This lesson focuses on Earth's magnetic field. We will go over some background material regarding how the magnetic field is generated and why it is important that this planet has one. We'll discuss how to measure the field and also important implications of the magnetic field that led to other discoveries. Namely, we'll use magnetic anomaly maps to reconstruct plate tectonic motion, and we'll explore the Neat-o Interdisciplinary Idea of magnetoreception in animals.
By the end of Lesson 3 you should be able to:
As you work your way through these online materials for Lesson 3, you will encounter additional reading assignments and hands-on exercises and activities. The chart below provides an overview of the requirements for Lesson 3. For assignment details, refer to subsequent pages in this lesson.
Lesson 3 will take us one week to complete. 10 - 16 Jun 2020
Requirement | Submitted for Grading? | Due Date |
---|---|---|
Reading discussion | Yes - we will discuss this in a discussion forum in Canvas. This will be part of your course discussion grade. | multiple participation spanning 10 - 16 Jun 2020 |
Paleomag problem set | Yes - turn in to Canvas assignment called "L3: Paleomag problem set." This will be part of your course data analysis grade. | 16 Jun 2020 |
If you have any questions, please post them to our Questions? Discussion Forum (not e-mail). I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
Please read the following articles, linked in Canvas. The first one, by William J. Broad, is a science article from the New York Times that discusses some of the recent research into the strength of Earth's magnetic field and also briefly delves into the history of magnetic field measurements. Broad also touches on our Neat-o Interdisciplinary Idea that animals use the magnetic field to navigate. In fact, he references the study done by Kenneth Lohmann and colleagues using sea turtles that you will also read as part of this assignment. The brief article from The Economist is a synopsis of a study done by Sabine Begall and colleagues in which they used Google Earth to try to assess the extent to which cows line themselves up preferentially with magnetic north while they graze. I have also included a recent article regarding measurements of magnetic fields induced by the small tsunami generated by the mag 8.8 earthquake in Chile that happened in April 2010.
A short video produced by Science discussing magnetoreception research. We Don't Know: Magnetoreception
Once you have finished the readings, engage in a class discussion that will take place over the entire week devoted to Lesson 3. This discussion will require you to participate multiple times over that period. See the Overview page of this lesson for specific dates.
You will be graded on the quality of your participation. See the grading rubric for specifics.
For an observer at the Earth's surface, the magnetic field is reminiscent of a permanent bar magnet whose poles are close to the geographic poles. Today the magnetic poles are about 11.5° away from the geographic poles. It is this difference that makes it necessary to set a correction on a GPS unit or a compass to account for the angle of declination (the azimuth of the horizontal component of the magnetic field) at your location. In the case where the magnetic and geographic poles coincide, which was most recently true in the late 1600s, the declination would be zero.
In order to build up a mental model of the Earth's magnetic field, let's start by considering a bar magnet like the one sketched below. You can also watch and listen to me draw a sketch of a bar magnet and there's a transcript of my explanation of a bar magnet's field lines.
This bar magnet is a good first order approximation of the Earth's field. Note that I drew the negative pole on top and the positive pole on the bottom. The pole that we call the "north" pole is actually a south magnetic pole because the north poles of magnets are attracted to it! At the poles, the strength of Earth's field is about 6 x 10-5 Tesla, and at the equator, its strength is about half that.
Why does a bar magnet have a magnetic field, anyway? At the atomic level, all atoms have electromagnetic properties because the electrons orbiting the nucleus of the atom produce a magnetic field perpendicular to the spin axis of the electrons as shown in the sketch below. You can also watch and listen to me draw the sketch of the alignment of electron spin axes in a magnetic material as well as read the transcript of me describing electron spin axis alignment.
However, most materials, such as wood, glass, gold, or plastic have atoms randomly oriented so that the teensy magnetic fields produced by each atom cancel each other out. Some special materials like iron and magnetite are composed of atoms where the spin axes of the orbiting electrons all line up in the same orientation. These teensy magnetic fields add to each other and the result is a material that is permanently magnetized. The Chinese figured out thousands of years ago that if you heated iron above a certain temperature (modern science calls this the Curie temperature) and cooled it slowly you could form a magnet out of it. Above the Curie temperature, the iron is so hot that the atoms become disordered and vibrate about. Once the iron begins to cool, the atoms vibrate less and less and they lock into place in accordance with the field of the Earth. After the iron is cooled all the way, the orientation in which it cooled is "locked in." You can pick up the iron and wave it around, and it will still be permanently magnetized according to the direction it was pointed in as it cooled.
So, the Earth is basically a great big permanent magnet. But how does it sustain its own magnetic field? Several models were put forward as early as 1600 to describe Earth's field. One idea was that the Earth's core functioned as a big iron bar magnet, like the first sketch above. In fact, the field lines observed at the surface of the Earth don't rule out this possibility. However, the temperature in the core is hotter than the Curie temperature for iron. The Curie temperature for iron is 770°C, whereas laboratory studies estimate that the temperature at the center of the Earth is about 6600 ± 1000°C. Furthermore, the exact direction and strength of the field fluctuate over time (for example, right now the field is getting weaker and drifting to the west—recall this from Broad's article), and this would not happen if the core were permanently magnetized and stationary. Therefore, the model that best fits our observations of the field is that of a self-exciting geomagnetic dynamo. What this means is that the outer core is composed of an electrically conducting fluid whose motions produce a magnetic field. This model was developed in the 1940s by Elsasser and Bullard and refined in the 1970s by Parker and Levy. A sketch of a simple self-exciting dynamo is shown below. You can also watch and listen to me draw the sketch of the dynamo as well as read the transcript of the screencast about the dynamo.
The Earth is different from this simple sketch because instead of just having a metal disk spinning about an axis with a hole in the middle, the Earth's outer core is a hot convecting fluid. Nevertheless, the sketch does have a couple of important features that are consistent with Earth's field. It can work in either direction, and it sustains its own field through its rotation.
Typically textbooks will tell you to demonstrate magnetic field lines by sprinkling iron filings on a piece of paper and holding it over a magnet; the iron filings will orient themselves in the direction of the field lines of the magnet. This works if you happen to have a lot of iron filings at your disposal. If you don't, try this instead: Get a magnet and hold it up to the screen of your old picture tube TV set (you can do this with an old computer monitor too, but be careful because a strong magnet can play havoc with your hard drive). You should see rings of color around the magnet corresponding to the magnetic field of the magnet. This works because the way the TV projects a picture on to its screen is to electrify particular combinations of the red, green, and blue lights in each picture cell to create the image you see. If you get really close to your TV, you can see the picture cells. When you hold your magnet up to the screen you are basically overriding the magnetic field it was generating on its own.
Word of caution: Don't leave the magnet there too long or this effect can be permanent and you'll have to take your TV into a repair shop to have it degaussed. This experiment is best performed without any interested toddlers around to observe you!
Another word of advice: This won't work on any of them gol' durn newfangled LCD TV's. You've got to have an old one. What better use for an old tv set sitting around in your basement than donating its body to science, anyway?
The Earth's magnetic field occasionally undergoes a spontaneous reversal in which the north and south poles switch places. The mechanism of reversals is still not completely understood, although simulations on supercomputers have been able to reproduce them. These reversals happen very fast geologically speaking.
How do we know reversals must happen fast? (I mean geologically fast)
Click here to see if you are right!
Below are some snapshots from the Glatzmaier-Roberts model of the geodynamo, which was first published in 1995. This model successfully reproduces the intensity of the Earth's field, its dipole character, and its present westward drift. It has also undergone a spontaneous reversal, as shown below (Figures from Glatzmaier and Roberts,1995).
We still don't have a perfect understanding about how the outer core's convection has sustained the field for at least 3,500 million years, but being able to simulate the most obvious features of the Earth's field correctly is an awfully good start.
Below is the 1999 Geological Society of America geologic time scale chart. The main thing I want you to see on this chart is that the periods of normal and reversed polarity have been marked so that they correspond with various ages on the time scale. These periods of time have mostly been set by careful correlation of marine floor magnetic properties.
In the screencast below, I point out the markings indicating episodes of normal and reversed polarity as shown on the 1999 GSA geologic time scale.
The study of the Earth's magnetic field as recorded in the rock record was an important key in reconstructing the history of plate motions. We have already seen how the recording of magnetic reversals led to the confirmation of the seafloor spreading hypothesis. The concept of apparent polar wander paths was helpful in determining the speed, direction, and rotation of continents.
To illustrate the idea of polar wander, imagine you have a composite volcano on a continent like the one in the sketch below. I assure you that the sketch will be better understood if you also watch the screencast in which I talk while I draw it.
This volcano erupts from time to time, and when its lava solidifies and cools, it records the direction of the Earth's magnetic field. A geologist armed with a magnetometer could sample down through the layers of solidified lava and thus track the direction and intensity of the field over the span of geologic time recorded by that volcano. In fact, geologists did do this, and they discovered that the direction of the north pole was not stationary over time, but instead had apparently moved around quite a bit. There were two possible explanations for this:
Before plate tectonics was accepted, most geologists thought that the pole must have moved. However, once more and more measurements were made on different continents, it turned out that all the different polar wander paths could not be reconciled. The pole could not be in two places at once, and furthermore, the ocean floors all recorded either north or south, but not directions in between. So how could lavas of the same age on different land masses show historic directions of the north pole differently from each other? Once seafloor spreading was recognized as a viable mechanism for moving the lithosphere, geologists realized that these "apparent polar wander paths" could be used to reconstruct the past motions of the continents, using the assumption that the pole was always in about the same place (except during reversals).
The example in my fabulous drawing gives a rather vague description of the idea behind using paleomagnetic data to reconstruct the former positions of the continents, but how is it actually done? We use magnetometers.
The angle between the Earth's magnetic field and horizontal is called the magnetic inclination. Because the Earth is a round body in a dipole field, the inclination is directly dependent on latitude. In fact, the tangent of the angle of inclination is equal to twice the tangent of the magnetic latitude, which is the latitude at which the permanently magnetized rock was sitting when it became magnetized. Therefore, given knowledge of your present location and a magnetometer reading of the inclination of your geologic item of interest, such as a basalt flow, you can calculate the magnetic latitude at the time of its formation, compare it to your present location, and determine how many degrees of latitude your present location has moved since that rock cooled.
Save the Lesson 3 Paleomag Problem Set to your computer. You will use this word processing document to record your work. The worksheet content is reproduced below but the link saves you from copying and pasting from the website. The worksheet is in Microsoft Word format. If you don't have access to Microsoft Word, let me know and I can give it to you in another format. You can use whatever text editor you like to work on this assignment. You can even do it by hand, as long as I can read your writing when you scan it. You will submit your worksheet to a Canvas assignment when you are done, so it must be in a format such as .doc, .docx, .pdf, .pages, .jpg, .png, .rtf or .txt so I can open it. If you have a format different than one of the ones listed, it still might work, but check with me first. If you do your calculations on a separate document or piece of paper, then submit those, too, so I can follow your calculations.
For the following problems let's assume that the magnetic poles coincide with the geographic poles to ease our calculations.
Example problem: State College, PA is located at 40.8° N, 77.9° W. Calculate its magnetic inclination.
Answer: Use the formula in which λ = the magnetic latitude and we are trying to solve for I. So,
Your turn!
1.1 Auckland, New Zealand is located at 36.9° S, 174.8° E. Calculate its magnetic inclination.
1.2 Look up the coordinates of your hometown and calculate the magnetic inclination there.
1.3 If I = 0°, where are you?
2.1 You are at a site in India whose coordinates are 23.3° N, 75.8° E studying some basalt outcrops and your magnetometer tells you that the magnetic inclination of the basalt is 30°. Calculate the latitude of this outcrop at the time the basalt erupted and cooled. (This problem is contrived on purpose to be the Deccan Traps, for those of you familiar with that location and where the Indian subcontinent was when they erupted)
2.2 If you can calculate the distance latitudinally that this site moved since this basalt erupted, do so. If not, say why you can't calculate it.
2.3 If you can calculate the distance longitudinally that this site moved since this basalt erupted, do so. If not, say why you can't calculate it.
2.4 Assume there was a 7% error in your magnetometer reading. How much would this error affect the distance you just calculated in 2.2 and/or 2.3?
The figure below is modified from Fred Vine's 1966 paper on seafloor magnetic reversals. Use it to answer the questions in part 3. Study the plot and verify in your head that you can find the names of seven epochs. These are geomagnetic epochs, which are not the same as "epochs" on the geologic time scale. I agree it is silly and confusing to use the same word for different things, but it's the way it is.
3.1 Which geomagnetic epochs correspond to times when the field is normally polarized?
3.2 Which geomagnetic epochs correspond to times when the field is reversed?
The East Pacific Rise profile below is also modified from Fred Vine's 1966 paper on sea-floor magnetic reversals. Use it together with the figure from Part 3 to answer the questions in Part 4.
4.1 Identify the 9 normal geomagnetic epochs and the 8 reversed epochs I have labeled with numbers. The blue numbers are meant to lie on top of the black bits that show normally polarized times, and the red numbers are meant to lie directly underneath the white bits that show reversed times. I want you to identify the epoch that corresponds with each number. It may be easier to identify repeating epochs if you start from the middle and work outward.
4.2 Which geomagnetic polarity epoch corresponds to the crust that is 100 km from the spreading ridge?
4.3 About how old is the crust that is 100 km from the spreading ridge?
4.4 Calculate the spreading rate for this ridge (assume it is constant over the time shown in the profile).
The South Atlantic ridge profile below is also modified from Fred Vine's 1966 paper on sea-floor magnetic reversals. Use it together with the figure from Part 3 to answer the questions in Part 5.
5.1 Identify the 9 normal geomagnetic epochs and the 8 reversed epochs I have labeled with numbers. The blue numbers are meant to lie on top of the black bits that show normally polarized times and the red numbers are meant to lie directly underneath the white bits that show reversed times. I want you to identify the epoch that corresponds with each number. It may be easier to identify repeating epochs if you start from the middle and work outwards.
5.2 Which geomagnetic polarity epoch corresponds to the crust that is 50 km from the spreading ridge?
5.3 About how old is the crust that is 50 km from the spreading ridge?
5.4 Calculate the spreading rate for this ridge (assume it is constant over the time shown in the profile).
The figure below is from Müller et al., 2007. Use it to answer the questions in Part 6. This figure shows the age of oceanic lithosphere around the globe ranging from warm colors (young) to cool colors (old).
6.1 How can you deduce the relative speeds of the spreading rates of the different mid-ocean ridges from this figure?
6.2 Compare the East Pacific rise with the South Atlantic ridge. Do the relative spreading rates agree with the calculations you made in Part 4, Question 4 and Part 5, Question 4?
6.3 Where is the oldest ocean crust?
6.4 Why isn't there any ocean crust on this map that is older than 280 million years?
Save an electronic version of your problem set in a format I can read. I gave you a list at the top of the page, but check with me if you aren't sure. Name your file like this:
L3_paleomag_AccessAccountID_LastName.doc (or other format)
For example, former Cardinals pitcher and hall of famer Dizzy Dean would name his file "L3_paleomag_jhd17_dean.doc"
Upload your problem set to the Paleomag Problem Set assignment in Canvas by the due date indicated on the Overview page.
I will use my general grading rubric for problem sets to grade this activity.
IAGA magnetic palaeointensity database
Müller, R. D., Sdrolias, M., Gaina, C., & Roest, W. R. (2008). Age, spreading rates, and spreading asymmetry of the world's ocean crust. Geochemistry, Geophysics, Geosystems 9 (4), @Citation Q04006. [Available through Library Reserves]
Vine, F. J. (1966). Spreading of the Ocean Floor: New Evidence. Science, 154, pp. 1405-1415.
Glatzmaier, G. A., & Roberts, P. H. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal, Nature, 377, pp. 203-209.
Have another reading or Web site on these topics that you have found useful? Share it in the next Teaching/Learning discussion!
The intricacies of Earth's magnetic field are an ongoing area of research. We understand many things about the field, such as how the clues it leaves behind in the rock record can be used to test the hypotheses of seafloor spreading and tectonic motion. There are also parts of it we don't understand, such as exactly how it has sustained itself for 3500 million years and what really causes a reversal to happen. Finding out how living things on the planet use the magnetic field is also a fairly new discovery. Do you think humans use the field unconsciously? Is this why some people are good at finding their way around and others get lost all the time? Perhaps, if you are one of those people who refuses to ask for directions, you can give the excuse that you are getting in touch with your ability to perceive the magnetic field.
You have finished Lesson 3. Double-check the list of requirements on the Lesson 3 Overview page to make sure you have completed all of the activities listed there before beginning the next lesson.