Download Sea Floor Spreading, Thomas

UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
Sea-floor Spreading
Developed by: K. Thomas, A. Knudson & J. Ta.
Teacher Material
California State Standards
 Earth Sciences, 3c. Students know features of the ocean floor (magnetic
patterns, age, and sea-floor topography) provide evidence of plate tectonics.
 Investigation and Experimentation, 1d. Formulate explanations using logic and
evidence.
Synopsis
The students have since learned about the Earth’s layers, plate boundaries, and how all
of the continents originated in Pangaea and drifted to their current location. They also
have reviewed convection currents and understand that is one way plates move. Here,
the students study the second mechanism for plate movement; they study sea floor
spreading through the context of discovering why the Atlantic Ocean is expanding using
observations of the features of the ocean floor. Students review the importance of using
evidence when making theories and are provided with a model that simulates the
events on the ocean floor.
Objectives
1. Students will analyze and synthesize the evidence provided in the activity to
develop a theory to explain why the Atlantic Ocean is expanding.
2. Students will construct models to demonstrate sea-floor spreading.
3. Students will become more familiar with the scientific method.
4. Students will develop writing skills by completing the required sections for the
activity and responding to the email.
5. Students will develop inquiry skills by analyzing, summarizing, and discussing
evidence.
Suggested Timeline
I. Why is the Atlantic Ocean Expanding? (35 minutes)
In groups:
- Students complete the activity that begins with the USGS email. They
develop a theory to explain why the Atlantic Ocean is expanding based on
the evidence found in the activity.
- Students construct models to demonstrate sea-floor spreading.
II. Discussion (15 minutes)
As a class:
- The teacher conducts the discussion (see suggested questions, below)
which reviews the questions at the end of the activity by asking similar
questions in different ways.
III. Homework
For homework:
Page 1 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
- Students respond to the USGS email by writing an explanation for why
the Atlantic Ocean is expanding, summarizing the evidence from the
activity.
IV. Warm-up (5 minutes)
Use the provided warm-up handout to start class the following day.
Materials
Handouts:
 Why is the Atlantic Ocean Expanding? (1 per person)
 A Model of Sea-Floor Spreading (1 per group)
 Homework (1 per person- hand out at end of class)
 Warm-up (1 per person- hand out the following day)
Model (for each group):
 Paper or poster board (8 ½ x 11 in.)
 Template A
 Ruler
 Scissors
 Tape
Teaching Tips
 In addition to the provided handouts and listed materials, the teacher will also need
to prepare maps that indicate the age and polarity of the rocks around the mid-ocean
ridge (see “Why is the Atlantic Ocean Expanding?”). Maps that include revised
versions of the following figures are suggested:

Introduce the “Why is the Atlantic Ocean Expanding?” activity to the class. While the
groups are working, walk around the class and help the students analyze and
synthesize the evidence. Once they complete the questions, the group should go on
to construct their model using “A Model of Sea-Floor Spreading”. After the class has
completed and labeled their models, direct the discussion (below).
Discussion Details
Page 2 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
Conduct this discussion after they have made and labeled their models. Referring to a
sea-floor spreading diagram throughout the discussion may help.
What is the one way that we have discussed thus far that explains plate
movement?
Convection currents moving plates that are floating on top of the asthenosphere
(mantle).
Another way is through something called sea-floor spreading.
Based on what you learned during the activity, what’s a good definition for sea
floor spreading?
Sea-floor spreading is the expansion of the ocean floor through the creation of
new crust.
Where is this new crust formed?
The mid-ocean ridge.
How is new crust formed?
At the mid-ocean ridge, molten material from the mantle comes out of the ridge,
cools, hardens, and becomes new crust.
What happens to the old crust that was originally near the mid-ocean ridge?
The new crust spreads out and pushes the old rock to the side- it is a continuous
process.
What do you think is causing the old crust to move towards the deep-ocean
trench?
New crust and convection currents.
If the ocean floor did not spread apart or move, what would you expect to happen
near the mid-ocean ridge as the magma is coming from the mantle?
It would pile on top of each other like a mountain.
What is some evidence near the mid-ocean ridge that helped you determine that
the ocean floor was expanding and spreading apart?
The age of the rock nearest the ridge was very young.
What can you tell me about the ages of the rock?
If you test the age of the rock near and around the mid-ocean ridge, you’ll find
that the rock is the youngest near and on the mid-ocean ridge, while the rock
farthest away is older.
What can you tell me about the polarity of the rocks around the mid-ocean ridge?
Page 3 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
The polarity of the rocks changes according to the position of the Earth’s
magnetic field when the rocks were formed. The polarity of the rocks at the ridge
have the same polarity as the Earth does today.
What effect do you think sea-floor spreading has on the size of the ocean floor?
It causes it to expand because it produces more crust.
Where does all of this crust go? Does the ocean keep getting bigger? What is
this process called? What is it?
Subduction – the process by which ocean floor sinks into a deep-ocean trench
and back into the mantle.
Deep-ocean trench – occurs at the boundary between two plates where the
oceanic crust bends downward underneath another plate.
What do you think happens when the crust enters the deep-ocean trench?
Gravity pulls the denser, older crust into the mantle, where it melts.
What effect do you think subduction has on the size of the ocean floor?
It prevents it from expanding quickly because the ocean floor sinks back into the
mantel.
We know that the Atlantic Ocean is continuously expanding. What’s stopping it
from expanding and taking over the entire world?
Subduction.
Imagine that you are a cashier in a supermarket. After you have scanned each
item, you put the item on a conveyor belt and send it to the bag boy who then
puts all of the items in a bag.
What would happen if you started to put food on the belt at a faster rate than the
bag boy could put the groceries in the bags?
The food would pile up on the conveyor belt.
There is a balance between the amount of crust that’s being made at the midocean ridge and the amount of crust that’s being subducted into the mantle.
This is why the Atlantic Ocean is not expanding at the same rate that the new
crust is being made.
So, let’s follow our analogy – What happens if the mid-ocean ridge creates more
crust than can be swallowed by deep ocean trenches? What do you think is going
to happen to the size of the ocean floor?
It’s going to expand.
The reason that it is not swallowing as much is because there are fewer
trenches.
Page 4 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
What effect do you think sea-floor spreading has on the position of the
continental plates?
As the ocean-floor expands, it carries the plates with them.
What happens if the deep-ocean trench swallows up more ocean floor than can
be formed at the mid-ocean ridge? What do you think is going to happen to the
size of the ocean floor?
The ocean floor will shrink (Pacific Ocean).
Page 5 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
(STUDENT HANDOUT BEGINS)
Name ______________________________ Period: ___________ Date: ___________
Why is the Atlantic Ocean Expanding?
You receive an e-mail with the subject, “The Atlantic Ocean is expanding!” Thinking this
e-mail is very strange, you begin to read it:
Dear Earth Science Student,
According to our research at the United States Geological Survey (or
USGS), the Atlantic Ocean is getting bigger, and we don’t know why! There
is something “strange” happening on the ocean floor near the mid-ocean ridge.
We heard that you are currently studying plate tectonics in earth science
class, and we were hoping that you’d be able to help us out.
John Johnson
Head Scientist, United States Geological Survey
Since you have been learning about plate tectonics, you decide to offer your help. You
borrow a deep-sea exploring vehicle called Alvin to explore the mid-ocean ridge located
in the middle of the Atlantic Ocean. The mid-ocean ridge is the largest mountain range
in the world and is formed when two oceanic plates are diverging in the middle of the
Atlantic Ocean.
Page 6 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
On your trip you make the following observations:
1. When you reach the mid-ocean ridge, you see signs of volcanic activity. The
magma seems to come out of the mid-ocean ridge, cool, and form new rock.
2. Looking at the plate boundary between North America and the Atlantic Ocean,
you see the ocean floor entering a deep ocean trench beneath the continental
crust of North America! As you recall from our plate boundaries activity, this
process is called subduction. The location where subduction occurs is called a
subduction zone.
Using the provided maps, make two additional observations:
1. Examine the map indicating the age of the rocks around the mid-ocean ridge.
What pattern(s) do you notice?
2. Examine the map that indicates the polarity of the rocks around the mid-ocean
ridge. What pattern(s) do you notice?
Page 7 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
Using your observations, answer the following questions:
What do you think is causing the ocean floor to increase in size?
Why do you think the rock farther from the mid-ocean ridge is older than the rock near
the mid-ocean ridge?
How do you think subduction affects the size of the Atlantic Ocean?
What do you think is going to happen to the crust once it enters the deep ocean trench?
Why is the Atlantic Ocean NOT expanding at the same rate that the new crust is being
made? (It is increasing in size, but the rate of growth is MUCH LESS than the rate that
new rock is formed at the mid-ocean ridge. How can you explain this?)
(STUDENT HANDOUT ENDS)
Page 8 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
(STUDENT HANDOUT BEGINS)
A Model of Sea-Floor Spreading
The creation of new sea-floor at mid-ocean spreading centers (ridges) and its
destruction in subduction zones is one of the many cycles that causes the Earth to
experience constant change.
The purpose of this activity is to make a simple model that shows the recycling of
oceanic crust through sea-floor spreading and subduction.
Materials
1 piece of paper or poster board 8 ½ x 11 in.
1 Template A
ruler
scissors
tape
Procedure
1. Place the long side of the paper towards you.
2. Draw a vertical line in the middle of the paper with a height of 11.5 cm, leaving
5cm on either side of the line. See figure1:
3. Draw a second vertical line of the same length to the right of the center line so
that it lies 3cm from the right edge of the paper.
4. Draw a third vertical line the same way on the other side of the paper.
5. Cut the vertical lines, without cutting the edge of the paper, so you have three
slits in the paper all the same height.
6. Using the Template A, cut two strips of paper by dividing the template in half
lengthwise. See figure 2:
7. Insert one end of each strip of paper through the middle slit of your first piece of
paper.
Page 9 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
8. Pull each strip towards the side slits nearest the margins of the paper. Pull
through the slits and then tape the strips to make two loops. See figure 3:
9. Circulate the loops of paper to simulate what happens during sea-floor
spreading.
You now have a working model of sea-floor spreading!!
Looking at the figure below,
1. label the following features of your model:
a. Subduction zone
b. Oceanic ridge
c. Oceanic crust
d. Deep-ocean trench
2. complete the labeling of:
e. the magnetic polarity of the rocks
f. the age of the rocks
3. Have your answers to the questions changed after constructing the model?
Why do you think it is important to construct models?
Discussion/Thinking Questions:
While the Atlantic Ocean is expanding, the Pacific Ocean is shrinking. From what you
have learned, how can you explain why the Pacific Ocean is shrinking?
What is the relationship between convection currents in the asthenosphere and the
movement of the oceanic crust? (STUDENT HANDOUT ENDS)
Page 10 of 12
UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools
http://www.nslc.ucla.edu/STEP/GK12/
(STUDENT HANDOUT BEGINS)
Name ______________________________ Period: ___________ Date: ___________
Using what you have learned, respond to the United States Geological Survey’s e-mail.
Dear Earth Science Student,
According to our research at the United States Geological Survey (or
USGS), the Atlantic Ocean is getting bigger, and we don’t know why! There
is something “strange” happening on the ocean floor near the mid-ocean ridge.
We heard that you are currently studying plate tectonics in earth science
class, and we were hoping that you’d be able to help us out.
John Johnson
Head Scientist, United States Geological Survey
(STUDENT HANDOUT ENDS)
Page 11 of 12
Teacher Material
Sea-floor Spreading
Kelly J. Thomas, GK-12 Fellow, 2004-2005
Science and Mathematics Inquiry
(STUDENT HANDOUT BEGINS)
Name: _____________________________Date: ____________ Period: ___________
Sea Floor Spreading
List the four main steps of sea floor spreading:
1.
2.
3.
4.
(STUDENT HANDOUT ENDS)
Page 12 of 12