Earth’s magnetic field, which is generated by convection currents in the highly-conductive liquid outer core, has been documenting our planet’s past for billions of years. Just like that of a standard bar magnet, our magnetic field behaves, on average, like an axial dipole: it has a north pole and a south pole and the field lines connecting them follow a characteristic geometry. Thanks to certain magnetic minerals that are incorporated into rocks as they form, the orientation of this magnetic field is written into the rock record.
On this week’s podcast, we take a look at paleomagnetism to understand these magnetic signatures and what they can tell us about the past.
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| A view of the Aurora Borealis as seen from Iceland. Image Credit and Copyright: Moyan Brenn |
On geologically short timescales of roughly a half to one million years, the polarity of the Earth’s magnetic field flips, and these reversals are recorded in the new oceanic crust forming at mid-ocean ridges. We speak to Dr. Frederick Vine, who, as a graduate student in the 1960s, made some of the earliest arguments for magnetic seafloor anomalies as evidence of continental drift. On longer timescales — hundreds of millions of years — the geometry of the magnetic field recorded by magnetic minerals in the crust tells us how the continents have moved with respect to each other, and Dr. Ross Mitchell describes how they come together and break apart in the supercontinent cycle.
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| A computer simulation/animation of the Earth’s magnetic field lines. Blue lines emanate from the south pole while orange lines emanate from the north pole. Image Credit: NASA Goddard Space Flight Center |
In addition to plate motions, paleomagnetic data also records episodes of true polar wander–occasions where the rotational axis of the Earth has shifted in response to changes in its mass distribution. Traces of all of these processes are embedded in the rock record, and as more paleomagnetic data is gathered, they offer an ever clearer picture of our planet’s dynamic history.
Here is a quantitative introduction to paleomagnetism:
http://www.geo.arizona.edu/Paleomag/book/chap01.pdf
Find out how magnetism is a consequence of quantum mechanics with Minute Physics and Veritasium:
https://www.youtube.com/watch?v=hFAOXdXZ5TM
Dr. Fred Vine explains his magnetic anomaly theory in the late 1960s:
https://www.youtube.com/watch?v=CRx66ZpEhOg
You can read the first paper by Vine and Matthews (1963) here:
http://users.unimi.it/paleomag/geo2/Vine&Matthews1963.pdf
Here’s a simple demonstration using a bar magnet and iron shavings:
https://www.youtube.com/watch?v=j8XNHlV6Qxg
Here’s a really good article on the supercontinent cycle:
http://io9.com/5744636/a-geological-history-of-supercontinents-on-planet-earth
If you’re curious about the history of plate tectonics, check out these books by Naomi Oreskes:
Plate Tectonics: An Insider’s History of the Modern Theory of the Earth: http://www.amazon.com/Plate-Tectonics-Insiders-History-Modern/dp/0813339812/
The Rejection of Continental Drift:
http://www.amazon.com/The-Rejection-Continental-Drift-American/dp/0195117336/
For more information on true polar wander, here’s a nice overview:
http://physicsworld.com/cws/article/news/2012/nov/12/how-earths-wandering-poles-return-home
Podcast and post by Meg Rosenburg