A Model of Seafloor Spreading
Revised from an activity by Ellen P. Metzger at San Jose State
University.
MATERIALS: For each student:
— 2 sheets of 8.5 x 11" binder paper (file folder cardboard could
be used in place of paper to make a sturdier model)
— scissors
— ruler
— transparent tape
— masking tap
— colored pencils or crayons
INTRODUCTION: The creation of new sea floor at mid-ocean
spreading centers and its destruction in subduction zones is one of
cause the Earth to experience constant change.
the many cycles that
PURPOSE: The purpose of this activity is to make a simple model that shows the evolution of oceanic crust
through sea-floor spreading and subduction.
PROCEDURES:
1) Place one sheet of binder paper so that the long side is towards
you (Fig. 1).
2) Draw a vertical line in the middle of the paper with a height of
11.5 cm leaving 5 cm on either side of the line. This line represents
a mid-ocean spreading center (See Figure 1).
3) Draw a second vertical line to the right of the center line so that it
lies 3 cm from the right edge of the paper. This line represents a
subduction zone.
4) Draw a third vertical line to the left of the center line so that it lies 3 cm from the left edge of the paper. This
line represents another subduction zone. When you are finished, your piece of paper should look like the
diagram in Figure 1. Label the mid-ocean ridge and subduction zones.
5) With a pair of scissors, cut the vertical lines so there will be three slits on the paper all the same height and
parallel to each other. To reinforce the slits you have made, place masking tape over each one and re-cut the
slit though the tape.
6) On the second sheet of paper draw 11 bands each 2.54 cm (1 "wide) perpendicular to the long edge of the
paper.
7) Choose one color to represent normal polarity and a second to represent
reversed polarity. Color alternate bands to represent periods of normal and reversed
polarity. Color the band on the far left as reversed polarity.
8) Cut the paper in half parallel to the long edge to get two strips of paper as shown
in Figure 2. Mark the bands on each strip with arrows to indicate alternating periods
of normal (up arrow) and reversed (down arrow) polarity.
9) Insert one end of each strip of paper through the spreading center line on your
first piece of paper (see Figure 3).
10) Pull each strip of paper towards the slits nearest the margins of the paper (the
subduction zones). Tape each strip to make a loop as shown in Figure 3.
11) Circulate the ribbons of paper (which represent oceanic crust) to simulate the
movement of ocean floor from the mid-ocean spreading center to the subduction zone. Start the movement of
the ribbons with bands representing normal polarity.
QUESTIONS: ANSWER THESE ON NOTEBOOK PAPER!
1) The Earth is about 4.6 billion years old. Based on observations of your sea-floor spreading model, why do
you think that the oldest ocean floor is only about 200 million years old?
2) On the real ocean floor, alternating stripes of normal and reversed polarity are not all of equal width. What
does this tell you about the lengths of time represented by normal and reversed polarity?
Part 3 – Calculating the Rate of Seafloor Spreading in the Atlantic Ocean
Revised from an Activity by Karen L. Bice
of Department of Geosciences of Pennsylvania State University
Estimated Time Required: 50 - 65 minutes
Anticipated Learning Outcomes
Students will gain an understanding of how geologists determine rates of sea floor spreading between two tectonic plates.
Students will gain experience applying some basic, useful mathematical concepts such as the calculation and use of
velocities and conversion from one set of units to another.
Background
Sea floor spreading is one component of the theory of plate tectonics. According to this concept, at ridges in many of the
world's oceans (mid-ocean ridges), new crustal rock is added to the edges of plates on either side of the mid-ocean ridge.
The mid-ocean ridge in the middle of the Atlantic Ocean is known as the Mid-Atlantic Ridge (MAR). At the MAR, new sea
floor rock (basalt) is added to the edges of the North American, South American, Eurasian, and African plates. The result
of the addition of new sea floor rock is that the Atlantic Ocean between these sets of continents is widening. North and
South America move farther away from Eurasia and Africa each year. The youngest sea floor rocks are found in the
middle of the ocean because that is where new sea floor is added. As you move away from the MAR, the sea floor rocks
become increasingly older. The oldest sea floor (the first Atlantic sea floor formed that is still preserved) is found closest to
the continents. One way that geologists can recognize the strips of sea floor basalt created at, and subsequently moved
away from, the MAR is by collecting and determining the magnetic properties and ages of rocks from the ocean floor.
Materials: (per student)
Ruler with mm subdivisions or 1/10 inch subdivisions (C-Thru rulers work best)
Calculator
Copies of map, worksheet, and geologic time scale (provided in activity or you can save paper by using one in your text.)
Procedures
A strip of map of a portion of the North Atlantic region is provided. The thin line that approximately parallels the easily
recognized coastline is the continental shelf edge. The continental shelf edge best defines the true edge of the sections of
continental crust, which at some time in the past were joined together to form a much larger continent known as Pangaea.
The bold line labeled "0" and located about half way between North America and Africa is the Mid-Atlantic Ridge (MAR),
where new sea floor forms. On either side of the MAR are strips of sea floor basalt labeled with their ages in millions of
years. For example, 55 million years ago, the strips of sea floor labeled "55" were formed at the MAR. Over the past 55
million years, these strips of rock have cooled and been moved away from the ridge where they were formed.
The distance between Point A on the North American continental shelf edge and Point B on the African continental shelf
edge is approximately 4,550 kilometers (km). By following the steps outlined below, students will determine how much the
distance between North America and Africa increases each year and how long ago the Atlantic Ocean began to open.
Students will need to determine a map scale using the given distance between Point A and Point B (4550 km), and the
distance they measure between these points on their copy of the map. The spreading rates that students calculate in step
#2 will vary between 18 and 24 km per My (1.8 to 2.4 cm per year) depending on which strip of sea floor basalt is used
and how carefully measurements are made. (Calculations made using highly sophisticated means show that the present
spreading rate in the Atlantic is about 2 cm per year.) Measurements between sea floor strips and the MAR should be
made as close to perpendicular to the MAR as possible. This is the approximate direction of movement of sea floor basalt
away from the ridge where it formed.
Sea floor spreading rates have not been constant through geologic time. Students will have calculated an average
spreading rate over some period of time. Because the oldest sea floor shown on the map is 156 million years old, it is best
to calculate an average spreading rate over some time period longer than about 50 million years. The relatively slow
spreading rates calculated from sea floor rock aged 20 or 35 My will yield dates for the opening of the Atlantic which are
older than they should be.
Because students will have calculated different spreading rates, the calculated times of opening of the North Atlantic
Ocean will vary as well. In general, these dates should cluster around 190 to 220 million years ago, around the time of the
Late Triassic and Early Jurassic periods. Certain geologic features in eastern North America are related to the initial
"pulling apart" of the continent of Pangaea.
Calculating the Rate of Sea Floor Spreading in the North Atlantic
Investigator___________________________________________________ Date__________
1. Study area: North Atlantic Ocean - Select one strip of sea floor rock. Record its age below and carefully measure the
distance it has moved from the mid-ocean ridge where it formed. Your ruler should be perpendicular to the MAR.
a. Sea floor age: ______________million years (My)
b. Distance to the Mid-Atlantic Ridge (MAR): ________millimeters (mm)
c. Calculate the scale of this map. Distance from Point A to Point B on the map: ______. Divide the actual distance
between North America and Africa (measured between points A and B), 4550 km by this number. 4,550 km /
____(distance on map) to get the scale 1mm = ______.
d. Calculate the actual distance from the strip you picked to the MAR. Answer in b. times your final answer (scale) in c. =
_____.
2. Using the age of the rock you have chosen and its distance from the MAR, calculate the half-rate of sea floor
spreading, the velocity at which one strip of this rock has spread away from the MAR.
a. Calculated half-rate (velocity) of sea floor spreading (distance / time = velocity in km/year): ______km per My
b. Calculated total rate (velocity) of sea floor spreading (2 X half-rate = total spreading rate): ______km per My
3. Calculated age of the North Atlantic Ocean: total distance (4,550 km) / total velocity (answer 2.b.) = time): 4550
km/_____km/MY = _____ My
4. Use a geologic time scale to see what Geologic Period occurred during the time period calculated in #3 to tell when the
North Atlantic began to open: ________.
5. Convert the total sea floor spreading rate from step #2 above to units that are easier to "imagine". This can be done
simply by filling in the spaces below and performing the multiplication. Check this calculation by making sure that units
"cancel out" to correctly yield the units desired (this procedure is known as dimensional analysis).
a. (_____km/My) X ( 0.6214 mi/km) X ( 5,280 ft/mi) X ( 12 in/ft ) X ( 0.000001 My/yr) = _____in/yr.
b. How much has the distance (in inches) between North America and Africa increased since you were born?
c. How much does the distance (in feet) increase during the average lifetime of an American (~82 years)?
d. How much closer (in feet) were these two continents when Columbus made his voyages? (Present year – 1492 =_____
X _______in/yr = ______.
Above is a strip map of a portion of the North Atlantic region showing the coastlines and continental shelf edges
of North America and Africa. The bold line labeled “0” is the Mid-Atlantic Ridge (MAR). Selected strips of sea
floor basalt on either side of the ridge are labeled with their ages in millions of years. The approximate distance
between Point A and Point B is 4,550 kilometers.
The Rate of Pacific Plate Motion
Directions: Use the map and the following information to determine the rate of motion of the
Pacific Plate over the Hawaiian hot spot. The volcano that formed the Island of Niihau is 4.89
million years old.
Step 1 - Rate is the distance traveled over
a period of time. The distance traveled is
equal to the distance from the present
location of the hotspot (southeast Hawaii)
to Niihau. Time is the age of the island.
Start by measuring the distance from
southeast Hawaii to Niihau.Use the scale
on the map.
Step 2 - To determine the average rate of
motion for the Pacific Plate, divide the
distance to Niihau by the age of the island.
Step 3 - Note that your rate is in
kilometers/million years (km/Ma). Convert
your answer to centimeters per year
(cm/yr). To convert the rate to cm/yr, follow this conversion:
km
1000 m
100 cm
1 Ma
cm
-- X ------ X ------ X --------------- = ---Ma
km
1m
1,000,000 years
year
Step 4 - How far will the plate move in 50 years? ______
Graphical Determination of the Rate of Plate Motion
The age of the islands and seamounts increases with distance
away from the Hawaiian hot spot. This activity determines the
average rate that the Pacific Plate has moved over the last 65
million years.
Seamount or
Island
Suiko
Koko
Midway
Necker
Kauai
Distance (km) Age
4,860
3,758
2,432
1,058
519
65
48
28
10
5
Directions:
1. Plot the data on the graph.
2. Draw a best-fit line through the data points.