Paleomagnetism lab

Relief map of the world

SONAR and Geology• During World War II, the need to locate and

navigate submarines drove the invention of SONAR. This process uses sound waves and their reflections to determine the distance between a ship and the bottom of the ocean. After the war, this technology was applied to making maps of the ocean floor. Many people were amazed to find that a mountain range runs down the center of the Atlantic Ocean, popping though the surface at Iceland and the Azores. This mountain range, known as the mid ocean ridge, circles the globe.

Compass and North

• If you have seen an orienteering compass, you know it has a carefully balanced needle that points to magnetic North. It does this because it is made from iron, and the Earth has a magnetic field that is related to the movement of material in its liquid outer core.

Molten rock and north

• Now imagine hundreds of these compass needles suspended in molten rock. While the rock is molten, they are free to orient themselves toward magnetic north, just like the compass needle. When the rock hardens as it cools, the particles are frozen in place, pointing in the direction of magnetic North.

North has not always been north• If the motion in the outer core changes, it causes

changes in the magnetic field of the planet. The field can reverse so that magnetic North is now magnetic South, and back again. These changes take place over thousands of years, so people don’t notice them. But new magma is affected by these changes. Every time magma erupts or solidifies under the surface, the iron particles in it align with the Earth’s magnetic field at that time. They make a record of magnetic field changes that is truly set in stone

Magnetometer

• One way to explore the sea floor is with a magnetometer. This device measures the magnetic orientation of the particles in the rock as it is towed across the water. After World War II, oil companies began using magnetometers to look for oil deposits. The data they uncovered proved very interesting to geologists for more reasons than oil.

Main goal

• discover how knowledge of magnetic changes that are sealed in the ocean floor can be used to learn about its history

• In this investigation, we will simulate the ocean floor using magnets covered in sand, and use a magnetic field sensor to determine the orientation of the hidden material.

Materials

• Magnetic field sensor• Laptop computer with Logger pro software• Cm ruler that runs the length of the pan• “Sandy bottom of ocean floor ” set-up in a pan

which needs– 1 rectangular aluminum baking pan (8x12 or larger)

--10 small ceramic magnets --Tape --Sand

Setting up ocean floorTo prepare the ocean floor, make up a pan for each group.1. Arrange the magnets in a row across the bottom of the pan

so that they alternate polarity except in one spot, where two magnets are oriented with the same polarity. Make each pan a little different, with no more than one pan having the magnets with the same polarity in the center of the pan.

2. Tape the magnets to the pan to prevent jumping.3. Mark the pan with tape at the points where the line of

magnets reaches the sides. Label the points W and E.4. Cover the magnets with sand.5. Be sure to keep a record of the magnet layout for each pan

and mark the pans.

Preparing the sensor

• To prepare the sensor, wrap a piece of tape around the barrel of the sensor about 2 cm above the black end with the white dot. This will help the students keep the sensor the same depth all the way through the investigation.

Procedure1. Turn on the computer and attach the Go-link and magnetic

field sensor.2. Set the switch on the sensor to LOW. 3. Open the file: Investigating Paleomagnetism.4. Obtain a sea floor model from your teacher. Be careful

not to disturb the sand.5. Set the cm ruler across the pan so that it runs down the

center of the sand pile, connecting the markings on the sides of the pan from west to east. Align the zero with the edge of the pan.

6. Make a sketch of the arrangement in your journal.7. Hold the magnetic field sensor away from the pan, the

computer, and any steel objects, and click the Zero button.

Procedure

8. Click on the COLLECT button. The KEEP button will then become active.

9. Hold the sensor at the 1 cm mark on the ruler. The sensor should be vertical with the white dot facing the person holding it. The bottom of the tape should be even with the ruler’s edge. This will be easier if one person holds the ruler in place.

10. When the value for the magnetic field has stabilized at the hundredth position, click the KEEP Button.

11. When the dialog box appears, fill in 1 and press ENTER. The value will be recorded in the data table.

Procedure

12.When the dialog box appears, fill in 1 and press ENTER. The value will be recorded in the data table.

13.Repeat steps 9 and 10 all the way across the pan, taking data each centimeter, and recording that distance measurement in the dialog box.

14.When you have taken a measurement at each centimeter, click on the STOP button.

15.Use CONTROL+ L to store the run.

Typical results

• The initial graph looks like a disorganized mess. There appears to be no pattern in the data.

Connecting data points

• Connecting the data points with a line shows a pattern that was not previously visible.

Changing to bar graphs• Application of the bar graph

option clarifies the pattern even more. Note the width of the section closest to the 10 cm line. This is the area which appears to be the center of the spreading. If students become frustrated with the pencil tool, they can just make a bar graph from the original data. It is not as dramatic a way to show how data can be analyzed in different ways, but it will work for the less coordinated.

Analysis Questions1. What type of pattern do you see in the data?2. From the Tool bar, select Predict. Use the pencil to

connect the dots in the order they were gathered from left to right.

3. What type of pattern do you see in the data?4. From the OPTIONS menu, select GRAPH OPTIONS. Check

the box for bar graph in the dialog box. Click DONE.5. Sketch this graph in your journal.6. What pattern do you see in the data?7. What inference can you make that is directly supported

by the data?

Making Sense

• There were gaps in the data and they were amazed that the magnetic field of the earth had not always been the same. As you have done, they looked at the data in many ways to try and make some sense from it. Finally, they saw, as you did, that there was one wide spot in the data and that the pattern spread away from that area. They reasoned that the rock must have originated in that area and been pushed to the sides as new rock formed.

Making Sense

• Comparison of this idea with a topographic map of the ocean floor showed that the wide spot coincided with the mid ocean ridge. They inferred that there was some connection between the location of the ridge and the eruption of new rock