Overview
The phenomenon of sea-floor spreading is one of the most remarkable features of planet Earth. Wherever two tectonic plates pull apart, the gap is filled continually with newly created oceanic crust. The site of new sea-floor creation is at the crest of the great oceanic spreading ridges, globe-girdling mountain ranges that would be seen as Earth's most dramatic features if they were not hidden beneath the waters of the oceans.

In many places, the spreading ridges are offset by great transform faults. They are so named because the pulling-apart motions of the plates along the ridges are transformed into strike-slip (side-by-side) motion along the faults. There, the plates are neither pulling away from nor approaching one another, but instead are slipping laterally past one another, producing neither gap nor overlap.

New oceanic crust is created at the spreading ridge crest from hot magma distilled from the mantle below. As the magma solidifies into basaltic ocean floor and cools, it records the currently prevailing direction of Earth's magnetic field. Because the magnetic field changes its polarity from time to time, this produces magnetic striping of the sea floor that can be detected with sensitive magnetometers towed behind research ships. It was the interpretation of these magnetic stripes that led to the discovery of sea-floor spreading.

Goals:
Using this program, you will be able to recreate sea-floor spreading and the pattern of magnetic stripes that are created by different configurations of plate boundaries. At the same time, you should be able to see how transform faults work and how their existence is dependent upon the operation of the spreading ridges.

Central Question:
What does the pattern of magnetic stripes on the sea-floor tell us about what is happening at mid-ocean ridges and divergent boundaries?

Materials for each team:
Access to a Macintosh computer
Sea Floor-Spreading program
Other resources:
Activity sheets
Map of ocean floor
ruler

Setting the stage:
Quickly review the types of plate boundaries. You may want to go through the instructions as a class before splitting into groups of three or four students per computer.

Exploration/Investigation
Once the students have read throught the introductory information, there are nine "suggestions" to complete. It is prudent to make certain, early on, that the students understand the basic concepts of sea-floor spreading.

Bringing it together:
Class discussion
You can have the students discuss their experiences with Sea-Floor Spreading and what they learned using the program.

Assessment:
Study Questions:
It is important that the students realize that it is at mid-ocean ridges that new oceanicc crust is created.
1) Is oceanic crust created symetrically on both sides of the spreading zone? How do we know?
2) Where does the new crust created at the speading zone come from?

Background:
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.

In this program, we deal only with the class of transform faults that offset two spreading ridge segments. As you run the program, you may notice that the lengths of the transform faults remain constant and, as a consequence, the offsets of the ridge segments do not change.

In addition, you may have noticed that 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.

The sense of motion along a transform fault (whether it is left- or right- lateral) is a direct consequence of the actively spreading ridges that are offset by the fault. Observations of the sense of motion of actual earthquakes along transform faults on the ocean floor confirm the operation of sea-floor spreading.

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, however, shows only a plan 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. Hence a few earthquakes due to this vertical motion are found along the fracture zones that extend beyond either end of the transform faults.

Changing Things:
You may change the time by clicking anywhere on the magnetIc polarity time scale or by choosing Set age... under the Options menu.

The next thing you will probably want to change is the shape of the plate boundary. First clear the stripes by choosing Clear Stripes under the Options menu, then click on the Set Boundaries button. Then, from top down, draw a ridge (vertical Iine) and click where you want to begin a transform fault (horizontal line). Draw the fault and click where you want to draw the beginning of the next ridge segment, and so on to the bottom of the screen. Then click Done to record your boundary.

The map area is shown as an oblique Mercator projection in which transform faults are always horizontal straight lines, but in which spreading ridges may deviate from the vertical.

This program, however, constrains the top most and bottom-most ridge segments to be vertical. All others may be obIique.

You may also set the spreading rate and direction by choosing Set Spreading... under the Options menu. If you set spreading to run backwards in time, you previously should have run spreading forward in time in order to generate some stripes. Going backward in time simply backs up the previous process.

You may clear the stripes from the screen without affecting the plate boundaries by choosing Clear Stripes from the Options menu. Resetting the age by clicking on the polarity scale always clears the stripes. If you don't want to do this, choose Set age... from the Options menu and do not cIick on the Clear Screen check box before clicking OK. A less-common feature of sea-floor spreading may also be simulated: a migrating transform fault. In this situation, a transform fault may shift its position with time, generating a kind of bologna-slicer effect in which previously generated stripes are sheared and displaced laterally. This can generate quite complicated magnetic stripe patterns. Make sure you understand how basic sea-floor spreading stripes are generated before you try migrating transforms.

To create a migrating transform fault, create a boundary with one or more transform faults, then choose Set Migrating Transform... under the Options menu. Click on a button to specify whether the fault is to migrate upwards or downwards, then click OK. If there is more than one transform fault, you must then click on one of them to specify which is to migrate. You may migrate only one fault at a time in this program. When spreading stops, migration of the transform is automatically turned off.

Activity Sheets:
Student Activity sheet can be downloaded and printed from HERE..