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GENERAL SCIENCE 1110L LAB LAB 9: Sea Floor Spreading

According to the theory of plate tectonics, (from the Greek, tekto, “builder”), the Earth’s crust is broken into many slowly moving plates. Sea floor spreading occurs at the mid-ocean ridge where two plates are moving away from each other. Here, magma rises up from below as the sea floor spreads out to either side as shown in Figure 1. This spreading occurs at about the same rate as your fingernails grow.

Mid-ocean Ridge

Oceanic Crust

Lithosphere

AsthenosphereMagma

Figure 1 The pattern of sea floor spreading can be observed by studying the magnetic field of the rock on the sea floor. At the mid-ocean ridge, magma rises up from the mantle below and cools. As it continues to cool, iron in the rock aligns itself with the magnetic field of the Earth, much like the needle in a compass. When the rock solidifies, this magnetic “signature” is locked in place.

Throughout history, the orientation of the Earth’s magnetic field has varied greatly. At times, the magnetic pole in the north has reversed completely and was located near the south geographic pole. Because new ocean floor is constantly moving away from the mid-ocean ridge, these reversals appear as bands of alternating magnetic fields as shown in Figure 1. On average, the Earth’s magnetic field reverses every several hundred thousand years with the most recent reversal occurring about 780,000 years ago.

In this experiment, you will use a model of a sea floor spreading zone. The mid-ocean ridge is running north to south down the center of the model. You will use a Magnetic Field Sensor to map the magnetic field of your model and use it to explain how this is evidence of sea floor spreading.

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OBJECTIVES In this experiment, you will

• Use a Magnetic Field Sensor to measure magnetic field. • Map sea floor spreading. • Interpret your results.

APPARATUS PC with Windows 7 Magnetic Field Sensor Vernier LabPro interface Model sea floor spreading zone Logger Pro Software 3.8.* Ruler

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PROCEDURE A 1...Connect the Magnetic Field Sensor to Channel 1 of the Vernier LabPro interface. The switch on the sensor should be set on the LOW 10 amplification setting.

Figure 1 Vernier Magnetic Field Sensor

2. Prepare the computer for data collection by opening the file “05 Sea Floor Spreading” in the Earth Science with Vernier folder.

3. Zero the Magnetic Field Sensor. This step removes the effects of the Earth’s magnetic field and any local magnetism from the results. a. Remove anything magnetic from the area where data collection will take place. This

includes moving the model sea floor spreading zone. b. Hold the sensor parallel to the table over the area where you will be collecting data, with

the white dot near the tip of the sensor facing upwards (as it will be during data collection).

c. When the magnetic field readings on the screen have stabilized, click the button.

4. Prepare the model sea floor spreading zone for data collection

a. Place the model sea floor spreading zone on your table with the side marked North at the top. The mid-ocean ridge is running north-south down the center of the model.

b. Position the ruler so that it is lined up with the East and West marks on the sides of the pan. Align the 0 cm mark with the left edge of the pan as shown in Figure 2.

5. Click the green button to begin data collection.

6. Position the tip of the Magnetic Field Sensor at the 0 cm mark. Important: For each reading, make sure the probe at the same height as the ruler and the white dot is facing up.

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7. When the reading has stabilized, click .

8. Type “0” in the edit box (for 0 cm) and press the ENTER key. The first data pair has now been saved.

9. Move the Magnetic Field Sensor to the 1 cm mark. When the reading has stabilized, click . Type “1” in the edit box and press the ENTER key.

Figure 2: SEA FLOOR MODEL

10. Repeat the Step 9 procedure at 1 cm intervals until you reach the other side of the pan.

11. When data collection is complete, click to end data collection.

12. Print a copy of your graph for each member of your lab group, as directed by your instructor.

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Figure 3 SIDE VIEW OF THE MODEL LOOKING TOWARDS THE WEST

PROCESSING THE DATA FOR YOUR MODEL 1. Study your graph. The mid-ocean ridge runs north to south down the center of your model.

Use a ruler or straight-edge to draw a line on your graph representing the mid-ocean ridge. Label the line “Mid-Ocean Ridge”.

2. Draw the bands of magnetic reversal on your graph. a. Study your graph. A change between normal magnetic field (“normal” meaning the north

magnetic pole is aligned with the north geographic pole) and reverse magnetic field (“reverse” meaning the north magnetic pole is aligned with the south geographic pole) occurred each time the line crosses zero. Using a straight-edge or ruler, draw a vertical line on your graph at each point where the line crosses zero.

b. In locations where the magnetic field values are positive, the magnetic field of the Earth was normal. In locations where the magnetic field values are negative, the magnetic field of the Earth was reversed. Lightly cross hatch the bands of ocean floor where the Earth’s magnetic field was reversed with a colored pencil or highlighter.

c. Across the top of your graph, label each band either “Normal” or “Reverse”. 3. Toward the bottom of your graph, draw two arrows indicating the direction of movement of the ocean floor. Remember from Figure 1 that the ocean floor is moving away from the mid-ocean ridge.

ANSWER 4 and 5 in your results section of the report

4. What is sea floor spreading?

5. Explain how the Earth’s magnetic reversals provide evidence of sea floor spreading.

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PROCEDURE B: Complete the following procedures on the "Age of the Atlantic Sea

Floor" map on page 6. Print out a copy of this page for each person to work on. The profile from Procedure B is along the line AB. 1. Label the Mid-Atlantic Ridge

2. Color the region of youngest rocks on the map. Use colored pencil.

3. Referring to the "Tectonic Plates" map in the Earth Science Reference Tables, and your ocean bottom profile, draw arrows on either side of the Mid-Atlantic Ridge indicating the direction in which the sea floor is moving.

4. Fold the map and tape it into your Lab Notebook

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EXTENSIONS

1. Using your print out, if the last magnetic reversal occurred about 780,000 ye ars ago, approximately how long ago did it take for the ocean floor at the center of your model to reach the outside edge of your model?

Figure 4: Actual map of Magnetic Reversals on either side of The Mid Atlantic Ridge

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North Atlantic Rift: This portion runs down through the middle of the North Atlantic past a "triple junction" (which is west of Spain) down to near the Equator where it passes another triple junction with a vast transform fault connected to the Carib Plate. At the Equator we pass to the next segment.

Lab Report Format:

Your lab report for this experiment should contain the following sections:

1. Title of experiment in center of the first page. Date to the left of the title. Experimenters name with partners name(s) under experimenters name to the right of the title.

2. Objective

3. Apparatus

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4. Original Data: Graph printed out by the computer. Each person needs a print out in their notebook.

5. Sample calculations: For this lab an example (ONE) calculation needs to be shown for each of the following: Calculate the age of the rocks at the edge of your model.

6. Results: State the age of the rocks at the edge of your model. Answer the two questions at the end of “Processing the Data” section of the handout. Label and color the"Age of the Atlantic Sea Floor" map as directed.

7. Conclusions: None for this Lab

PROPER MATERIALS, ETC. FOR YOUR REPORTS 1. ALL DATA IS TO BE RECORDED DIRECTLY IN YOUR LAB NOTEBOOK. NO SCRATCH PAPER IS TO BE USED. 2. YOU ARE TO USE BLUE OR BLACK INK ONLY FOR RECORDING DATA AND DOING YOUR REPORTS IN YOUR NOTEBOOK. 3. REMEBER, ONLY THE FRONT OF THE PAGES IN YOUR LAB NOTEBOOK ARE TO BE USED FOR DOING YOUR LAB REPORT. I WILL NOT LOOK AT ANY INFORMATIION ON THE BACKSIDE OF THE NOTEBOOK PAGES. 4. DO NOT TEAR OUT ANY PAGES FROM YOUR NOTBOOK. 5.DO NOT ERASE OR USE WHITEOUT FOR MISTAKES!!!! All observations taken under the same experimental condition are equally valid and should be retained for analysis. Do not erase readings. If you must change a reading, draw a single line through it and then record the new measurement next to the old one.