Sea Floor Spreading Activity Sheet

Concepts of Sea-floor Spreading
Sea-floor spreading has many surprising properties. One of the most notable is that the process is so symmetrical about the ridge crest. In other words, almost exactly the same area of sea-floor is added to the plate on either side of the ridge. For this reason, the magnetic stripes formed on either side of the ridge are also generally symmetrical (mirror-images) about the ridge.

Remember, there are two distinctly different types of relative motion along transform faults:

Motion is called right-lateral when something attached tot he plate on the other side of the fault appears to move to the right as seen from where you are standing on this side of the fault. If the object appears to move to the left, the motion is called left-lateral. If you were to cross to the other side of the fault, in order to face this side, you would have to turn around, and so the relative motion would appear the same. As a result, whether a fault is right- or left-lateral does not depend on which side of the fault you are on.

Earthquakes occur where sea-floor spreading takes place, at mid-oceanic ridges. We would expect to see earthquakes along the plate boundary either where the two plates are pulling apart at the spreading ridges or along the transform faults.

The animation that you will see in the program, however, shows only a plan (bird's eye) view of horizontal motions. There are also vertical motions involved, since spreading ridges are in fact high mountains. New oceanic crust is created at the crest of the ridge, and as the crust ages, it moves outward and downward toward the flanks of the ridge.

This produces vertical relative motion between the two plates even beyond the ends of the transform faults, as shown in this three-dimensional view. Earthquakes due to this vertical motion are found along the fracture zones that extend beyond either end of the transform faults.

Getting started:
To start sea-floor spreading animation click on the Start button in the lower right portion of the main screen. To stop the action, click on the Stop button. Pressing any key on the keyboard will toggle between Start and Stop and may be used as an alternate to clicking with the mouse.

The animation will stop when it reaches the present time (O M.Y.). At this point, the polarity stripes will appear as they actually do today for a plate boundary of this shape.

You may reset to any time within the past 24 M.Y. by clicking anywhere on the magnetic polarity time scale or by choosing Set age... under the Options menu Then click Start to begin spreading.

When the program begins, you will see a blank gray area with a zig-zag boundary line dividing it roughly in half. The gray area represents a continental mass that is about to be broken apart by the newly-created plate boundary.

Below the gray area is a magnetic polarity time scale that shows which polarity (normal or reversed) occurred at which times. The program begins with the time set to approximately 3.3 million years (M.Y.) ago, when the magnetic field was in the normal polarity. Look for the arrow just above the magnetic polarity time scale that indicates the time and polarity graphically.

At bottom left, a small cartoon shows when the magnetic field is normal (the compass points toward geographic north, as it does today) or reversed (compass points toward geographic south).

When you click on the Start button, time will begin to advance towards the present, and rifting will occur along the spreading ridges (vertical dotted lines). The gap produced will be filled with black if the magnetic field is in its normal state, or with white if it is in its reversed state.

As the process continues, note the motion of the time arrow on the magnetic polarity scale, the correspondence between the polarities on the scale and the polarity state displayed in the cartoon, and production of parallel bands of magnetic stripes on either side of the spreading ridges. Of particular interest are the two transform faults that offset the spreading ridges. Note how the stripes slide past one another along these faults.

When you are ready to experiment further, try Concepts of Sea-floor Spreading under the Help menu and the various options under the Suggestions menu.

Suggestion #1
Before starting the spreading, using a ruler, measure the offset of the ridges produced by the two transform faults.
A. When spreading has stopped, remeasure the ridge offsets. Have they changed?


B. Measure the offset of the edge of the continent (gray area) on the left side of the pattern. Is it the same as the original length of the corresponding transform fault? Why or why not?


Go to Suggestion #2
A. What differences do you see in the pattern of stripes from the previous experiment?


B. Many more earthquakes are found along the transform faults in the interval between the ridges than along the extensions of the faults to the right and left beyond the ridges. Why??


Go to Suggestion #3
Before starting, place markers just above and below the midpoint of each transform fault by clicking where you wish to place them. When all four markers are in place, start spreading.
A. Which transform fault displays left-lateral displacement? Which one displays right-lateral displacement?


B. Watch the marker just below the top transform as it approaches the topmost ridge. How does the relative motion along the fault change as the marker passes the ridge? Why would this point experience fewer earthquakes after it passes the ridge?



Go to Suggestion #4
Start spreading an observe how the striped pattern is formed along the diagonal spreading ridges. Let it continue to spread until it stops at 0 M.Y. (present time).
A.What effect doe a diagonal ridge have on the stripe pattern?


B. Why is the central normal (black) stripe the widest by far of any on the screen?



Go to Suggestion #5
This complex plate boundary is similar to a portion of the Mid-Atlantic Ridge that separates South America from Africa. Start spreading, then stop at 4 M.Y. Place a marker directly on the longest ridge segment (dotted line) by clicking on that point with the mouse. Start spreading again and stop at 2 M.Y. Place another marker on the same ridge crest, and continue spreading until it stops.
A. Measure the distance in cm. between your 4 M.Y. marks. If 1 cm.= 100 km., at what rate (in cm/yr) is the sea widening?


B. Choose Set Spreading.... under the Options menu and set time running backward. Start spreading and continue until your inner markers rejoin and stop there. The age indicator should show just 2.0 M.Y. Explain how the magnetic stripes can be used to reconstruct the relative motion between continents on either side of a spreading ridge as a function of time.


C. Why does the Mid-Atlantic Ridge have almost exactly the same shape as the edges of the continental shelves of the adjoining coasts of Africa and South America?


Go to Suggestion #6
Two continents (shown in gray at the edges) have rifted apart under the influence of a spreading ridge (dotted line in middle). Spreading began at time A and continued steadily until time B, at which point spreading stopped.
A. Using the magnetic polarity time scale below, identify the time at which spreading began and the time at which it stopped.

B. Use the program to recreate the above pattern.


Go to Suggestion #7
Start spreading. Stop when you reach 4 M.Y. Next, choose Set Migrating Transform... under the Options menu and set migration to proceed downwards. Start spreading once again and let it continue until it stops at 0 M.Y.
The rhombic region containing diagonal stripes is the zone traversed and sheared by the migrating transform fault. Though relatively rare, migrating faults can cause very complex patterns.
A. At the end of the process, how many transform faults are active?

Clear Stripes (under Options menu) to check your answer

Go to Suggestion #8
Before the concept of sea-floor spreading was accepted by most geoscientists, the only class of faults with horizontal displacement that was recognized was the transverse fault. Transverse faults do not terminate at spreading ridges, but just extend for some distance until they die out. Offsets of oceanic ridges were assumed to be caused by transverse faults, and so the offsets would be expected to increase with time.
Click OK to set up a (non-spreading) oceanic ridge that is somehow outlined by magnetic stripes. Then click Start to begin motion along the transverse fault.
A. What differences do you see between transform and transverse faults?



Go to Suggestion #9
You can draw your own boundaries, set times for beginning and ending of spreading, set the rate of spreading, and migrate faults.
The more closely spaced the boundaries, the more complex the patterns produced. Migration or shifting of boundaries part-way through the spreading process can make the pattern even more complicated. Fortunately, much of the ocean floor is marked with fairly simple magnetic-stripe patterns and so can be interpreted in a fairly straight-forward manner.