7.5 Weathering Breaks Down Rocks

7.5
Weathering Breaks Down Rocks
PRESCRIBED LEARNING OUTCOMES
Page 198
Time
• explain how Earth’s surface changes over time
30–45 min
KNOWLEDGE
Key Ideas
• the features on the surface of Earth are formed by tectonic activity,
particularly at convergent, divergent, or transform fault tectonic
boundaries and by the processes of wind, water, and ice that wear
down surface features over time
Rock materials are broken
down into smaller pieces by
mechanical, chemical, and
biological weathering.
SKILLS AND ATTITUDES
•
•
•
•
•
• use analogies to visualize science concepts
ICT OUTCOMES
• work cooperatively using information technology tools
• use a variety of information technology tools to create, modify,
explore, and present electronic documents that express ideas or
concepts
• synthesize information from a variety of electronic sources for
presentations
• apply the principles of good design when developing electronic
documents
• develop interactive hypertext documents for presentations
NEL
Observing
Classifying
Making Inferences
Creating Models
Lesson Materials
of internal pressure produces microfissures, cracks, and flaking. Another
process is thermal expansion.
Temperature changes will cause
different minerals to expand and
contract at different rates. Also, the
surface of a rock will expand due to
heat more than the interior of the
rock. This constant heating and
cooling (and the resulting expansion
and contraction) can cause stresses,
small fractures, and surface flaking.
Alternate cycles of wetting and
drying can also cause expansion and
contraction within the rock, causing
stresses and flaking.
• Chemical weathering is the altering of
the minerals inside rocks by chemical
reactions, where elements may be
removed or added. Therefore, the
chemical composition of the
weathered material is changed. Water
in all its forms is the most important
agent in chemical weathering as it
can act as a solvent or as a chemical
reagent. One process of chemical
Chapter 7
weathering
mechanical weathering
ice wedging
chemical weathering
biological weathering
Skills and Processes
SCIENCE BACKGROUND
• Weathering is the breakdown and
alteration of rocks at Earth’s surface.
As the physical properties of rocks
vary widely among rock groups as
well as among the same rock type,
not all rocks nor the entire surface of
one rock will show signs of
weathering evenly.
• Weathering is the first step of many
other geological processes. For
example, the products of weathering
are a source of sediment for erosion
and deposition. Weathering
contributes to soil formation by
providing mineral particles, sand, and
clay.
• There are three categories of
mechanisms for weathering:
mechanical (or physical), chemical,
and biological.
• Mechanical weathering is caused by
natural forces from within or from
outside the rock. One process is ice
wedging or frost action. Water within
a rock freezes, causing it to expand
between 8 and 11 %. This increase
Vocabulary
per group
• ice-cube tray
• tap water
• sand
• foil
per group or teacher
• chalk
• glasses or cups
• tap water
• vinegar
per student
• paper for sketching
• pencils
Program Resources
SM 7.5 Weathering Breaks
Down Rocks
BLM 0.0-3 Venn Diagram
(comparing three Items)
Science Skills and Processes
Rubric 1: Creating Models
Science Skills and Processes
Rubric 2: Creating
Models—SelfAssessment
Nelson Science Probe 7
Web site
www.science.nelson.com
Old rocks can be recycled into new rocks.
375
Related Resources
Earth in Change: The
Earth’s Crust (video).
Mississauga, ON: Marlin
Motion Pictures Inc.,
1989. 1-800-865-7617
weathering is dissolution. Here,
minerals or parts of the chemical
composition of a mineral are dissolved
in water. For example, when carbon
dioxide gas dissolves in water, it
produces carbonic acid. Just a small
amount of this solution will increase
the corrosive effects of water, which in
turn can cause flaking, pitting, and
dulling of the surface of limestone and
marble, and can even cause the rock
to crumble. Oxidation or rusting is
another process. This occurs when
oxygen, helped along by moisture,
combines with iron-bearing minerals.
Rusting weakens and softens the rock,
making it more vulnerable to other
weathering processes.
• Biological weathering is the breaking
down of rocks because of chemical
and/or physical aspects of an
organism. Therefore the processes of
biological weathering can be chemical
or physical. For example, plants can
grow in soil deposited in cracks in
rocks. The roots of the plant expand
and increase the crack more.
Chelation is a chemical process that
results in the decomposition of rocks
by organic substances produced by
plants or animals. Lichens are an
example of an organism that produces
such substances. The guano produced
by large colonies of seabirds (e.g.,
gannets) contains organic acids that
can also cause chelation.
TEACHING NOTES
1 Getting Started
• Check for Misconceptions
– Identify: Students may not think of ice, water, wind, and plants as
agents capable of wearing down large natural formations such as
mountains.
– Clarify: The text provides familiar examples that illustrate how
these agents break down rocks and minerals. For example, students
are reminded of the force of expanding frozen water when a full
bottle of pop or water is left in the freezer, the force of the roots of
plants in cracks in rocks, and the effect of acid rain on gravestones
or building stone. While these are small-scale examples, students
should be reminded that these same processes are occurring
continuously on larger formations, such as mountains.
– Ask What They Think Now: Ask students to think about any
evidence they’ve observed that these weathering processes are taking
place over the short term (e.g., from their personal experiences or
from the three Try This activities) and then imagine the effect these
processes would have over tens of thousands of years.
• Read the first paragraph with students and have them look at
Figure 1. Ask them for other examples of natural or human-made
structures that can crumble or decay over time. For example, fences
rot, paint peels off houses, hillsides become rockier. Ask students
what they think causes these changes.
2 Guide the Learning
• Read the Learning Tip on page 198 with students, and have them
identify the three types of weathering.
• Use Reading and Thinking Strategies: Synthesize to guide the
reading and summarizing of the first page.
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Unit C: Earth’s Crust
NEL
• Continue with the same reading strategy for the section on
Mechanical Weathering.
• Consider modelling ice wedging by completely filling a plastic bottle
with water, screwing the lid on tight, and placing in a freezer over
night. Show students the result and ask them how this is a model for
ice wedging.
• Have students discuss the Learning Tip on page 199. Ask, Does this
information fit with what you already know about rocks? What new
things did you learn?
• Introduce Try This: Model Mechanical Weathering. After the activity,
ask students how this model is the same and how it is different from
weathering by glaciers. Students should see that this is a “quick” model
of a geological process that occurs over thousands of years.
TRY THIS: MODEL MECHANICAL WEATHERING
Purpose
• Students will demonstrate that the weathering of rock by glaciers is not caused
directly by the ice, but by the rocks that are trapped in the ice at the bottom of
the glaciers.
Notes
• This activity can be done in groups. Prepare the ice cubes in advance. Have
students double the foil as some students may press harder than needed to
cause grooves.
Suggested Answers
• The regular ice cube should leave no marks on the foil, but those with sand
should scratch/groove the foil quite easily.
• Continue with the same reading strategy for Chemical Weathering.
Have students use the Learning Tip on page 201 to explain to a
partner how the cave in the photo was formed. Before students
complete the Try This: Model Chemical Weathering, ask them to
predict what will happen and why.
TRY THIS: MODEL CHEMICAL WEATHERING
Purpose
• Students will model how acid precipitation can weather structures that contain
calcium carbonate.
Notes
• This activity can be done in small groups or as a class demonstration.
• It can take up to 6 h before you can see a noticeable reaction on the surface of
the chalk, or it can happen almost immediately, depending on the type/brand of
chalk used. The vinegar reacts with the chalk in the same way that acid rain
erodes statues.
• Have students discuss their observations as a class. If the activity was done in
groups, display all the chalk pieces and discuss similarities and differences in
their surfaces.
Suggested Answers
• The chalk in the vinegar should be broken down/dissolved to a much greater
extent than the chalk in water.
NEL
Chapter 7
Old rocks can be recycled into new rocks.
377
• Continue with the same reading strategy for Biological Weathering.
Assign Try This: Observe Biological Weathering as a before- or
after-school activity. As an alternative, ask students to find examples
of biological weathering on their way home or around their
neighbourhood. Ask students to think about the time required for
the effects of such weathering to become noticeable.
TRY THIS: OBSERVE BIOLOGICAL WEATHERING
Purpose
• Students will observe how living things can break down rock into smaller pieces.
Notes
• Students could also do their observations and sketches in other areas than the
schoolyard. This would provide more variety for comparison. Students may
consider monitoring a specific location to see if there are any observable
changes over a longer period of time.
Suggested Answers
• Typical observations might include lichens on the surface of rocks, plants
growing in sidewalk cracks, vines attached to the sides of buildings, and tree
roots growing in rock crevices.
• For students who need additional support with the reading in this
section, use SM 7.5 Weathering Breaks Down Rocks.
At Home
3 Consolidate and Extend
Students could look for
evidence of different
forms of weathering
around their homes, such
as plants (especially
dandelions) growing in
cracks in a driveway,
lichens growing on roof
shingles, or the paint on a
car affected by bird
droppings.
• As a class, make a list of cause-and-effect statements to show what
students learned about weathering. For example:
– Ice wedging causes cracks in rocks to widen and pieces to fall off.
– Sand and small pieces of rock carried by wind wear down the
surfaces of rocks.
– Fast-flowing water rubs rocks against each other, causing the
surfaces of the rocks to wear away and become smooth.
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Unit C: Earth’s Crust
• Assign the Check Your Understanding questions. Provide students
with BLM 0.0-3 Venn Diagram to answer question 1.
NEL
CHECK YOUR UNDERSTANDING—SUGGESTED ANSWERS
1. Answers will vary. A Venn diagram that shows the relationships among
mechanical, chemical, and biological weathering should have three circles with a
definition and examples of each kind of weathering in each circle. The parts of
the circles that overlap should include elements common to two or three kinds
of weathering. For example:
Chemical
Biological
• caused by chemical reactions
• happens as a result of
• water dissolves some rock
living things
materials
•
lichens
• carbon dioxide gas and
produce acid
water form weak acid
that weathers
that dissolves limestone
rocks
and makes caves
• acid rain damages
statues and buildings
• breaks down
• oxygen in air rusts
rocks
iron in rocks
• slow
• plants
• involve
process
grow in
water
rocks and
• non-living
help split rocks
forces
Social Studies
Connections
Students may learn about
the importance of
preserving and/or
restoring historical
structures (e.g., statues,
buildings) that have been
affected by weathering.
Students may also discuss
the impact of human
activities in producing
gases (pollution) that
cause acid precipitation,
and consider what can
and should be done to
reduce the production of
these gases.
Technology Connections
Mechanical
• caused by physical forces
• ice-wedging causes cracks to widen
• particles carried by wind wear down
surfaces
• rocks in fast-flowing water rub against
each other
• rocks in glaciers scratch surface
2. Water can be involved in both mechanical and chemical weathering. Water
causes mechanical weathering by freezing and thawing inside cracks in rock
surfaces. In time, the cracks can get bigger and pieces of rock can break off.
Fast-flowing water can cause rocks to rub against each other and smooth their
surfaces. Pounding waves of water can break up rock into smaller pieces. Water
causes chemical weathering when it contains an acid, like dissolved carbon
dioxide. This acidic water causes a chemical reaction that can dissolve rock such
as limestone or marble.
Have students work in
small groups to research
one of the kinds of
weathering, and then use
a computer graphics or
hypertext program to
create an animation of
the selected type of
weathering. Student
animations should explain
the causes and effects of
the weathering and
provide examples of
where this type of
weathering is typically
found, using both
animations and text.
3. Old gravestones can get weathered from chemical weathering (acid rain),
mechanical weathering (ice wedging), and biological weathering (lichen growing
on the stone).
4. Many human activities can increase the rate of weathering. For example,
humans put lots of carbon dioxide into the air from car exhaust, large factories,
and gas lawn mowers. This makes the rain more acidic and causes more
chemical weathering. Failure to repair cracks in sidewalks or buildings gives
plants a chance to grow in the cracks and widen the cracks even more.
NEL
Chapter 7
Old rocks can be recycled into new rocks.
379
7.5
Reading and Thinking Strategies: Synthesize
Weathering Breaks Down Rocks
LEARNING TIP
Preview the section and
read the headings. How
many types of weathering
will you be learning about
in this section?
An old cemetery can be an interesting place to visit. You can see
how small, slow changes make a big difference after many years. For
example, almost 200 years ago, the gravestone in Figure 1 was polished
and new. Today, the edges are chipped and the surface of the stone
has tiny holes. In a few more years, the writing will be worn away.
Eventually, the stone will crumble apart and disappear into the soil.
Figure 1
This old gravestone is starting to show wear. Compare it with a new gravestone to see
how rock weathers over time.
The process that slowly breaks down natural materials, such as
rocks and boulders, into smaller pieces is called weathering.
Weathering also breaks down human-made structures, such as roads
and buildings. Weathering can be caused by physical forces or by
chemical reactions.
The term “weathering” indicates that the changes to the rock
material are caused by the weather. Weather includes changing
temperature, wind, rainfall, and snowfall. Weathering slowly breaks
down all rock materials in contact with the air.
There are three kinds of weathering: mechanical, chemical,
and biological.
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Unit C
Earth’s Crust
Meeting Individual Needs
NEL
• Read the first page with the students and discuss the
information it provides. Ask questions to elicit from
students the information that this section is about
weathering; it explains what weathering is (gives a
definition of weathering) and gives examples of
weathering.
• Generate or revisit criteria for point-form notes. For
example,
– write in point form, not sentences
– use own words when possible
– stick to one main idea per point
– include only main ideas, important supporting details,
and important facts
– organize information in a way that makes sense
• Model with students how to summarize and organize
the important information about weathering in point
form under the headings Definition and Examples.
• Have students read the subsection Mechanical
Weathering, and guide them in writing notes to
summarize the content following the format for the first
section.
• Ask students to follow the same process for the
remaining sections.
Extra Support
• For students who need additional support with the reading in this
section, use SM 7.5 Weathering Breaks Down Rocks.
Extra Challenge
• Have students research weathering in different climates. For example,
they could compare the kinds of weathering that are common in a
desert climate, a tropical climate, and a northern or Arctic climate.
ASSESSMENT FOR LEARNING
What To Look For in Student Work
Suggestions for Teaching Students Who Are Having Difficulty
Evidence that students can
• describe how rock materials are broken
down by mechanical, biological, and
chemical weathering and give examples
• create models (e.g., mechanical weathering
by glaciers)
• make inferences based upon observations
(e.g., compare chalk soaked in tap water
and in vinegar)
• create products that are complete
(e.g., Venn diagram)
• use technical language correctly
(weathering, mechanical weathering, ice
wedging, chemical weathering, biological
weathering)
Ask questions after the reading of each section to help students focus on the
important information: for example, What is mechanical weathering?
How does ice wedging break down rock? How does wind break down rock?
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