Download Seafloor Spreading - Teacher Site Home

Exploring Earth
Student Handout
Why? The theory of plate tectonics states that the rigid outer layer of the earth is divided into several
plates that “float” or drift on the mantle. Because these plates move slowly on the mantle, boundaries that
shift and move are created between them. The three different kinds of boundaries are divergent
boundaries, convergent boundaries and transform boundaries. The following sites and maps that you will
investigate show or illustrate what takes place at each type of boundary. Use these animations and maps as
well as the knowledge you have gained from class to answer the following questions.
Structure of the Learning Activity: This  symbol indicates that you must record an answer on your
answer sheet.
Learning Outcomes: By the end of this lesson, you should be able to:
 Differentiate between the types of plate boundaries
 Describe what is taking place at each type of boundary
 Identify the sea floor feature created at each type of boundary
 Speculate what will happen in the future to different locations on Earth based on the type of
boundary location
Using the “Exploring Earth: Plate Tectonics” link on Mrs. Butts’ website, enter the following Key
Codes and answer the following questions on your answer sheet.
KEY CODE: ES0803
1a. Describe what is taking place at a divergent boundary after watching the animation.
1b. How can you tell what the polarity of the Earth was at the time of the making of the ocean floor?
KEY CODE: ES0804
Watch the animation of the transform boundary.
2. Describe what is taking place at a transform boundary including what sea floor feature is found at
this type of boundary.
Watch the animation of the convergent boundary.
3a. Describe in detail, including what you see taking place on the continent, what is happening at a
convergent boundary.
3b. Which types of crust are involved in this type of boundary?
3c. What sea floor feature is found here?
Watch the animations of the divergent boundaries.
4. What sea floor feature is created at a divergent boundary?
KEY CODE: ES0802
Step1: Matching Coastlines.
Look at that first map, on Step 1 (look at the bottom of the page for the “Steps”)
5. Based on the coastlines of South America and Africa, the two appear to have once been joined
together (like jigsaw pieces.)
5a. Based on your knowledge of plate tectonics, are they moving away from each other or towards
each other?
5b. Which sea floor feature must be between these two to have caused this movement?
5c. How can you tell?
Step 2: Evidence for Joined Continents
6. In addition to their shapes, what other data support the hypothesis that Africa and South America
were once joined together?
Step 6: Other Ocean Floors
7. Where else on Earth do mid-ocean ridges exist?
8. What can you infer about continents on opposite sides of the same mid-ocean ridge?
9. What evidence would you look for if you wanted to show that two landmasses were once connected?
Using the “Plates & Boundaries Map” link on Mrs. Butts’ website, answer the following questions
on your answer sheet.
Plates & Boundaries Map
10a. Based on the map of the plate boundaries, predict, in detail, what will ultimately happen to the
Mediterranean Sea.
10b. Based on this map and your knowledge of plate boundaries, in your opinion, what will happen
between Africa and Arabia (the Red Sea Rift?)
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, 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 the Learning Activity: This  symbol indicates that you must record an answer on your
answer sheet.
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
11. 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.
11a. What are the two variables used to draw the profile chart, and on which axes are they plotted?
11b. How many years are represented by each interval (step) on the y-axis?
11c. What distance is represented by each interval (step) on the x-axis?
12a. Where along the profile are the youngest rocks?
12b. Describe how the age of seafloor bedrock changes as you travel from the spreading center to the
South American and African coastlines.
12c. Is the age-distance profile roughly symmetrical on either side of the spreading zone?
12d. 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?
13. 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 easily grasped. 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 !)
13a. 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. SHOW YOUR WORK!
13b. 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?
13c. 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.
13d. Explain how and why the appearance of this profile would change if the seafloor had been
spreading at a much greater (faster) rate?
14a. 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.
14b. Write a brief comparison of the spreading rates in the South Atlantic and South Pacific
14c. Which ocean has the most subduction around its edges, the Atlantic or the Pacific?
14d. 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?
15. Below you see another profile across the Indian Ocean from Australia to Antarctica (from about
130°E, 33°S to 134°E, 63°S).
15a. Parts A and B of the above graph show the same slope. What does that tell you about the rate on
either side?
15b. Parts C and D have a different slope. Are the plates moving slower or faster at C and D
compared to A and B?
15c. 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?
16. Review all of the spreading rates that you have calculated so far.
16a. 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.
16b. 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.
17. Now, using your boundaries map AND your calculated spreading rate of the Indian Ocean, predict
how long it will take, in millions of years, for Australia to collide with Indonesia (remember, you have
access to a world map!) The distance between the two is 3,455 km.