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Activity Instructions - A Magnetometer and Changing Magnetic Fields
Changes in the Direction of the Earth’s Magnetic Field
If all the compasses in the world started pointing south rather than north, many people might think something very strange, very unusual, and possibly very dangerous was going on. Doomsayers would have a field day proclaiming the end is nigh, while more rational persons might head straight to scientists for an explanation.
Fortunately, those scientists in the know—paleomagnetists, to be exact—would have a ready answer. Such reversals in the Earth's magnetic field, they'd tell you, are, roughly speaking, as common as ice ages. That is, they're terrifically infrequent by human standards, but in geologic terms they happen all the time. As the time line at right shows, hundreds of times in our planet's history the polarity of the magnetic shield ensheathing the globe has gone from "normal," our current orientation to the north, to "reversed," and back again.
The Earth is not alone in this fickleness: The sun's magnetic shield appears to reverse its polarity approximately every 11 years. Even our Milky Way galaxy is magnetized, and experts say it probably reverses its polarity as well. Moreover, while a severe weakening or disappearance of the magnetic field would lay us open to harmful radiation from the sun, there's little evidence to date that "flips" per se inflict any lasting damage.
It might sound as if scientists have all the answers regarding magnetic reversals. But actually they know very little about them. Basic questions haunt researchers: What physical processes within the Earth trigger reversals? Why do the durations and frequencies of both normal and reversed states seem random? Why is there such a disproportionately long normal period between about 121 and 83 million years ago? Why does the reversal rate, at least during the past 160 million years, appear to peak around 12 million years ago?
All these questions remain unanswered, though experts like Dennis Kent, the Rutgers University geologist who supplied NOVA with updated figures for the time line, are hard
at work trying to answer them. In the meantime, not to worry. Reversals happen on average only about once every 250,000 years, and they take hundreds if not thousands of years to complete.
Even the weakening currently under way may be a false alarm. The field often gets very weak, then bounces back, never having flipped. As Ron Merrill, a magnetic-field specialist at the University of Washington remarked when asked whether we're in for a reversal: "Ask me in 10,000 years, I'll give you a better answer." So hang on to your compass. For the foreseeable future, it should work as advertised.—Peter Tyson (http://www.pbs.org/wgbh/nova/magnetic/timeline.html; downloaded 10/13/2009)
DVD Activity: We will now view ~15 minutes of the Magnetic Storm movie produced by NOVA. These segments of the program:
- relate when magnetic field reversals have occurred in the past and speculates when the next reversal might take place.
- feature a computer model showing the flipping of the poles.
- discuss how cooling of Earth's core may be the cause of the weakening magnetic field.
- explore how a changing magnetic field might affect life on Earth.
A map of the Earth’s Magnetic Anomalies is posted on the wall. In groups, look at the map and discuss the following:
- How are magnetic data collected?
- What do you think these magnetic anomalies represent, in other words, they are anomalous compared to what?
- What types of rocks are indicated by magnetic highs?
- What patterns do you see on the map and can you give any explanation for them?