Download Why is there Lithosphere

Why is there Lithosphere?
Original Source: Dr. Leslie Sautter (Dept. of Geology, College of Charleston)
Grade Level:
SC Standards:
5th and 8th Grade
National Standards:
Earth and Space Science – Content Standard D
5th Grade - IIIA2a,b,c
8th Grade - IIIB1a,b,c,d,e,f,g
.
Focus Questions
Materials
How does the outer layer of the earth
known as the lithosphere fracture under
pressure, as compared to the asthenosphere
that underlies it?
Why does the upper mantle have a different
texture from the mantle below it?
Objectives
For each group of students:
copy of the STUDENT PAGES
2 room-temperature plain chocolate candy
bars, such as Hershey’s
1 rectangular graham cracker
pencil
For the teacher:
overhead transparency copies of
Figures 1, 2, and 3.
Students will:
examine the effects of temperature on the
mantle’s texture.
use their observations to interpret how
mantle texture relates to the response to
stresses.
understand why the upper mantle is part of
lithospheric plates, whereas the underlying
asthenosphere mantle behaves differently.
Time Frame
20-30 minutes
Suggested Learning Environment
Collaborative learning groups of 3-5, depending
on amount of materials available
Relevant pages in Of Sand and Sea
Key Words
lithosphere
asthenosphere
rigid, brittle
plastic
lithospheric plates
upper mantle
crust
tectonic layers of the Earth
Chapter I, The Ocean Planet, pp. 7-9
Teacher Preparation
Prior to conducting this activity, students
should review the structure of the earth’s
layers: crust, mantle, outer core, and inner core.
This activity may be done as a demonstration, or
if enough materials are available, students may
explore together in small groups. Check for any
students with chocolate allergies. Instructions
are for small groups of 3-5 students.
COASTeam Program, Project Oceanica, College of Charleston
Why is there Lithosphere?
TEACHER PAGES
NOTE: For this activity, the “crust” represents
only the oceanic crust. The focus of the activity is
to study the effect of temperature on the mantle’s
texture.
The reason the upper mantle is included in the
lithosphere is that it is colder and more rigid than
the mantle beneath it, so that when stresses occur
at or near the Earth’s surface, the upper mantle
fractures the same way that crust fractures. They
break together as a single layer. This layer forms a
fractured piece of the outer earth which we refer
to as a lithospheric plate.
Refer to Figures 1-4 to understand the set-up of
the model and how stress should be applied.
Students will conduct the activity by following the
steps listed in the STUDENT PAGES. The
procedure follows, and notes for the teacher are
bulleted (diamond-shaped bullet), in blue italics.
Procedure
Students should follow the steps below, found on
the STUDENT PAGES, while taking notes in their
science notebook. Information for the teacher
is provided in blue italics.
Assessment questions are included for students to
answer in a science notebook or on separate pages
to hand in. Guidelines for evaluating student
answers are included.
1.
Teacher Background Information
This activity is a good introduction to the structure
and texture of the Earth’s lithosphere and
asthenosphere (Fig. 4). It demonstrates that the
lithosphere is brittle (i.e., rigid), and when stressed,
fractures into pieces we refer to as plates, or
lithospheric plates. The lithosphere is located on
the outer portion of the earth, on top of the
asthenosphere. Because the lithosphere is above
the asthenosphere, it is much colder, and behaves
differently to applied stresses. Even though the
warmer asthenosphere is solid, when stresses are
applied it has the ability to flow (i.e., it has a
“plastic” or non-rigid nature), and does not fracture
as readily as the lithosphere.
Your teacher will provide each student group
with two small, square chocolate candy bars and
a graham cracker. Do not eat the materials
(yet!), and leave the wrappers on the chocolate
bars.
A volunteer must agree to sit on one of the
chocolate bars (wrapper on) for about 10
minutes.
a.
What is the purpose of sitting on one of the
candy bars?
 Sitting on the candy bar will warm it up.
b. Why don’t we just heat the candy bar in the
microwave?
 Sitting on the candy bar will not melt it.
2. Before removing the candy bar from under the
volunteer student, consider how these three
items (cracker and two chocolate bars) can be
arranged to model the outer portion of the
earth (i.e., excluding the outer and inner core).
Remember that the two chocolate bars have the
same composition.
Use this activity to reinforce the concept of the
tectonic layers of the Earth, namely that the two
top tectonic layers are the lithosphere and the
asthenosphere. The lithosphere consists of the
crust and the uppermost portion of the mantle,
referred to simply as the upper mantle.
lithosphere = crust + upper mantle
a.
Draw your model in your science notebook.
Shade and label the different layers,
according to the different compositions.
COASTeam Program, Project Oceanica, College of Charleston
2
Why is there Lithosphere?
TEACHER PAGES
may wish to draw it on the board or show it
using an overhead projector.
3. The cracker represents the earth’s oceanic
crust and both chocolate bars are the mantle –
because they have the same composition.
a.
6. After about 10 minutes, the chocolate bar
should be warm enough for the activity. With
clean hands, unwrap both chocolate bars and
construct your proposed model.
Of the three layers, where would the
warmest layer be located?
 The bottom layer would be warmest.
READ THIS NEXT PARAGRAPH BEFORE YOU
BEGIN TO APPLY STRESS TO YOUR MODEL!
b. Why?
 The earth gets hotter with increasing
This is very important! Project Figure 2 and
describe the next steps to the students.
depth. (This is known as geothermal heat.)
c.
Your teacher will show on the overhead projector
the proper way to work with your model. One
student should hold the model in two hands in front
of him/her so everyone in the group can observe.
Slowly and carefully apply downward stress to the
edges and upward stress to the center.
When any portion of the model breaks – or
fractures – STOP applying the stress.
Where should the room temperature
chocolate bar be located?
 It should be above the warm chocolate bar
and below the crust. (See Figure 1)
4. The room temperature chocolate bar represents
the uppermost portion of the mantle, or the
“upper mantle.” The warm chocolate bar will be
referred to as a portion of the asthenosphere.
The asthenosphere is also part of the mantle.
So, now you know what each of the 3 layers of
your model represents.
a.
a.
 If the model worked properly, the upper
Revise and label your initial drawing to
reflect these new observations. Labels
should show the earth layer represented
(not the type of candy bar).
two layers (graham cracker “crust” and room
temperature chocolate bar “upper mantle”)
will have fractured, but the warm chocolate
bar “mantle” will not have fractured, but will
have bent.
 Students should label the crust, upper
mantle and mantle as in the Figure 1.
7. Use your observations of the model to answer
the following questions.
5. The lithosphere is the outer portion of the
Earth comprised of the crust and upper mantle.
a.
Draw what you observe in your science
notebook. Label this drawing with
appropriate terms for the layers and title it
as “Post-Stress.”
a.
Which layer or layers represent(s) the
lithosphere?
 The graham cracker (crust) and room
temperature chocolate bar (upper mantle)
are the lithosphere.
Which Earth layer or layers fractured when
stress was applied (NOTE: Use the Earth
layer terms, not “cracker” or “chocolate
bar”)?
 The crust and upper mantle fractured.
b. Which layer or layers did not fracture?
 The warmer mantle did not fracture..
b. Place a bracket } on your drawing next to
the layers that represent the lithosphere.
 See Figure 1. Note that students do not
c.
need to receive a copy of Figure 1, but you
Which layer or layers would you consider to
be rigid and brittle?
COASTeam Program, Project Oceanica, College of Charleston
3
Why is there Lithosphere?
TEACHER PAGES
 The crust and upper mantle are rigid, or
 Because the crust and upper mantle are both
brittle. The lithosphere is rigid/brittle.
brittle, they fracture together as a unit. This
unit is a lithospheric plate.
8. The term plastic texture of solid rock means
that it has the ability to move or flow when
stress is applied. In other words, the rock is
not liquid, but it is still able to flow, because it
has a plastic texture. The layer of the mantle
that is plastic is called the asthenosphere. The
top of the asthenosphere lies approximately
between 100 and 200 km below the earth’s
surface. Geologists are uncertain where the
base of the asthenosphere is located, but
estimates are that it extends to a depth of
between 600 and 700 km. In other words, the
asthenosphere is NOT the remainder of the
mantle, but is just the hot, plastic portion of
the mantle below the lithosphere.
a.
10. Stress causes fracturing of the brittle
lithosphere. These broken pieces of lithosphere
are called plates, or lithospheric plates.
Continued stress (and other factors that won’t
be described here) cause plate motion. Because
of the rigid nature of the lithosphere, plates
move independently of the underlying plastic
asthenosphere.
a.
 There are two lithospheric plates in the
model after the upward stress fractured it
into two pieces.
Which layer or layers would you consider to
be the asthenosphere with its plastic
texture?
11. Review figures of the lithosphere and
asthenosphere from texts, web sites, or CDROMs and compare them to your model.
 You may wish to project Figure 4, or draw a
would be the asthenosphere.
 Tell students they have only the oceanic crust in
 The warm mantle (warm chocolate bar)
similar diagram.
their model.
b. Add the term asthenosphere to your
diagram.
c.
How many lithospheric plates are illustrated
in your Post-Stress drawing?
12. Discuss the results of applying stress on the
lithosphere and asthenosphere, based on your
observations of the model.
 Be sure the discussion include the appropriate
Compare your group’s results with results of
other student groups. Were any other
results different?
terms for the layers, not the model’s terms
(cracker and chocolate).
 Student results may vary, as with any
scientific experiment. Hopefully many
students will have the “expected results.”
 If there is a warm chocolate bar still
available, You may want to show how plastic
the chocolate is when it’s warm by molding it
into different shapes, as shown in Figure 3.
Of course, once the chocolate has cooled it
will become brittle again.
13. If your teacher gives permission, and you have
been handling the model with clean hands, it is
now okay to eat the Earth model.
 Certainly, this is optional! You may wish to have
unused chocolate bars available!
Summarize your observations and discoveries by
answering the questions on the next page.
9. Why do you think geologists group the crust and
upper mantle together as the lithosphere, when
each layer is made of different materials?
COASTeam Program, Project Oceanica, College of Charleston
4
Why is there Lithosphere?
TEACHER PAGES
Questions (Assessment)
1.
Draw a side-view (called a cross-section) of the lithosphere and asthenosphere. Label the tectonic
layers. Also, label the individual layers of the cross-section.
 Use Figures 1 and 4 to assist with the assessment of student answers.
2. Why does the upper mantle fracture differently from the mantle below it?
 The upper mantle is relatively cold, as compared to the mantle that is below it. The deeper,
hotter layers bend and flow when stress is applied. This is called plastic texture. The colder
upper mantle is rigid and brittle and fractures when stress is applied.
 Note: The earth is hotter at increasing depths due to geothermal heat.
3. How do the differences in texture of the mantle and the crust relate to the formation of lithospheric
plates?
 The crust and upper mantle are both rigid and brittle. They fracture together when stress is
applied. This is the lithosphere. The hotter mantle below flows due to its plastic nature.
Therefore, the lithosphere is broken and can move as a unit, called the lithosphere.
4. Are the upper mantle and mantle below it the same composition?
 Yes, they are the same composition (which is why the same chocolate bar is used). They behave
differently under stress because they are different temperatures.
5. Write a one-paragraph summary of how the different textures of the mantle affect the formation of
lithosphere and asthenosphere. Include why the mantle has different textures.
 Students should incorporate the information from questions 1-3 in this paragraph. The key is in
understanding that the composition is the same, but the different temperatures cause different
textures – either rigid/brittle or plastic. The rigid/brittle mantle breaks, whereas the plastic
mantle flows.
Source: Dr. Leslie Sautter, Project Oceanica, Dept. of Geology and Environmental Geosciences, College of Charleston, SC.
website: http://oceanica.cofc.edu email: [email protected] phone: 843-953-5586
Funding for the COASTeam Program was provided by the South Carolina Sea Grant Consortium. http://oceanica.cofc.edu/coasteam/
COASTeam Program, Project Oceanica, College of Charleston
5
Why is there Lithosphere?
TEACHER PAGES
oceanic crust (room temp. cracker)
upper mantle (room-temp. chocolate bar)
lithosphere
asthenosphere (warm chocolate bar)
Figure 1. Diagram of the graham cracker and chocolate bar model. Each layer is labeled
appropriately and the crust + upper mantle are indicated as the lithosphere.
Figure 2. (a) Stress should be applied to the model as indicated by the arrows. (b) If the model works
properly and the student stops applying stress as soon as any fracturing occurs, the upper two layers (the
cracker + room temperature chocolate bar) should break, but the warm chocolate bar should not break.
COASTeam Program, Project Oceanica, College of Charleston
6
Why is there Lithosphere?
TEACHER PAGES
Figure 3. The teacher may want to demonstrate the plastic nature of the “asthenosphere,” as modeled
by the warm chocolate bar. It can be molded and shaped without breaking by adding stress.
Figure 4. The layers and textures of the lithosphere, both oceanic lithosphere and continental lithosphere,
are indicated. (Figure from Of Sand and Sea, by P. Keener-Chavis and L. Sautter, p. 8)
COASTeam Program, Project Oceanica, College of Charleston
7