Plate Tectonics and the Magnetic Field

In-Class Activity 1_Active Interior and Crustal Change

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Figure 1 Pattern created by magnetic stripes along the Mid-Atlantic Ridge South of Iceleand

Provenance: Source: http://www.london-oratory.org/chemistry_folder/Revision/Earth&Rocks/New/student%20workbook%202.htm
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Julia Kahmann-Robinson PhD and Marjorie Chan PhD, University of Utah Department of Geology & Geophysics

Purpose

• Understand magnetic reversals and their support of plate tectonics
• Explain how the Earth's magnetic field is generated
• Determine whether or not Mars is an 'active' planet

Preparation

1. Discuss and review the "puzzle" evidence of continental drift and plate tectonics. See examples of government sites such as: http://volcanoes.usgs.gov/about/edu/dynamicplanet/wegener/ http://pubs.usgs.gov/gip/dynamic/dynamic.html
    
2. Other evidence such as fossil correlation and magnetic reversals on Earth may be discussed prior to this activity.
3. Reference: Connerney, et al., 2005, Tectonic implications of Mars crustal magnetism: Proceedings of the National Academy of Sciences, v. 102, no. 42, p. 14970-14975, available online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1250232/

Materials Needed:

Compasses and labeled magnets (positive-negative) to meet the size of the class.

Engage

Have students observe the following image (Figure 1) and explain that the banding is associated with normal and negative polarity reversals in Earth's Magnetic Field as recorded by basaltic rocks at the Mid-Atlantic Ridge. Note the "ridge axis of the Mid-Atlantic Ridge" in the image. This is used as evidence for continental drift.

Ask students the following:

1. How is this image proof that the Earth's crust is moving?
2. Why are the rocks recording a reversal/change in polarity?

Explore

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Figure 2 Normal and Reverse polarity of the Earth in reference to the Earth's magnetic & geographic poles.

Provenance: Source: http://college.cengage.com/geology/resources/protected/physicallab/thelab/plates1/activities/activity2/activity2.htm
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In order to understand Question #2 above, explore the following with students using a compass and magnets:

1. Using the compass, ask students to consider ways the direction of the needle could be opposite i.e. change polarity.
2. To test their ideas, have students see how the magnets cause the compass to behave differently. Does this "test" support their ideas? Ask students to consider if Earth itself was a bar magnet like the magnets they are using.
3. If Earth were a bar magnet, ask students to explain changes in polarity as they consider Figure 2. More importantly, why do our compasses point North in this context?
   

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   Figure 3 Crustal magnetism at Meridiani Terra

   Provenance: Crustal magnetism at Meridiani Terra Mars. Warmer colors are positive anomalies (Connerney, 2005).
   Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.

Compare the crustal magnetism of Mars at Meridiani Terra (Figure 3) to that of the Mid-Atlantic Ridge. Is the Mars magnetism weaker or stronger (are bands as distinct and strong)?

Explain

• The Earth itself is a magnet due to the convection of Earth's inner core causing electrical currents and a resulting electromagnetic field.
• The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15° with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15°with the geographical axis and points only approximately in the North-South directions at a place. In other words, a freely suspended magnet does not show exact geographical South and North because the magnetic axis and geographical axis of the Earth do not coincide.

Elaborate

Different stations on Earth are recording the changes in the electro magnetic field on Earth. One such station is found in Sweden.

1. Navigate to the following website: http://www.irf.se/Observatory/?link[Magnetometers]=Data/
2. Ask students to note the legend and discuss the "description" tab
3. Ask students observe the "real time" data of the EM field ask them to postulate reasons for why there is activity in the data.

Evaluate

In a class discussion environment, ask the class...based upon this activity, if they would consider Mars an active planet.