Consortium for Ocean Leadership. Adapted with permission.
Over time, Earth’s magnetic poles change strength and location. They also completely reverse directions episodically. The north magnetic pole is currently moving northwest at 40 kilometers per year. It moved from 81.3° N, 110.8° W in 2001 to 82.7° N, 114.4° W in 2005. (Learn more at www.ngdc.noaa.gov/geomag/faqgeom.shtml.)
The strength and direction of Earth’s magnetic field at any time in geologic history is recorded by sediments and oceanic crust deposited or formed at that time. After scientists drill and collect a cylindrical “core” sample from the ocean floor, they use an instrument called a magnetometer to determine the magnetic orientation of each of the core’s layers.
This number can be measured in degrees (360°) or inclination (dip), the angle of magnetism with relation to Earth’s surface up to 90° or –90° (for example, 0° is horizontal, at the equator). A magnetometer also indicates pole reversals and the motions of plates on the Earth’s surface.
Before simulating this process in class, your teacher will need to make a “core” for each small group of students ahead of time. See “Preparation for Making the Magnetic Cores” online at www.oceanleadership.org/files/JanuaryActivity12_26_07.pdf for instructions.
- Computer with Internet access
- Foam swim tubes or noodles
- Three bar magnets (any size) for each core
- Compass, preferably round
- Meter stick
- Unlined 8.5” x 11” paper
- Safety scissors
- Transparent tape
- Masking tape
- Working in small groups, cut a sheet of paper into three similar-sized lengthwise strips. Use transparent tape to join the three strips end-to-end to make one long “data strip.”
- Write the word “Top” at one end of the strip.
- Start at the top of the strip and label every five centimeters (5 cm, 10 cm, and so on) along the edge until you reach the bottom.
- Tape your data strip down on the table.
- Lay your core alongside the data strip, with the top of the data strip lined up with the top of the core.
- Place your compass on the data strip and orient the “face” of the compass (not the needle) so that the line running from north to south is parallel with the core.
- Starting at the top of the core and the data strip, draw a circle on the data strip to represent the compass. Record by drawing an arrow within the circle to indicate the direction of the arrow. Label N, E, S, and W on the circle.
- Slide the compass slowly down the data strip. Record and draw a circle every time the compass points directly north, east, south, or west. Continue this procedure until you reach the bottom of the core.
- Discuss: How many times did a polar reversal take place on the data strip? Were the reversal transitions evenly spaced? Where are the oldest and youngest parts of the core on the data strip?
To learn more about paleomagnetism, visit www.oceanleadership.org/education/deep-earth-academy/students/careers/career-profiles/paleomagnetism-specialist.