investigating seafloor spreading - cosee se teacher.pdf · investigating seafloor spreading ......

COASTeam Program, Project Oceanica, College of Charleston

1

Grade Level: 8th

SC Standards: IIIB3a,b,c,j,k; IIIB4c

National Standards: Earth and Space Science Content Standard D

Investigating Seafloor Spreading Original Source: Dr. Leslie Sautter (Dept. of Geology, College of Charleston)

Focus Questions 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?

Objectives 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.

Key Words 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

Materials 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

Time Frame One to two 50-minute class periods, depending

on how much of the construction of the seafloor

model and graphing is assigned as homework.

Suggested Learning Environment 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.

Relevant pages in Of Sand and Sea Chapter I, The Ocean Planet, pp. 9-13

Investigating Seafloor Spreading TEACHER PAGES


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Teacher Preparation

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 half-

seafloor 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).

Procedure

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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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 mid-

ocean ridge. When the two half-seafloors are

laid out together, 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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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:

Rp = D/T where:

Rp is the plate’s rate (velocity) of motion

D is the distance from the mid-ocean ridge

T is time, or the age of the rock on the seafloor

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:

Rss = 2(Rp) where:

Rss is the model ocean basin’s seafloor

spreading rate

Rp is the half-seafloor’s plate rate

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.




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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.


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 = 40,000 m = 4,000,000 cm = 4 cm

1 m.y. 1,000,000 yr 1,000,000 yr 1 yr

Answer: 40 km/my = 4 cm/yr

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).


16. For the model seafloor in this activity, the

“seafloor spreading rate” is twice (2x) the half-

seafloor “plate rate.”

a. Complete Table 5 (column d) by calculating

the Seafloor Spreading Rate during each

color band. Now your units are cm/yr.


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).


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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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.

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.

21. 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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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.



8

Questions (Assessment)

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: [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/

http://oceanica.cofc.edu/

mailto:[email protected]

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.

cardboard tube continent

|-------------------------------------------------92 cm----------------------------------------|

Figure 2. Layout of the full seafloor model (Parts II and III) with continental pieces.

ridge (2 cardboard tubes)

continent continent

R O Y G B V

V B G Y O R

R O Y G B V



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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



11

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.

Color Band a) Width of Color Band

on paper

(cm)

b) Youngest Age of

seafloor

(m.y.)

c) Oldest Age of

seafloor

(m.y.)

d) Total Seafloor Age In Color

Band

(m.y.)

RED

15.00

0.00

20.10

20.10

ORANGE

15.00

20.10

40.10

20.00

YELLOW

7.00

40.10

67.70

27.60

GREEN

30.00

67.70

131.90

64.20

BLUE

10.00

131.90

147.70

15.80

VIOLET

15.00

147.70

180.00

32.30

TOTAL “Half Seafloor”

92.00



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Table 2. Using the model scale.

The paper model uses the scale 92 cm = 2,300 km.

If:

92 cm on the model = 2,300 km in the actual ocean

Then:

1 cm = 25 km Now, enter the number above into all the blank spaces below: 1 cm on the model = 25 km in the ocean the model represents We can express the scale as being “25 kilometers per centimeter.” So, the scale’s conversion factor 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.



13

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.

Color Band a) Seafloor Width, measured on

the model

(cm)

b) Seafloor Width, represented by

the model

(km)

c) Distance from ridge of

oldest rock

(km)

RED

15.00

375.00

375.00

ORANGE

15.00

375.00

750.00

YELLOW

7.00

175.00

925.00

GREEN

30.00

750.00

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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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.

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).

Color Band a)Seafloor Width (cm)

b) Age Difference

(yr)

c) Half-Seafloor Plate Rate

(cm/yr)

d) Seafloor Spreading

Rate (cm/yr)

RED

37,500,000.00

20,100,000.00

1.87

3.73

ORANGE

37,500,000.00

20,000,000.00

1.88

3.75

YELLOW

17,500,000.00

27,600,000.00

0.63

1.27

GREEN

75,000,000.00

64,200,000.00

1.17

2.34

BLUE

25,000,000.00

15,800,000.00

1.58

3.16

VIOLET

37,500,000.00

32,300,000.00

1.16

2.32

Color Band a) Seafloor Width

(km) (from Table 3)

b) Age Difference

(m.y.) (from Table 1)

c) Half-Seafloor Plate Rate (km/m.y.)

RED

375.00

20.10

18.66

ORANGE

375.00

20.00

18.75

YELLOW

175.00

27.60

6.34

GREEN

750.00

64.20

11.68

BLUE

250.00

15.80

15.82

VIOLET

375.00

32.30

11.61



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Table 6. Summarizing your data.

Complete the table below, using your calculations from Table 3. Use this table to

make your graph, below.

Color Band a) Oldest Age of Seafloor in Color Band

(m.y.) (from Table 1)

b) Distance from ridge of oldest rock

(km) (from Table 3)

RED

20.10

375.00

ORANGE

40.10

750.00

YELLOW

67.70

925.00

GREEN

131.90

1675.00

BLUE

147.70

1925.00

VIOLET

180.00

2300.00

s

Graph 1. Graphing Half-

Seafloor Plate Rate.

Use data from Table 6 to graph

the variations in half-seafloor

plate rates.

Half-Seafloor Plate Rate

Seafloor Spreading

0

500

1000

1500

2000

2500

0 50 100 150 200

Age of Seafloor (m.y.)

Dis

tan

ce f

ro

m R

idg

e (

km

)

Half – Seafloor Plate Rate

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