Investigating Seafloor Spreading
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Procedure
Follow the steps below while taking notes in your science notebook.
Part I: Constructing the Seafloor Model
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.
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
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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.”
5. Cut the corrugated cardboard into two pieces so that one edge of
each piece is equal to the striped paper’s width (Figure 1, on the
overhead projector).
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.
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The cardboard pieces represent continents and the cardboard tubes,
are located at the model’s mid-ocean ridge. When the two halfseafloors 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).
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.
a. How is an igneous rock formed?
b. What do the small crystal sizes of basalt tell you about how it
cooled?
c. What does the dark color of basalt tell you about its
composition?
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.
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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.
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:
where:
Rp = D/T
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
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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:
where:
Rss = 2(Rp)
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 halfseafloors on either side of a mid-ocean ridge, then add them together,
rather than doubling one half-seafloor’s rate.
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.
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.
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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.
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
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.”
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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).
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).
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?
b. If one segment of the line is steeper than the other, what does
that mean?
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c. Based on your data, did the rate of seafloor spreading remain
constant through time? How do you know?
d. How old is your model ocean?
e. What is the distance from the ridge to the continent (in km) for
your half-seafloor?
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.
g. What is the average rate of seafloor spreading for the entire
ocean basin?
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?
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.
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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?
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.
This model approximates the ages of the North Atlantic’s seafloor.
The model ocean basin is scaled to the North Atlantic as well.
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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”?
b. What type of crust is produced at the ridge?
c. Where is the source of the magma that formed the seafloor
rock?
d. What color represents the oldest seafloor?
e. What color represents the youngest seafloor?
f. As distance from the ridge increases, what happens to the age of
the seafloor?
Summarize your observations and discoveries by answering the
questions on the next page.
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Questions
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.
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.”
2) When was the plate movement slowest? Again, answer with a range
of time.
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.
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DATA CALCULATION TABLES
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
b)
c) Oldest
d) Total
Color Band Youngest Age of Seafloor Age
on paper
Age of
seafloor
In Color
seafloor
Band
(cm)
(m.y.)
(m.y.)
(m.y.)
RED
15.00
0.00
20.10
ORANGE
15.00
20.10
40.10
YELLOW
7.00
40.10
67.70
GREEN
30.00
67.70
131.90
BLUE
10.00
131.90
147.70
VIOLET
15.00
147.70
180.00
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 = ________ km
Now, enter the number above into all the blank spaces
below:
1 cm on the model = ______ km in the ocean the model
represents.
We can express the scale as being “_____ kilometers per
centimeter.”
So, the scale’s conversion factor is:
______ km/cm
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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
b) Seafloor Width, c) Distance from
represented by
ridge of
the model
oldest rock
(km)
(cm)
RED
15.00
ORANGE
15.00
YELLOW
7.00
GREEN
30.00
BLUE
10.00
VIOLET
15.00
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(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.
Color Band
a) Seafloor
Width
(km)
(from Table 3)
b) Age
Difference
(m.y.)
c) HalfSeafloor Plate
Rate
(from Table 1)
(km/m.y.)
RED
ORANGE
YELLOW
GREEN
BLUE
VIOLET
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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) HalfSeafloor
Plate Rate
(cm/yr)
RED
ORANGE
YELLOW
GREEN
BLUE
VIOLET
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d) Seafloor
Spreading
Rate
(cm/yr)
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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.)
b) Distance from ridge
of oldest rock
(km)
(from Table 1)
(from Table 3)
RED
20.10
ORANGE
40.10
YELLOW
67.70
GREEN
131.90
BLUE
147.70
VIOLET
180.00
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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
Distance from Ridge (km)
2500
2000
1500
1000
500
0
0
50
100
150
Age of Seafloor (m.y.)
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