Original Source: Dr. Leslie Sautter (Dept. of Geology, College of Charleston)
8 th
IIIB3a,b,c,j,k; IIIB4c
Earth and Space Science Content Standard D
Why isn’t the entire seafloor the same age?
How can seafloor age increase as distance increases from a mid-ocean ridge?
What do the age differences of the ocean floor tell us about motion of the seafloor?
Students will: calculate and graph the variable rates of seafloor spreading through time, using a seafloor model. learn the concept of seafloor spreading. describe the processes involved in creating new seafloor at a mid-ocean ridge.
mid-ocean ridge magma, molten rock basalt, igneous rock, mafic oceanic crust fissure plate, lithosphere divergence, divergent plate boundary plate rate seafloor spreading tensional forces
Per student group: photocopies of STUDENT PAGES. sheet of white paper (from banner roll),
~150 cm long, and at least 40 cm wide corrugated cardboard. One edge of the cardboard should be the same width as the white paper. The length of the other edge should be at least 20 cm, but does not have to be any specific length. tape (any kind) scissors metric measuring device crayons, paint, or markers – 6 colors (red, orange, yellow, green, blue, violet) graph paper (divisible by tenths)
1 cardboard tube, cut to the width of the seafloor model’s paper (at least). Mailing tubes or wrapping paper tubes work well. small sample of basalt (if available)
For the Teacher: overhead transparencies of Figures 1-4 from TEACHER PAGES overhead transparencies of Tables 1-6 and
Graph 1 from STUDENT PAGES overhead transparencies of Tables 1-6 and
Graph 1 from Teacher Answer Key
(optional), from TEACHER PAGES
One to two 50-minute class periods, depending on how much of the construction of the seafloor model and graphing is assigned as homework.
Collaborative learning groups of 2-4 are recommended, depending on amount of materials available. At least two complete models (i.e., constructed by two collaborative groups) will be required for later portions of the activity.
Chapter I, The Ocean Planet, pp. 9-13
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TEACHER PAGES
Students will construct a model of a fictitious ocean that is 4,600 km wide and 180 million years old.
Using this model, students will calculate halfseafloor plate rates as well as full-ocean seafloor spreading rates based on the data provided. They will also graph and summarize their results. The seafloor spreading rates that will be calculated are similar to rates found in several ocean basins, including the Atlantic and Indian Oceans.
This activity may be confusing unless the teacher has reviewed the material in advance and guides the students through the process.
Students should be familiar with converting from kilometers (km) to centimeters (cm). The teacher will need to review the conversion prior to conducting the activity.
Students should also understand how to use the seafloor model’s scale to calculate the distances of the ocean floor the model represents (see Table 2).
The steps that follow are included in the STUDENT
PAGES. Additional information on the TEACHER
PAGES is included in blue italics .
Part I: Constructing the Seafloor Model
(The teacher may assign a portion of the construction as homework to save class time.)
1.
Your teacher will show on the overhead projector an illustration of the seafloor model for you to use as a guide for creating the model
(Figure 1). Look at Table 1 and use the data from the “Width of Color Band on Paper”
(column a) to mark and color one of the long sheets of white paper provided by your teacher.
Borders between colors do not need to be perfectly straight as shown in the figure, but they should be perpendicular to the paper’s long edge.
 Project Figure 1 to help guide the students.
 Here is where students could be assigned the time-consuming coloring duties as homework.
2.
Label each boundary between color bands with the appropriate rock age in millions of years
(m.y.). Widths of the color bands vary (some are narrow, some are very wide).
Using units of m.y.:
Units of “million years”, or “m.y.” are used to avoid having long numbers with 7 or more digits, as with 1,000,000 yr (or “one million years”).
For example 23,000,000 yr would be expressed as 23.00 m.y., and
131,530,000 yr = 131.53 m.y.
What is 46.78 m.y. expressed in “yr”?
46.78 m.y. = ______________ yr


The answer is 46,780,000 yr.
Be sure that students understand how to use the units of “m.y.”
3.
Trim the paper about 5-7 cm beyond the edge of the violet color band, so that the total length of the paper is about 100 cm, or 1 m in length
(including the white edge at the end).
4.
Cut the paper down the middle of its length , so that you have two striped strips that are nearly identical to one another. Each piece represents a “half-seafloor.”
 Be sure students cut the paper along its length.
5.
Cut the corrugated cardboard into two pieces so that one edge of each piece is equal to the
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Investigating Seafloor Spreading
TEACHER PAGES striped paper’s width (Figure 1, on the overhead projector).
 Figure 1 should be projected again.
6.
Tape the cut edge of a cardboard piece to the white end of one paper sheet. The paper should be taped under the cardboard to hide the white edge, so that the violet color band is adjacent to the edge of the cardboard.
Repeat for the other piece of cardboard and striped paper.
7.
Cut the cardboard tube so that the two pieces are the widths of the half-seafloors. Tape the other red end of each striped paper to a cardboard tube (refer again to the projected figure).
What does your model represent?
You have now created two “half-seafloors” that are nearly identical to one another. The color bands indicate seafloor of varying ages (see Table 1).
Each color band represents rocks with ages that range over a segment of time. For example, the
YELLOW color band is made of seafloor rock with ages ranging from 40.10 to 67.70 million years old.
The cardboard pieces represent continents and the cardboard tubes, are located at the model’s midocean ridge. When the two half-seafloors are laid out t ogether, with the mid-ocean ridge in the middles and the continents on the ends, they will represent a complete “slice” of an ocean basin that is bounded by continents.
8.
On a long table, or the floor, arrange the model
“seafloor” so that the striped sheets of paper lie end to end, with the mid-ocean ridge in the middle and the continents at the edges (see

Figure 2, on the overhead projector).
Project Figure 2 to help guide the students.
When oceanographers began to sample the seafloor and determine rock ages, mid-ocean ridge rocks were dated to be less than 1 million years old (1 m.y.). The type of rock collected was basalt .
9.
Your teacher may provide you with a sample of basalt. Basalt is a black igneous rock with small crystals that can rarely be seen by the naked eye. Examine the sample provided by your
 teacher.
If students have not yet studied igneous rocks, you may opt to skip the questions associated with this step. a.
How is an igneous rock formed?

Igneous rocks are formed by the cooling and crystallization of a magma. b.
What do the small crystal sizes of basalt
 tell you about how it cooled?
Small crystal sizes indicate that the magma cooled rapidly, preventing large growth of crystals. c.
What does the dark color of basalt tell you
 about its composition?
Igneous rocks that are rich in iron (Fe) and magnesium (Mg) are typically dark gray to black. We call these rocks mafic igneous rocks.
As rocks were collected farther from the ridge, scientists discovered that basalt ages were increasingly older. Eventually, enough data were collected to construct a map similar to the striped paper model.
10.
As a class, decide what to name your ocean basin, and label your seafloor model. Everyone in the class has a “slice” of the same ocean

 basin.
This step is useful for the evaluation portion of the activity, when students compare their model ocean basin to the Pacific Ocean.
Remind students that the earth has one ocean, but there are several ocean basins.
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TEACHER PAGES
Part II: Plate Rates and Seafloor Spreading
The seafloor of any ocean basin is the upper layer of a plate . New seafloor – or oceanic crust – is formed as the plate moves away from a fissure , or large, elongate fracture in the seafloor. As the fissure widens, magma rises up from the underlying mantle onto the seafloor. The hot magma quickly cools when it comes in contact with the cold seawater, and crystallizes into solid rock along the fissure. The mafic composition of the magma produces a mafic igneous rock with very small crystals (due to the rapid rate of cooling), forming basalt.
The motion of two plates moving away from each other at mid-ocean ridges is called divergence , and results in a divergent plate boundary . This motion is called seafloor spreading, named by the two oceanographers who first studied it, Harold Hess and Donald Dietz.
The rate that a plate moves, or the “ plate rate ,” can be determined using the formula for velocity:
R p
= D/T where:
p
But there are two sides to this story! Divergent plate boundaries have seafloor on both sides of the mid-ocean ridge. In your model ocean, the two sides of the ridge are mirror images of one another, so by doubling the rate of one half-seafloor, the rate of divergence, or the seafloor spreading rate of the entire ocean basin can be calculated:
R ss where:
= 2(R p
)
R ss
R p
It is important to know that not all plates have the same rate of motion , so scientists calculate the rate of each of the two half-seafloors on either side of a mid-ocean ridge, then add them together, rather than doubling one half-seafloor’s rate.
 Emphasize to students that the two plates on either side of a mid-ocean ridge often have different rates, but their model was constructed so that the two half-seafloors have identical rates.
11.
Complete Table 1 (column d) by calculating the
“Total Seafloor Age” represented by each of the 6 color bands of your model. These ages are expressed in units of m.y.
For example, the YELLOW color band total age
 is 67.70 m.y. - 40.10 m.y. = ________ m.y.
Refer to Teacher Answer Key, Table 1.
12.
Using the information provided in Table 2, calculate the model’s scale in the form:
1 cm = ___ km.
This scale means that 1 cm on your model is equivalent to _______ km in the ocean it represents.
Fill in all the blanks on Table two with the value you calculate. The scale’s conversion factor
 you will use in Table 3 is ____km/cm.
Refer to Teacher Answer Key, Table 2.
13.
Using the scale’s conversion factor from Table
2, calculate the distance represented by each color band, and complete Table 3 (column b).
Complete column c by adding the distance of each color band to the distance from the previous line. In other words, this number is a cumulative distance from the mid-ocean ridge.
 Refer to Teacher Answer Key, Table 3.
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TEACHER PAGES
14.
Transfer the data from Tables 1 and 3 to the appropriate columns on Table 4 (as indicated).
Then, calculate the Half-Seafloor Plate Rate for each color band, completing column c. Note that rates in Table 4 should be expressed as km/m.y.
 Refer to Teacher Answer Key, Table 4.
15.
Because seafloor motion occurs at a rate similar to the growth of fingernails (and faster!), oceanographers most often use the units cm/yr.
Thus, the data from Table 4 must be converted so that the units are cm/yr. For example, if you have calculated a rate of 40 km/m.y., the conversion would be as shown below.
40 km =
1 m.y. 1,000,000 yr 1,000,000 yr 1 yr
Answer: 40 km/my = 4 cm/yr


40,000 m = 4,000,000 cm = 4 cm
Students may need assistance with the conversion from km/m.y.
to cm/yr , as shown in the example above.
Remember that there are 1000 m in a kilometer and 100 cm in a meter. a.
Convert your data from Table 4 to complete

Table 5 (columns a, b, and c).
Refer to Teacher Answer Key, Table 5.
16.
For the model seafloor in this activity, the
“seafloor spreading rate” is twice (2x) the halfseafloor “plate rate.” a.
Complete Table 5 (column d) by calculating the Seafloor Spreading Rate during each
 color band. Now your units are cm/yr.
Refer to Teacher Answer Key, Table 5.
Again, in real ocean basins, rate of motion for each plate must be determined separately, and added together to calculate the ocean’s spreading, or growth rate.
17.
Complete Table 6 (column b) by transferring the data from Tables 3, as indicated. These data will be used to graph the seafloor’s velocity

(rate).
Refer to Teacher Answer Key, Table 6.
18.
Using the data from Table 6, plot your data on
Graph 1 with “distance from ridge (km)” on the y-axis (the independent variable) and “age of seafloor (m.y.)” on the x-axis (the dependent
 variable).
Kilometers and millions of years are used when plotting these data, so the numbers are not so large. a.
Be sure to include the zero point, as it represents the mid-ocean ridge, where distance = 0 km, and age = 0 m.y. b.
Note that both axes are labeled, including the units used. c.
Connect data points with a line, starting at the origin (0,0).
 Refer to Teacher Answer Key, Graph 1.
19.
Compare your graph with the Half-Seafloor
Plate Rate data from Table 4 (column c). In your science notebook answer the following questions: a.
What does the slope of each line segment on
 your graph represent?
The slope of each line segment represents
D/T or velocity (plate rate). b.
If one segment of the line is steeper than
 the other, what does that mean?
It means that the plate rate was faster for that period of time. c.
Based on your data, did the rate of seafloor spreading remain constant through time?
How do you know?
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TEACHER PAGES
 The line segments vary in slope (on the graph), indicating that the plate rate was not constant through time. d.
How old is your model ocean?
 The model ocean is 180 m.y. old. e.
What is the distance from the ridge to the
 continent (in km) for your half-seafloor?
The distance is 2300 km (represented on the model by 92 cm). f.
What is the overall average rate of seafloor spreading for the half-seafloor? Express
 your answer first in km/m.y., then in cm/yr.
To calculate the average rate, students should divide the total distance (ridge to continent) by the total amount of time (age).
Thus, the answer is
2300km/180m.y. = 12.78 km/m.y. or
230,000,000cm/180,000,000yr = 1.28 cm/yr g.
What is the average rate of seafloor
 spreading for the entire ocean basin?
Students should multiply the answer for the half-seafloor (previous question) by 2.
Thus, the answer is
2 x 1.28 cm/yr = 2.56 cm/yr h.
Seafloor spreading rates of the North
Atlantic Ocean have varied between 2 and 4 cm/yr, whereas the East Pacific Rise in the
Pacific Ocean has a rate of over 10 cm/yr.
How does your model ocean compare with
 these actual spreading rate averages?
The model most closely approximates the
North Atlantic.
Part III: Simulating Seafloor Spreading
20.
Roll up each half-seafloor until the cardboard continent is reached. Place a wooden dowel through each of the 2 half-seafloor tubes.
Four students are needed to demonstrate the process of seafloor spreading. Select your group.

21.
Arrange the two half-seafloor rolls with the dowels as shown in Figure 3a ( not as shown in
Fig. 3b!). Two students face each other, and each holds the ends of two dowels.
Make sure that students have their paper rolls oriented as shown in Figure 3a, not as shown in
Figure 3b.
A third and fourth student each hold the end of one of the continental pieces. Place the edges of the two continental pieces together, with the paper rolled up beneath them, hidden from the surface.
Before unrolling the seafloor , simulate the initial rifting (fracturing) of the continent to produce a small fissure (only 4-6 inches wide).
Violet paper should be exposed in the fissure.
A fissure is an elongate fracture. When a fissure occurs, the heated rock below melts
(due to the release of pressure) to form magma.
The magma rises to fill in the crack. The composition of the magma is mafic. As the magma cools and crystallizes, basalt is formed. a.
Explain what the violet paper represents. b.
Based on your data table, how old is the rock that was originally formed when the fissure was produced?


The violet paper represents seafloor that is
180 million years old, or formed 180 m.y.a.
The seafloor formed at the fissure is composed of basalt.
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TEACHER PAGES
22.
Continue to “create new seafloor” by very slowly diverging the two plates. Be sure that the same color of paper (= same age of rock) is
“forming” at the ridge at the same time. As new seafloor is “formed at the ridge,” call out the ages. When the divergence simulation is completed, the “red” seafloor will be located at
 the mid-ocean ridge.
Emphasize that students are simulating the growth of their model ocean by having rifted a continent, and by diverging the two plates. New seafloor was formed in the process.
This model approximates the ages of the North
Atlantic’s seafloor. The model ocean basin is scaled to the North Atlantic as well.
23.
Answer the following questions in your science notebook. a.
Do the different colors represent rocks of different compositions? What composition or compositions of rocks form as an ocean
 floor “grows”?
No, the colors do not represent different compositions of rock. All the rock produced at the ridge is basalt. b.
What type of crust is produced at the ridge?
 Oceanic crust is formed at the ridge. c.
Where is the source of the magma that
 formed the seafloor rock?
The magma that formed the basalt most likely is located in the mantle, directly beneath the ridge axis. d.
What color represents the oldest seafloor?
 The oldest rock would be the first rock that had extruded, which is represented by the purple color band. e.
What color represents the youngest
 seafloor?
The red color band extruded last and represents the youngest rock. This rock could have been formed last week! f.
As distance from the ridge increases, what
 happens to the age of the seafloor?
The age of the seafloor increases with increasing distance from the ridge.
Summarize your observations and discoveries by answering the questions on the next page.
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TEACHER PAGES
Use your observations of the Earth’s lithospheric plates and the relationship of plate boundaries to seafloor features to answer the following questions. Your teacher will provide information of how to format your answers.
Use the students’ responses to questions 9, 19 and 23 (written in their science notebooks) to assess their understanding of the activity.
 Were all tables and graphs completed accurately?
Students will apply the knowledge gained from this activity in the next activity “Age of the Ocean Floor,” which uses actual seafloor age data.
1) Examine your graph of the Half-Seafloor Plate Rate (Graph 1). During what range of time did the plate move fastest? Answer with a range of time, such as “between 10 and 20 million years ago.”
 The seafloor has been spreading most rapidly during the last 40 m.y.


Students may be more specific, stating that between 20 and 40 million years ago the rate was fastest.
2) When was the plate movement slowest? Again, answer with a range of time.
From 40 to 67 m.y. ago, the seafloor spreading rate was at its slowest.
3) Write a 1 page description of how your ocean grew, starting 180 million years ago. In your description, explain what occurred during seafloor spreading (including the type of rock formed) and how the spreading rates varied through time.
 Key points that students should include: the initial rifting of the continent and earliest formation of oceanic crust 180 million years ago. cooling and crystallization of magma to form basalt and oceanic crust.
Formation of oceanic crust at the mid-ocean ridge
Divergence from the mid-ocean ridge the different time periods (the ranges of time that different rates occurred).
Source: Dr. Leslie Sautter, Project Oceanica, Dept. of Geology and Environmental Geosciences, College of Charleston, SC.
Contributions to this activity were made by Elizabeth King Rhodes and Shelley Aksomitas. website: http://oceanica.cofc.edu
email: oceanica@cofc.edu
phone: 843-953-5586
Funding for the COASTeam Program was provided by the South Carolina Sea Grant Consortium. http://oceanica.cofc.edu/coasteam/
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Figure 1. Layout of the half-seafloor model with continent attached.
The “seafloor” is 92 cm in length and represents 2,300 km of the fictitious ocean basin. The letters
“R,O,Y,G,B,V” represent red, orange, yellow, green, blue and violet, respectively. The cardboard tube is attached to the Red end of the paper.
R O Y G B V
|-------------------------------------------------92 cm----------------------------------------|
Figure 2. Layout of the full seafloor model (Parts II and III) with continental pieces.
ridge (2 cardboard tubes) continent
V B G Y O R R O Y G B V continent
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Figure 3. Set-up of seafloor rolls for Part IV.
Assemble model this way…. ….not this way!
Figure 4. Depiction of the full seafloor model as it is “diverging.”
continent
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TEACHER PAGES
Teacher Answer Key
Table 1. Calculating seafloor ages for each color band.
Subtract the youngest age from the oldest age for each color band on your model. This “Total Seafloor
Age in Color Band” indicates the period of time that elapsed while the plate was moving away from the mid-ocean ridge.
RED
ORANGE
YELLOW
GREEN
BLUE
VIOLET
TOTAL “Half Seafloor”
15.00
15.00
7.00
30.00
10.00
15.00
92.00
0.00
20.10
40.10
67.70
131.90
147.70
20.10
40.10
67.70
131.90
147.70
180.00
20.10
20.00
27.60
64.20
15.80
32.30
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Table 2. Using the model scale.
The paper model uses the scale 92 cm = 2,300 km.
92 cm on the model = 2,300 km in the actual ocean
1 cm = 25 km
1 cm on the model = 25 km in the ocean the model represents
We can express the scale as being “
25 kilometers per centimet er.”
So, the
is:
25 km/cm
To calculate, use the ratio:
92 cm
=
2300 km
1 cm x km
Solve for x:
x = (1 cm * 2300 km)
92 cm
or:
x = 2300 cm*km
92 cm
x = 25 km (centimeters cancel out)
Thus, 1 cm = 25 km
This is the same thing as saying there are 25 km per cm on the map, or having a scale conversion factor of 25 km/cm.
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TEACHER PAGES
Table 3. Calculating distances.
Use the scale’s conversion factor from Table 2 to calculate the width of seafloor for each color band segment (column b). Add the distances together (column c) so that the distance increases from the ridge along the model seafloor.
RED
ORANGE
YELLOW
GREEN
15.00
15.00
7.00
30.00
375.00
375.00
175.00
750.00
375.00
750.00
925.00
1675.00
BLUE 10.00 250.00 1925.00
VIOLET 15.00 375.00 2300.00
Column b values are calculated by multiplying column a values by 25 km/cm (the scale of the map):
15.00 cm * 25 km/cm = 375.00 km
Column c values are the cumulative distance, meaning the distance from each segment is added to the previous value.
For example:
The value of 750.00 km in column c was calculated by adding the ORANGE “Seafloor Width” value (column b) of 350.00 km to the RED “Distance from the ridge” value (column c) of 350.00 km, or:.
375.00 km + 375.00 km = 750.00 km
Then the YELLOW “Seafloor Width” of 175.00 km was added to the previous distance of 750.00 km to calculate the cumulative distance of 925.00 km.
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TEACHER PAGES
Table 4. Calculating half-seafloor plate rates.
Calculate the model’s half-seafloor plate rates through time, first using the units “kilometers per million years”, or km/m.y.
(from Table 3)
(from Table 1)
RED
ORANGE
375.00
375.00
20.10
20.00
18.66
18.75
YELLOW 175.00 27.60 6.34
GREEN
BLUE
VIOLET
750.00
250.00
375.00
64.20
15.80
32.30
11.68
15.82
11.61
Table 5. Calculating seafloor spreading rates in cm/yr.
Plate rates are usually expressed using the units “centimeters per year,” or cm/yr. Seafloor spreading rate is two times the half-seafloor plate rate, since there are two plates involved and they are moving apart at the same rate (in your model ocean).
RED
ORANGE
YELLOW
GREEN
BLUE
VIOLET
37,500,000.00
37,500,000.00
17,500,000.00
75,000,000.00
25,000,000.00
37,500,000.00
20,100,000.00
20,000,000.00
27,600,000.00
64,200,000.00
15,800,000.00
32,300,000.00
1.87
1.88
0.63
1.17
1.58
1.16
3.73
3.75
1.27
2.34
3.16
2.32
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TEACHER PAGES
Table 6. Summarizing your data.
Complete the table below, using your calculations from Table 3. Use this table to make your graph, below.
(from Table 1)
(from Table 3)
RED
ORANGE
YELLOW
20.10
40.10
67.70
375.00
750.00
925.00
GREEN
BLUE s
VIOLET
Graph 1. Graphing Half-
Seafloor Plate Rate.
Use data from Table 6 to graph the variations in half-seafloor plate rates.
2500
2000
1500
131.90
147.70
180.00
1675.00
1925.00
2300.00
1000
500
0
0 50 100
150 200
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