your study. Describe how you will use the information to plan your next
steps.
The Good Earth/Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Chemical Weathering
The Good Earth/Chapter 9: Weathering and Soils
Chemical Weathering
The decomposition of rock due to the chemical breakdown of
minerals is called chemical weathering
• Dissolution
− Minerals in a rock are dissolved by water
− Example – acid rain removes detail of a
statue
• Hydrolysis
− Hydrogen ions (H+) in water replace other
ions in silicate minerals
− Example – Feldspar reacts with water and H
ions to form clay which is used in
1,200 meters
manufacturing glossy magazine paper
• Oxidation
− Oxygen reacts with iron and other metals to
form new mineral compounds
− Example – the rust on your car
The Good Earth/Chapter 9: Weathering and Soils
Chemical Weathering
• A sugar cube disappearing in a cup of coffee is an
example of dissolution
• Carbon dioxide is a small portion of the air we breathe,
but when combined with rainwater it creates a weak acid
called carbonic acid (H2CO3)
H2O
rain
+
CO2
from air or

H2CO3
carbonic acid
bacterial
decomposition
• Carbonic acid can dissolve limestone and marble
• Limestone is common on Earth’s surface – dissolution of
limestone creates karst landforms (sinkholes, caves)
The Good Earth/Chapter 9: Weathering and Soils
Chemical Weathering
stalactites
stalagmites
All rocks contain some cracks. Water carrying
dissolved limestone is transported through caves
and may precipitate the limestone out of solution as
stalactites and stalagmites.
Sinkholes are large
depressions on the surface
where limestone bedrock
has been dissolved resulting
in collapse on the overlying
surface (above: Alabama’s
“December Giant”)
The Good Earth/Chapter 9: Weathering and Soils
Chemical Weathering
• When hydrogen ions replace other ions in silicates
(hydrolysis), the silicates become weaker and more likely
to break down.
• Where do the hydrogen ions come from?
H2CO3

H+
+
HCO3
-
(break down of carbonic acid)
Hydrolysis and dissolution can occur together to chemically weather
a rock more quickly
More hydrogen ions = more rapid chemical weathering
The Good Earth/Chapter 9: Weathering and Soils
Chemical Weathering
All rain water is mildly acidic.
Pollutants (sulfur dioxide, nitrogen oxides) significantly decrease the pH of rain water
(increase acidity).
pH scale is logarithmic – each increment represents a 10 times increase/decrease in
hydrogen concentration per volume of solution.
The Good Earth/Chapter 9: Weathering and Soils
Weathering Concept Survey
Venn Diagram: Physical vs. Chemical Weathering
Complete the following Venn diagram to compare and contrast the similarities
and differences between physical and chemical weathering. Identify 8
characteristics that are either shared by both physical and chemical
weathering or are unique to one of the two. Then place the number
corresponding to each characteristic in the diagram. Characteristics 1 and 2
have been plotted for you as examples.
1. Causes the breakdown of
rocks and minerals
2
2. Causes iron bearing rocks
to turn red/orange
3.
4.
1
5.
6.
7.
8.
Physical weathering
Chemical weathering
The Good Earth, Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Biological Weathering
and Decay
The Good Earth/Chapter 9: Weathering and Soils
Biological Weathering and Decay
• Believe it or not, critters (plants and animals) can
remove and/or break down rocks and minerals.
− Macroscopic (by organisms we can see)
− Includes the actions of: plant roots, animal burrows,
termites, and other boring organisms
− Microscopic (by organisms we cannot see with the naked eye)
− Primarily caused by decomposition of material that
converts solid material to gases with or without water
− Works mostly on organic material such as dead plant or
animal matter
sugars in organic material + oxygen  carbon dioxide + water
Sugars in organic material  carbon dioxide + methane gas
The Good Earth/Chapter 9: Weathering and Soils
Biological Weathering and Decay
Q: Is this an example of
micro or macroscopic
biological weathering?
A: Macroscopic
– tree roots are
forcing apart the
rock
The Good Earth/Chapter 9: Weathering and Soils
Biological Weathering and Decay
• Other examples of
macroscopic
biological
weathering:
− Chitons (a type of
mollusk) wear away at
limestone
− Sea urchins excavate
holes in bedrock
beneath shallow
water
The Good Earth/Chapter 3: Near-Earth Objects
Biological Weathering and Decay
Checkpoint 9.9
Give an every day example of
microscopic or macroscopic biological
weathering. Explain your example –
how does it work, where might it occur.
The Good Earth/Chapter 9: Weathering and Soils
Biological Weathering and Decay
Checkpoint 9.11
Write a paragraph that argues for OR
against the following statement:
Biological weathering processes could be
considered examples of physical or
chemical weathering.
The Good Earth/Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Weathering Rates
The Good Earth/Chapter 9: Weathering and Soils
Weathering Rates
What controls how quickly a rock weathers?
Rock composition
Rock properties
Climate
In low porosity rocks weathering is restricted to the
outer rind
Weathering concentrated along fractures.
Degree of weathering decreases downward.
Why?
The Good Earth/Chapter 9: Weathering and Soils
Rates of Weathering Concept Survey
Say whether each of the following
statements relates to the influence on
weathering by rock composition, rock
properties, or climate.
1.
Exposed rock in the mountains of Alaska
2.
3.
A lack of fractures in an outcrop
4.
High porosity (a large amount of space between
grains in a rock)
5.
Rocks on the coast of a tropical island
6.
Minerals that dissolve in the presence of water
Quartz is rarely affected by dissolution, hydrolysis,
or oxidation
The Good Earth/Chapter 9: Weathering and Soils
Weathering Rates
Compositional control
1. Weathering is faster in rocks composed of weaker material
or material that is easily converted to weaker material (such
as feldspars)
2. Weathering is faster in rocks made up of minerals that
dissolve in water (salt, gypsum)
3. Weathering is slower in rocks made of resistant materials
(quartz)
Rock property control
1. Weathering is faster in rocks that allow air and water in
(porous, fractured)
2. Fractures are natural weathering surfaces
3. Igneous and metamorphic rocks generally have low porosity
– do you think they are particularly susceptible to
weathering?
The Good Earth/Chapter 9: Weathering and Soils
Weathering Rates
Climate control
(Climate = a regions average annual temperature,
precipitation, and vegetation)
1. Chemical weathering is faster in warmer climates
2. More water (rain, humidity) = more dissolution and hydrolysis
reactions
3. More shade provided by vegetation can impede evaporation
and allow more water to find its way into cracks thereby
increasing rates of weathering
4. Carbon dioxide released from plants can combine with water
to make carbonic acid
5. Higher elevations may have more freeze/thaw cycles,
increasing rates of physical weathering
6. Extremely cold regions don’t have much thawing, therefore
not much wedging occurs
The Good Earth/Chapter 9: Weathering and Soils
Weathering Rates Checkpoint 9.13
Assuming that the rock type is similar in each of the
locations on the map below, predict whether physical
or chemical weathering (or neither) dominates in
each location. Explain your answers.
The Good Earth, Chapter 9: Weathering and Soils
Weathering Rates Checkpoint 9.15
Analyze four of the World Heritage Sites described
in Table 9.1 below (omit Angkor Wat). Rank the
locations from 1-4 on their potential for weathering
(1 being highest potential). Justify your ranking.
The Good Earth, Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Soils: An Introduction
The Good Earth/Chapter 9: Weathering and Soils
Soils: And Introduction
Soils from different areas of the U.S.
What do you notice about them? Why do you think they look different?
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction
• Soil = a stratified mixture of regolith that includes enough
organic material, water, and air to support plant life
− Organic material is supplied by decaying plants and animals
− Organics get mixed with soil by burrowing animals, worms, and
insects
− Water moves through the soil and leaches it (dissolves iron)
− Water can transport fine clay particles to lower layers
Soil Profile: A series of distinct soil horizons (horizontal layers)
created by
1. Organic Activity
2. Leaching and Precipitation
3. Transport of Clays
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction
Characteristic soil profile:
O – Organic debris, dead leaves, plant and
animal remains make up 30% of this layer.
Usually at top. Why?
A – Topsoil, dark organics mixed with mineral
grains by organic activity. Lacking in fine
particles and soluble ions.
E – Subsurface layers that have lost most of their
minerals. Can be embedded in A horizon or
replace A horizon.
B – Ions leached from A are precipitated here.
Includes clay particles that were carried down
from A. Little organic material is present. Red
color due to oxidation (rainy areas), or
accumulation of calcium carbonate (arid areas)
forming a white layer.
C – Lowest layer, consists of soil parent material,
either weathered bedrock (regolith) or
unconsolidated sediments
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction
• Soil formation is controlled by
− The rock in the source area
− Temperature and amount of rain in a region (climate factors)
− Biological activity occurring in an area
Q: How might soil formation differ on bare rock surfaces
exposed in cold climates (e.g. Alaska) vs. warm, wet
climates?
A: In cold climates soil may take thousands of years to
develop due to slow rates of chemical weathering. In warm,
wet climates soils may develop in a few hundred years
owing to rapid chemical weathering. The thickest soils exist
in tropical regions that have year-round warm temperatures
and rainfall.
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction
• Soil fertility – thick soil is not necessarily fertile soil
− Fertility changes over time depending on leaching and replacement of
nutrients by weathering
− Heavy rainfall can carry away soil nutrients
− Example: rainforests have dense vegetation but the extreme rainfall
carries nutrients away, leaving only the top few centimeters of soil fertile.
There are thousands of
soil types based on the
texture and composition
of the soil. At right, a
map showing ten of the
twelve U.S. soil orders.
Which of the soil types
on the map would you
think are the most fertile?
Why?
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction Conceptest
How is the thickness of soil in a region
related to weathering?
a.
Weathering breaks down materials near the Earth’s
surface and therefore reduces the thickness of soil.
b.
Weathering increases the thickness of soil because
it provides more materials to be incorporated into
the soil.
c.
Soil thickness is dependent on the character of the
regolith and therefore is not related to weathering.
The Good Earth/Chapter 9: Weathering and Soils
Soils: An Introduction Checkpoint 9.18
From what you learned about geologic time
in chapter 8, approximately when did the first
regolith form on Earth? When did the first
soils form?
The Good Earth/Chapter 9: Weathering and Soils
Go back to the Table of Contents
Go to the next section: Soil Erosion and
Conservation
The Good Earth/Chapter 9: Weathering and Soils
Soil Erosion and Conservation
• Soil forms very slowly and is often depleted faster than it
can be formed
− Soil erosion rates are affected by climate factors (water and wind)
and land use practices
• Erosion of soil by wind and water occurs when soil
particles are detached from soil column and transported
away. This is controlled by:
− the amount and frequency of rainfall
− wind velocity
− character of the soil
− vegetation cover
− slope of the land surface
The Good Earth/Chapter 9: Weathering and Soils
Soil Erosion and Conservation
Examples of soil erosion features:
a. Rain can dislodge soil particles.
b. Water carves channels (rills) in a plowed field. Visible rills indicate an
erosion rate of at least 12 tons per acre.
c. Wind erosion – dust rises above a cultivated field.
d. A massive dust storm approaches a Great Plains town.
The Good Earth/Chapter 9: Weathering and Soils
Soil Erosion and Conservation
Each dot represents erosion of 200,000 tons of soil due to water (blue) and
The Good Earth/Chapter 9: Weathering and Soils
wind (red).
Soil Erosion and Conservation
Humans are now more important as agents in moving
soils and sediments than all other natural processes
operating on Earth combined.
Agriculture can accelerate soil loss:
-Poor soil management causes widespread soil erosion
-Highest erosion rates are in parts of Africa, South
America, and Asia where farming practices aren’t well
regulated and rainfall is abundant
-Lowest erosion rates are generally in Europe and N.
America where governments encourage preservation of
soils by good agricultural practices
The economic cost of soil erosion in
U.S. alone is estimated to be tens of
billions of dollars a year!
The Good Earth/Chapter 9: Weathering and Soils
Soil Erosion and Conservation
Many people are doing
their part to conserve
what we’ve got.
Erosion rates of soil
from U.S. cropland.
What
patterns/features do
you notice on this
graph?
U.S. farmland under
conservation tillage (methods
of conservation including
covering soils, limiting contour
plowing and terracing, and
ensuring a steady supply of
nutrients. The Good Earth/Chapter 9: Weathering and Soils
Soil Erosion and Conservation
Checkpoint 9.21
Examine the graph at
left.
Which conditions are
most likely to result in
wind erosion of soil?
Which conditions are
most likely to result in
water erosion of soil?
Explain your choices.
The Good Earth/Chapter 9: Weathering and Soils
The End
Go back to the Table of Contents
The Good Earth/Chapter 9: Weathering and Soils