1. Magnetic Stripes on the Seafloor
Part 1: Seafloor Magnetic Stripes &
Earth’s Magnetic field
AUTHORS: Leah Ziegler (original author), Scripps Institution of Oceanography, and Dave van Dusen, San Diego School of Creative and Performing Arts
Modifications and Additional Annotations by: Ashlee Henig, Scripps Institution of Oceanography, and Stephen Halpern, San Diego High School
WHY: This presentation Introduces students to the Earth’s Geomagnetic Field and its properties. Did you know that the Earth is like a big Magnet? As scientists were studying the shape of the seafloor and its various features, they noticed a striped pattern of rock magnetization in the rocks of the seafloor. Studies of these magnetic patterns eventually led scientists to the hypothesis of seafloor spreading. This presentation and the activities help students understand how the seafloor becomes magnetized and what the magnetic stripes mean.
SUMMARY: The discovery of seafloor magnetic stripes on the seafloor was an important piece of evidence used by scientists in the discovery of seafloor spreading and the development of the theory of plate tectonics. In this lesson, Earth’s magnetic field and its reversals are reviewed and used to explain the causes for the magnetic stripes observed in oceanic crust. Main points of this presentation are: 1) The Earth has a magnetic field that reverses throughout time; 2) The direction (polarity) of the magnetic field is captured in the rocks of the seafloor as they cool; 3) A symmetric pattern of magnetic reversals can be observed on each side of the seafloor spreading centers. This presentation introduces students to the magnetic field and it’s reversals (flips in direction). The reversals are recorded in the cooling rocks of the seafloor forming a record of when the magnetic field switched polarity. Scientists noticed that this pattern was symmetric about the oceanic ridge, providing clues for the mechanism of seafloor spreading.
PICTURE/GRAPHICS CREDITS: Credits given on each individual slide (see below for this slide)
WEBSITES USED IN THIS PRESENTATION:
ADDITIONAL READING: Wikipedia article on Plate Tectonics (magnetic striping section) and Vine–Matthews–Morley hypothesis.
CONTEXT FOR USE: Activity is good for middle or high school classes. The worksheet may be too challenging for middle schoolers, but they can do the rest of the activities. It is used as part of the Seafloor Spreading Centers Unit to introduce the evidence of seafloor spreading.
MISCONCEPTIONS: ?
EVALUATION TIPS: The worksheet can be used for evaluation of understanding (Worksheet_SF_MagneticStripes.pdf).
TEACHING NOTES. Slide notes are contained within each slide. Please view the lesson plan for additional information on this activity, including required files and materials.
LESSON DOCUMENTS:
*GPTS_activity_details.pdf
*GPTS_labels.doc
*Lesson_Plan_mag_stripes.pdf
Lecture_SF_MagneticStripes.ppt
Lesson_Plan_SF_MagneticStripes.doc
Worksheet_SF_MagneticStripes.pdf
Worksheet_SF_MagneticStripes_key.pdf
Files from SCC 2010 Solid Earth and Plate Tectonics Unit
This Slide:
Figure: this figure shows the magnetic “stripes” observed on the seafloor near Iceland (the Reykjanes Ridge is the name of the oceanic ridge there). The black and white pattern illustrates different magnetizations of the rock – they form a striped pattern like a zebra! These stripes are not actually visible on the seafloor but illustrate the magnetic properties of the rock in a given area (the black and white is a convention that scientists use to distinguish between two different magnetizations). Scientists studying the seafloor and looking for seafloor features noticed this striped pattern from the shipboard magnetometer readings. As we will see soon, this discovery turned out to be a very important piece of evidence in the evolution of the theory of Continental Drift to Plate Tectonics.
Figure credit:
Scripps Classroom Connection

2. Earth’s magnetic field is a Dipole Field, like a bar magnet’s field
Figure (left): Earth with an imaginary bar magnetic in the center. Green lines show the imaginary lines of attraction that form the Earth’s magnetic field.
Figure (right): A bar magnet with iron filings aligned with the magnetic dipole field. Dipole means “two poles.”
The magnetic field of the Earth creates imaginary lines of attraction around the Earth, called the Magnetic Field. The Magnetic Field pulls on a compass needle making it point in a certain direction (you might notice that the imaginary bar magnet drawn inside the figure of the Earth, on the left, has the South pole in the northern hemisphere. This is a convention that scientists invented to account for a compass needle pointing North. Since opposites attract, the north end of the compass points toward a south magnetic pole, so we draw the south magnetic pole in the northern hemisphere). The compass needle aligns with the Earth’s magnetic field just like the iron filings align with the magnetic field (imaginary lines of attraction) of the bar magnet from the demonstration.
Figures taken from:
(CARISMA is the Canadian Array for Realtime Investigations of Magnetic Activity)
Earth’s magnetic field is a Dipole Field, like a bar magnet’s field
Scripps Classroom Connection

3. Earth’s field sometimes flips (Reverses)
What scientists found was that the geomagnetic pole is always changing. It wiggles around close to the North geographic pole, but sometimes wanders farther, all the way to the South pole. Then the North geomagnetic pole will stay there and wiggle around the South Geographic pole for a while. These flips (called ‘reversals’) have happened many many times. The last reversal occurred 780,000 years ago.
Scripps Classroom Connection

4. What was Earth’s magnetic field in the past?
Rocks that cool from lava get magnetized by Earth’s field!
Photo of lava at Hawaii, taken by L. Ziegler. The lava erupts as magma, then cools down to form rocks.
The earth’s magnetic field is recorded in cooling rocks. Volcanic rocks are formed by hot lava cooling and solidifying into solid rock. These rocks have minerals that are magnetic and just like the iron filings or a compass, when the rocks cools the minerals align with the magnetic field. It’s like having a rock filled with lots of little tiny compasses. When the rock cools below a special temperature (the Curie temperature – different for different materials), the magnetic minerals are locked into place, pointing in the direction of the magnetic field at the time they cooled. If the magnetic field is normal at the time the lava cools into rock, the minerals will point in one direction; if the field is reversed when the rocks cool, the magnetic minerals will align in the opposite direction. Scientists can read these magnetizations in a lab and learn the direction that they magnetic field pointed at different times in the past.
Extra details for the educator: Iron in rocks responds to the Earth’s magnetic field only when the rocks are really hot – like when they first form into solid rocks from liquid magma. After the rock cools below the Curie temperature, there is not enough energy in the rocks for any remagnetization to happen as the Earth’s magnetic field changes. Igneous rocks have a ‘frozen in’ record of what the Earth’s magnetic field was like when it first formed millions or even billions of years ago. So it’s like a fossil record of the magnetic field.
The magnetization direction
Depends on the direction of the magnetic field
Gets locked in when the rock cools below a certain temperature
This is how the volcanic seafloor got its stripes!
Scripps Classroom Connection

5. Your compass points near geographic North, the same way it does today
Normal Polarity:
Your compass points near geographic North, the same way it does today
Figure: This figure shows the convention scientists use to document normal and reverse polarity rocks. A normal polarity time period is colored as black on a timeline, while reverse polarity periods are left as white.
Here, define Normal and Reverse polarity – the pictures on the top show the two fields. Then as a reminder, rocks record whether there was a Normal or Reverse polarity magnetic field. Scientists often color graphs or maps of rocks ‘black’ for normal polarity and white for reverse polarity. (This leads into the next slide).
Figure of Earth’s magnetic field taken from:
Extra details for educator: Some students might catch that for the ‘normal’ polarity figure, the arrow is pointing downward (toward the south pole!). This is confusing but true. Remember that with bar magnets, opposites attract. We call it the North geomagnetic pole because that’s where the North pole of the compass points. But does the North pole of a compass get attracted to another ‘north’ magnetic pole? Nope. So the ‘bar magnet’ inside the Earth actually has a ‘south’ end at the North pole. This could be demonstrated with two bar magnets: call one your compass and one the Earth’s dipole, and show that opposite poles attract.
Reverse Polarity:
Your compass points the opposite way it does today (near the South geographic pole)
Scripps Classroom Connection

6. Timeline of magnetic field reversals
Figure: A timeline of geomagnetic reversals in the last 2 million years. Black – normal polarity periods. White - reverse polarity periods. Above, the rocks with “N” or “R” (normal or reverse) are shown.
The picture at the bottom shows the polarity changes through time. The x axis is in ‘Millions of years ago’, (not pronounced ‘Ma’ as in mama, just said as the two letters: M-A). 0 Ma is ‘0 million years ago’ which means, today! ‘2’ means ‘2 million years ago’. From the previous slide: remember black means Normal polarity, white means Reverse. When was the last reversal - ie, the last time the field flipped? (780,000 years ago…or ‘.78 Ma’). How many times did the Earth’s field reverse in the last 2 million years (7 – you count each time the barcode CHANGES color).
Since the rocks of the seafloor are cooled from lava or magma, they record the Earth’s magnetic field at the time they formed. Therefore, a striped pattern of magnetic reversals occurs within the rocks of the seafloor. (The following worksheet illustrates that the magnetic stripes on the seafloor form a symmetric pattern across the oceanic ridge, aka seafloor spreading center).
Figure from:
The little pictures of rocks with ‘N’ and ‘R’ are just a reminder that rocks which formed during black periods would be magnetized with a ‘Normal Polarity’ and the rocks which formed during the white periods would record a ‘Reverse Polarity’.
Scientists found a striped “barcode” pattern like this on the seafloor!
Scripps Classroom Connection

7. Magnetic Reversals Timeline Activity
“VGP Latitude”:
The value scientists measure in the lab to find out if a rock was formed during Normal or Reverse polarity
Scientists only call the place your compass points TODAY the geomagnetic north pole. All the places the compass used to point are called ‘Virtual Geomagnetic Poles’ abbreviated to ‘VGP’, which is another way of saying ‘where the geomagnetic pole would have been’ – The pole location is given in terms of geographic latitude and longitude.
Each rock has a VGP latitude and longitude which tell where the North magnetic pole was when that rock formed. If the VGP latitude is POSITIVE (meaning northern hemisphere), then the magnetic field was NORMAL like it is today. If the VGP latitude is NEGATIVE (meaning southern hemisphere), then the magnetic field was REVERSED.
This is the info you need to know for the ACTIVITY (GPTS_Activity_Details.pdf). Each rock is labeled with a VGP location and an age from the label sheet (GPTS_labels.doc). Follow instructions in the ‘activity details’ sheet, but leave this information up for students to reference. This is not important for students to put in their notes, as it is technical.
Students should complete the GPTS Rock activity
Then
Students should complete the worksheet:
Worksheet_SF_MagneticStripes.pdf
Before proceeding with the lecture
(VGP: Virtual Geomagnetic Pole)
VGP Latitude of YOUR rock sample:
POSITIVE Latitudes = Northern Hemisphere = Normal
NEGATIVE Latitudes = Southern Hemisphere = Reverse
Scripps Classroom Connection

8. Magnetic Stripes on the Seafloor
AUTHORS: Leah Ziegler (original author), Scripps Institution of Oceanography, and Dave van Dusen, San Diego School of Creative and Performing Arts
Modifications and Additional Annotations by: Ashlee Henig, Scripps Institution of Oceanography, and Stephen Halpern, San Diego High School
WHY: This presentation Introduces students to the Earth’s Geomagnetic Field and its properties. Did you know that the Earth is like a big Magnet? As scientists were studying the shape of the seafloor and its various features, they noticed a striped pattern of rock magnetization in the rocks of the seafloor. Studies of these magnetic patterns eventually led scientists to the hypothesis of seafloor spreading. This presentation and the activities help students understand how the seafloor becomes magnetized and what the magnetic stripes mean.
SUMMARY: The reversals of the Earth’s magnetic field are recorded in the newly formed rocks of the seafloor. Throughout time, a record of the changes in magnetic field can be observed in a symmetric pattern across the seafloor spreading centers. Main points of this presentation are: 1) The Earth has a magnetic field that reverses throughout time; 2) The direction (polarity) of the magnetic field is captured in the rocks of the seafloor as they cool; 3) A symmetric pattern of magnetic reversals can be observed on each side of the seafloor spreading centers. This presentation introduces students to the magnetic field and it’s reversals (flips in direction). The reversals are recorded in the cooling rocks of the seafloor forming a record of when the magnetic field switched polarity. Scientists noticed that this pattern was symmetric about the oceanic ridge, providing clues for the mechanism of seafloor spreading.
PICTURE/GRAPHICS CREDITS: Credits given on each individual slide
WEBSITES USED IN THIS PRESENTATION:
ADDITIONAL READING: Wikipedia article on Plate Tectonics (magnetic striping section) and Vine–Matthews–Morley hypothesis.
CONTEXT FOR USE: Activity is good for middle or high school classes. The worksheet may be too challenging for middle schoolers, but they can do the rest of the activities. It is used as part of the Seafloor Spreading Centers Unit to introduce the evidence of seafloor spreading.
MISCONCEPTIONS: ?
EVALUATION TIPS: The worksheet can be used for evaluation of understanding (Worksheet_SF_MagneticStripes.pdf).
TEACHING NOTES. Slide notes are contained within each slide. Please view the lesson plan for additional information on this activity, including required files and materials.
LESSON DOCUMENTS:
*GPTS_activity_details.pdf
*GPTS_labels.doc
*Lesson_Plan_mag_stripes.pdf
Lecture_SF_MagneticStripes.ppt
Lesson_Plan_SF_MagneticStripes.doc
Worksheet_SF_MagneticStripes.pdf
Worksheet_SF_MagneticStripes_key.pdf
* Files from SCC 2010 Solid Earth and Plate Tectonics Unit
Part 2: The Seafloor Tape Recorder
Scripps Classroom Connection

9. Newly Forming Ocean Crust Gets Magnetized
“Stripes” of new crust are Normal and Reverse polarity
Stripes are symmetrical on both sides of the mid-ocean ridge
Figure shows a time series of seafloor spreading where the Earth’s magnetic field is recorded in newly erupted lava that cools to form seafloor. This new seafloor is then spread to both sides of the spreading center, creating a symmetric pattern of seafloor magnetic stripes. The top of the figure is the farthest back in time when less seafloor had been created. The bottom is closer to present time when many reversals of the magnetic field have been recorded in the seafloor.
Visit this website, where it has an animation of sea-floor spreading and a button to click in the upper right which ‘flips’ the magnetic field:
Website has a few discussion prompts as well.
Show students how the magnetic polarity changes and is recorded in the new seafloor by clicking the button of the Earth’s magnetic field in the top right corner. Make it very clear how each magnetic reversal that is recorded is spreading to BOTH sides of the oceanic ridge forming a symmetric pattern.
Figure from:
Scripps Classroom Connection

10. Conclusions The Earth’s magnetic field switches throughout time!
Rocks can record the magnetic field direction.
The Magnetic Reversal Timescale.
Seafloor Magnetic Stripes are Symmetrical!
This conclusion slide is optional for the students. If you prefer, simply reiterate these main concepts without showing this text slide to the students.
The Earth has a magnetic field in the shape of a dipole that reverses polarity (switches direction) throughout time. Normal polarity is what we observe today (compass points North). Reverse polarity occurs when the magnetic field is opposite of what it is today (a compass would point South).
Rocks that cooled from lava can record the direction of the magnetic field at the time when they formed (actually at the time that the cooled below the Curie temperature). Because the rocks that form the seafloor have magnetic minerals in them, they record the polarity of the magnetic field at the time that section of the seafloor forms. This leads to a pattern of magnetic stripes on the seafloor (remember – these are not actual visible stripes, but rather systematic back-and-forth changes in the magnetization of the rocks which can only be observed with readings from a magnetometer.)
The Magnetic Reversal Timescale shows a graphical representation of the changes in polarity of the magnetic field throughout time. Normal polarity is plotted in black, while reverse polarity is plotted in white. You can remind students that this is what they made with the rock VGP activity, and what they plotted on their worksheets.
Seafloor magnetic stripes are more or less symmetrical across the oceanic ridge. (Students don’t learn this until later but - this is because as new seafloor is created it is magnetized and then spread to both sides of the spreading center. This geometry creates a “mirror-image” pattern on the seafloor).
Scripps Classroom Connection