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Hello, I'm Leslie Gordon and welcome to another episode of USGS CoreCast. Today we have another episode in celebrating Earth Science Week. If you listened to our last two episodes, you've learned some things about the Earth's north and south poles: the Arctic and Antarctic, respectively. Today we're going to talk about why we have poles. Not just the geographic poles around which the Earth rotates, but the Earth's magnetic poles. Today my guest is geologist, Dr. Duane Champion, part of the USGS paleomagnetics group, here in Menlo Park, California. So, Duane, can you tell us a little about the Earth's magnetic field and what is it? And what causes it? And why the Earth's magnetic field is important?
Sure Leslie. The Earth has a self-sustaining dynamo caused by the rotation of the Earth and thermal convention in the outer core of the Earth where the liquid iron and sulfur turn in response to the earth's rotation to the east, and forms the magnetic field as we see it at the surface. The magnetic field protects the surface of the Earth from some parts of the sun's radiation, which of course, is what life on Earth depends upon, but the solar wind and some energetic particles that come from the sun are not particularly healthy and the magnetic field protects us from that radiation.
So today's magnetic field is it stable, or does it change with time?
The magnetic field is really changing at a low level all the time; while preserving an orientation very close to the Earth's spin axis: the north and south poles. Through time, through thousands of years it will be stable, but the magnetic field does reverse polarity. The last time it did so was 780,000 years ago, but since that time the magnetic field has had, perhaps, 15 to 20 episodes when it was quite unstable, became quite weak, oscillated in position before it returned being orientated close to the rotational axis of the Earth.
How do you know that the Earth's magnetic field has reversed in the past?
In our laboratory and other laboratories in the world, we study how rocks have recorded the magnetic field. Sediments record the magnetic field as they're deposited in lakes and the ocean. But in particular, volcanic rocks as they come out and cool at the Earth's surface do a wonderful job recording the magnetic field that is available to them at the time they cooled. And we can take samples from those rocks and bring them to the laboratory, measure them in instruments we call magnetometers, and learn about their specific direction of magnetization, their intensity of magnetization, and whether they record polarity...the normal polarity of the present time or reversed polarity that has also occurred in the past.
What is it in rocks that allows it to record a magnetic signature?
Most rocks, in particular volcanic rocks, contain a minor proportion (about 1 to 2 percent) of a mineral called magnetite, which is an iron-titanium oxide. And when this mineral cools in the presence of a magnetic field, it does an excellent job recording exactly the direction and intensity and polarity of the magnetic field that it is cooling in. This mineral is similar to the mineral that's on magnetic that the recording industry uses to record...or at least used to record audio and video.
So, in the paleomagnetics lab here in Menlo Park, you've documented and demonstrated how the Earth's magnetic field has reversed itself several times in the past. And the question is: what's happening now? Is it going to reverse itself again soon?
The magnetic field has reversed numerous times, countless times, frankly, in the past. Our laboratory was founded 50 years ago by 3 scientists who were able to prove that the magnetic field had reversed. It was a possibility at the time they started and they proved that the magnetic field had reversed multiple times in the last millions of years of time. Actually, the magnetic field at present time is in kind of a grey area, in that it has been losing intensity on a global-basis for about 150, 200 years. And there is actually an active scientific discussion, right now, whether or not we're in the early stages of a magnetic reversal or not. The information is not clear, at the present time, but it's something we're investigating.
Well, thank you very much Duane. Thank you for taking the time to participate with us this afternoon. Now that we've talked about the magnetic signature stored in the rocks, in our next episode of CoreCast, we'll be talking about the rocks themselves. So stayed tuned for our next episode and happy Earth Science Week to all our listeners. This is Leslie Gordon for USGS CoreCast. Bye-bye.
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"You Got To Go Down" by Blind Gary
Mentioned in this segment, and other resources:
Good overview of Earth's magnetic field and geomagnetic studies
Info about Paleomagnetic research and laboratory
Title: Earth Science Week, Continued: Geomagnetism and the Self-Sustaining Dynamo Called Earth
Description: USGS scientist Duane Champion explains the Earth's geomagnetic qualities and the potential for and possible consequences of a geomagnetic shift.
Date Recorded: 10/17/2007
Audio Producer: Leslie Gordon , U.S. Geological Survey
Usage: This audio file is public domain/of free use unless otherwise stated. Please refer to the USGS Copyright section for how to credit this audio.
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