Magnetometer Extension Activities
|This activity models real-world uses of a magnetometer instrument. Students will see how sea floor spreading at mid-ocean ridges deposits a record of the history of the reversals of Earth's magnetic field. They will also learn how some ore deposits are found.||Materials:
|Adapted from "Plate Tectonics: The Way the Earth Works" by the Lawrence Hall of Science.|
Student Learning Outcomes:
|Hands-on Activity, Reading & Writing Exercise|
National Standards Addressed:
Click here to see a more detailed explanation of the relationship between this Terrabagga activity and the National Science Education Standards.
Overview: There are two parts to this activity: investigate a seafloor spreading model and investigate an ore deposit model. They are sufficiently independent that you can have your class do either or both, and the order in which you do them doesn't matter. The two activities reinforce each other, and can be done concurrently; one half of the class can be working with one model while the other half is working with the other, and they can switch models halfway through the class period. We recommend doing the two activities concurrently.
Depending on the size of your class, you may want to build more than one of each of these models. You may also want students to use their magnetometer on a few different models you've created.
Each student should complete a student worksheet for each of the two activities.
Constructing a Seafloor Spreading Model
- Open a file folder. Choose one side to tape magnets to.
- On this side, draw 2 lines, that divide the folder space into 4 equal parts (one line should be 11 inches and one line 8 1/2 inches if you are using a standard file folder).
- Arrange the magnets along the long line so that their polarities are symmetric on each side of the short line. For example, you may decide to place 3 magnets on either side of the short line in this order: N-S-N-line-N-S-N. Another arrangement might be N-N-S-line-S-N-N, etc.
- Tape the magnets in place.
- Close the folder and tape it shut as well. Draw the short line on the top of the folder - this represents the mid-ocean ridge.
Constructing an Ore Deposit Model
- With the folder shut, draw a grid on the top of the folder (squares about 2.5 cm or 1 inch on a side work well).
- Open the file folder. Arrange any number of magnets anywhere on the bottom portion of the folder. The magnets will represent the locations of ore deposits. North or south polarity can face up.
- Tape the magnets in place.
- Now close the file folder and tape shut.
- Print the student worksheets - links to them are found in the "Materials and Worksheets" section above
- Each worksheet includes a short background reading assignment, a few multiple choice questions, and an area to sketch results of student exploration of the model
- The students are told in the worksheets to highlight/underline/circle important elements in the reading assignments. You can use these results to assess reading comprehension.
- The responses to the multiple choice questions can also be used to assess comprehension of the readings. Correct answers are:
- Seafloor spreading: 1) C, 2) B, 3) A
- Ore deposits: 1) D, 2) B, 3) B, 4) A
- The sketches can be used to assess how well students did locating and labeling the locations of "ore deposits" and the "seafloor" spreading
The donut magnets used for this activity can be found at Radio Shack stores across the nation. They are relatively inexpensive at 5 for $2 or so. These magnets have a definite north and south pole - with the top face of the magnet being one polarity and the bottom face of the magnet being the opposite polarity.
Nickel and iron ore deposits are the most common metal deposits that are detectable with a magnetometer. You may want to discuss mining with your class in relation to this activity: where the metals used in products come from, environmental concerns related to mining, the importance of recycling metals, and so forth.
Metal "deposits" created by humans can also be detected by magnetometers. Examples include large, metal, underground storage tanks and clusters of buried metal drums (such as a forgotten toxic waste disposal site). Such "deposits" raise many issues relating science to social issues: how abandoned waste disposal sites might be located for cleanup, the possible leakage of fluids from underground storage tanks into groundwater, and so on.
You may want to print a map and tape or glue it to the outside of the folder instead of a simple grid. A map can provide context for the activity and make it more interesting and relevant, especially if it shows an area that students are familiar with. For example, if you know of the locations of mines in your vicinity (especially iron or nickel mines!), you could use a map that includes the location of the mine and place one of the magnets under the mine. Large underground storage tanks also could be detected with a magnetometer. You could include a map (can even be a fairly simple sketch) of the neighborhoods near your school, with magnets under gas stations to indicate the underground gasoline storage tanks at those sites. If you do include a map, remember to indicate a coordinate grid on the map that students can use in reporting the locations of the "deposits" they find.
New seafloor is constantly being created at mid-ocean ridges (such as the Mid-Atlantic Ridge). As the lava that oozes forth at these ridges solidifies, the resulting rock is slightly magnetized by the Earth's magnetic field. Earth's magnetic field is not constant over geologic time scales! The polarity of Earth's magnetic field reverses at irregular intervals. At some times in the past, our modern compasses would have pointed towards the Antarctic as our magnetic North Pole! At other times, the magnetic North Pole was in the Northern Hemisphere as it is today. Since new seafloor, and thus magnetized rock, is constantly being created at the mid-ocean ridges, we can determine the past history of magnetic field reversals of Earth's magnetic field.
Rock that is created at the mid-ocean ridges is constantly being supplanted by newer rock. Thus, rock that is 10 km from a ridge is older than rock that is 5 km from the same ridge, which is in turn older than rock that is 1 km from the ridge. Geologist commonly refer to a conveyor belt carrying rock away from the ridges as an illustrative analogy for this process. The rock ages are symmetric about the ridge, since the material spreads away from the ridge on both sides at roughly equal rates. Material that is 5 km away from the ridge on one side is about the same age, and thus the same magnetic polarity, as material on the opposite side of the ridge at a distance of 5 km.
Scientists use dating methods that measure relative abundances of various isotopes of elements in the rocks to determine their ages. Magnetometers towed behind ships near mid-ocean ridges determine the pattern of magnetic polarity in the rocks at various distances from the ridges. Combining these two sets of data allows us to determine the historical pattern of magnetic reversals of the Earth's magnetic field.
Reversals of Earth's magnetic field occur at extremely sporadic intervals. For example, during past 4 million years the longest interval between reversals was 700,000 years, while the shortest was 20,000 years. In the more distant past, the longest intervals between reversals may have been as long 6 million or possibly even 35 million years.
The answers to the questions on the Student Activity sheet are as follows: C, B, A, D, B, B, A (in the order the questions are asked).
RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:
References & Background Information:
- Mid-Ocean Spreading Ridge
- Convective Motion in the Mantle Drives Seafloor Spreading
- Seafloor Spreading
- Age of Atlantic Ocean Seafloor Crust, Globe
- Graphic of Rocks Recording Magnetic Reversals Symmetrically about a Spreading Ridge
- Magnetic Field Reversals
- Reversals of Earth's Magnetic Field During the Past 5 Million Years
- Reversals of Earth's Magnetic Field During the Past 160 Million Years
- How do plates move?
- The Earth's Crust, Lithosphere and Asthenosphere
- The Earth's magnetic field
- The force of magnetism
- Magnetic field
- Magnetic material
- Magnetism overview