Seafloor Spreading
Student Handout
Why? In the plate tectonic model, Earth’s tectonic plates rift apart at oceanic spreading centers. There,
upwelling magma cools, and crystallizes forming new igneous rocks along the edge of the plates. As
seafloor spreading continues the new rocks move away from the spreading zone. Here, in GeoMapApp,
we analyze seafloor age data and calculate spreading rates in different areas of the world. We find a wide
range of results, all of which support the tectonic model of Earth.
Structure of GeoMapApp Learning Activity: This  symbol indicates that you must record an answer
on your answer sheet, the last page of this lab.
Learning Outcomes: By the end of this lesson, you should be able to:
 Analyze data related to the age of seafloor crust
 Calculate seafloor spreading rates using profiles of seafloor age versus distance
 Analyze and compare spreading rates at various locations on Earth and at various times in Earth’s
history
 Speculate about the effects of tectonic activity on seafloor spreading rates
1. Using GeoMapApp, a map with ages of rock
shown in different colors was chosen. Then a
profile was made across the Atlantic Ocean from
South America to Africa.
1a. What are the two variables used to draw the profile chart, and on which axes are they plotted?
1b. How many years are represented by each interval (step) on the y-axis?
1c. What distance is represented by each interval (step) on the x-axis?
2a. Where along the profile are the youngest rocks?
2b. Describe how the age of seafloor bedrock changes as you travel from the spreading center to the
South American and African coastlines.
2c. Is the age-distance profile roughly symmetrical on either side of the spreading zone?
2d. In a sentence or two, describe what is meant by the symmetry of the age profile – what does it tell
us about the ages and motion of rocks on either side of the spreading zone?
3. Rate is described as a change in distance over a period of time. The average rate of seafloor
spreading during a particular period of time can be determined by analysis of the profile chart as shown in
the following worked example. Pay attention because you’ll be doing a similar calculation later on:
First we pick 2 points on the profile and determine the distance and age represented by each point (in this
example, we’re using points A and B on the chart below).
Then we calculate the distance span and time span between the points A and B.
To determine the rate at which the seafloor is moving eastward from the spreading center during the last
50 Ma (Ma stands for mega-annum. It is the scientific abbreviation for million years) we divide the
distance the seafloor moved by the time it took to move from place A to place B. In the example above,
the rate of seafloor spreading is
Distance seafloor traveled / time it took to travel
or
(1080 km / 50 Ma) = 21.6 km / Ma
It’s hard to visualize 21.6 km, and even harder to imagine a time span of 1 million years. Spreading rates
are more often reported in millimeters (mm) per year, a unit more easy to grasp. One mm is about the
thickness of a pencil line – your thumbnail is about 10 mm across, and there are 25.4 mm in an inch.
There are 1000 mm in 1 m, and there are 1000 m in one km. There are, then, 1 million mm in 1 km, and
1,080,000,000 mm in 1080 km.
Our spreading rate, then, can be calculated as:
1,080,000,000 mm / 50,000,000 years = 21.6 mm/yr
(notice that the number of km/Ma reduces to same number of mm/yr !)
3a. Your turn! Using the methods described above, calculate the rate at which the seafloor was spreading
during the time span between points C and D on the chart above. Enter your answer in the data table on
your student answer sheet.
3b.  Are the rates calculated on either side of the spreading zone in this area similar? How could you
have known that they were similar by simply looking at the graph?
3c. How much wider is the South Atlantic getting each year? (That is, what is the combined spreading
rate?) Enter your answer in the data table on your student answer sheet.
3d.  Explain how and why the appearance of this profile would change if the seafloor had been
spreading at a much greater (faster) rate?
4a. Using the methods described above, determine the spreading rate along a profile drawn from
133°W, 27°S to 90°W, 32°S in the South Pacific Ocean shown below.
Remember to calculate the combined spreading rate of both sides of the profile. You may assume that the
right side of the spreading center has the same rate as the left. Show your work and record the results in
the table on your answer sheet. Please note the different scale on the Y-axis.
4a .  Write a brief comparison of the spreading rates in the South Atlantic and South Pacific
4b.  Which ocean has the most subduction around its edges, the Atlantic or the Pacific?
4c.  It is assumed that the motion of the plates is generated by ridge push, the pushing aside of plates
from a mid-ocean ridge, as well as slab pull, the pulling of the plate as a plate subducts. Based upon
your data, which seems to be the most powerful process?_Why?
5. Below you see another profile across the Indian Ocean from Australia to Antarctica (from about 130°E,
33°S to 134°E, 63°S).
6a.  Parts A and B of the above graph show the same slope. What does that tell you about the rate on
either side?
6b.  Parts C and D have a different slope. Are the plates moving slower or faster at C and D
compared to A and B?
6c. India smashed into Asia, changing the movement of adjacent plates. Based upon the changes in
the rate of movement, when do you think that the collision happened?
7. Review all of the spreading rates that you have calculated so far.
7a. At the present time, are seafloor spreading rates around the globe roughly the same? Explain your
answer with supporting evidence from the profiles and from the spreading rates you’ve calculated
throughout this activity.
7b. Is the spreading rate at any particular spreading center necessarily constant over time? Explain
your answer with supporting evidence from the profiles and from the spreading rates you’ve
calculated in this activity.
Name___________________________class____
Seafloor-Spreading
Student Answer Sheet
1a. X-axis variable: _____________________ Y-axis variable: _______________________________
1b. Years per interval on the y-axis: _____________
1c. Distance per interval on the x-axis: _____________
2a. Where along the profile are the youngest rocks? ________________________________________
_____________________________________________________________________________________
2b. Change in age of seafloor bedrock from the spreading center to coastlines. ____________________
_____________________________________________________________________________________
2c. Is the age profile roughly symmetrical? ___________
2d. Significance of symmetry? __________________________________________________________
_____________________________________________________________________________________
3a. Spreading rate between points C and D
Work: Please note that the scales differ from the previous example
Location
Description
Approx. Start
Long, Lat
Approx. End
Long, Lat
South Atlantic
37°W,22°S
10°W,12°S
South Pacific
133°W,27°S
90°W,32°S
Spreading Rate
Left of Spreading
Center (mm/yr)

(4a)
Spreading Rate
Right of Spreading
Center (mm/yr)
21.6
(3a)
(4a)

Combined Seafloor
Spreading Rate
(mm/yr)

(4a)
(3c)
3b. Rates similar? __________
How does your graph indicate similarity? ___________________________________________________
_____________________________________________________________________________________
3c.
 How much wider is the South Atlantic getting each year? (Enter your answer on the data table
above)
3d.  Appearance of graph had the seafloor been spreading at a much greater rate: __________________
_____________________________________________________________________________________
4a.
South Pacific profile – Show your work below, and enter your answers in the table above
Work:
4b.
 Which ocean has the most subduction?_______________
4c.
 Which is more effective at moving plates, ridge push or slab pull?______________
How do you
know?______________________________________________________________________
6a.
 Comparison of the spreading rates on either side: _______________________________________
_____________________________________________________________________________________
6b.
 C and D faster or slower than A and B? _______________________________________________
6c. date of collision?_______________________
7a. Are current spreading rates the same?______Explain: ____________________________________
_____________________________________________________________________________________
7b. Are spreading rates constant over time?__________Explain _______________________________
_____________________________________________________________________________________