Many years ago I read an article in which the author jokingly referred to something called the “International Stop Continental Drift Society.” Believe it or not, ISCDS was an actual organization in the early 1980s that produced a tongue-in-cheek newsletter for geologists. If it were still around, I’d join in a second: stopping continental drift, like any number of other futile and pointless endeavors, is a cause I could really get behind. Besides, given the complex subject matter, I’d probably learn a lot more from a humorous article than a dry textbook.

In our family, I’m the science guy; my wife tends more toward arts and literature. But she also took a college class that covered plate tectonics, a subject I knew very little about. It gave me a warm feeling in my heart to hear her excitedly talking about continental drift and what happens when the edge of one tectonic plate dives below another one. That’s the kind of stuff we should find interesting, especially since we get plenty of firsthand experience with seismic activity here in San Francisco. But one topic from Morgen’s class stuck out as being particularly interesting: the theory of mantle convection.

Passing the Mantle
The mantle is the thick layer of rock below the crust of the earth. It’s not quite molten, but it’s softer than the crust, and because of the enormous pressure it’s under, it behaves almost like a very thick liquid, with the tectonic plates “floating” on top. The big question that has confronted geologists and seismologists since the existence of tectonic plates was postulated is why they move. And the most reasonable theory to explain that at the moment is that the mantle is fluid in a way—though moving extremely slowly. How slowly? Think in terms of hundreds of millions of years for a given portion of the mantle to circulate from its lowest point to its highest point and back. And that appears to be exactly what’s happening: an unfathomably slow but powerful circular movement within the mantle.

You may be familiar with the term convection to describe water or air currents. The idea is simply that hot portions of a fluid rise, and as they cool, they sink back down. The hot bits going up and the colder bits going down need to stay out of each other’s way, so a somewhat circular motion builds up. It isn’t perfectly uniform, though; watch a Lava Lamp for a while and you’ll see the unpredictable convection currents in action. The theory of mantle convection says that a layer of the earth 1,800 miles (3,000km) thick is doing exactly that: responding to heat from the molten core below, moving upward, then cooling and sinking back down. This movement in turn causes the plates above to shift, accounting for many earthquakes and volcanoes, not to mention the formation of some mountains.

Moving Pictures
Sci-fi adventures notwithstanding, no one has been able to dig under the crust and explore to find out exactly what’s happening down there, but seismological data and computer models give us a fairly good picture of the currents beneath the earth. Not good enough to predict earthquakes—at least not yet—but that problem is analogous to predicting the movement of a piecrust resting on a boiling cherry filling. Tricky, to say the least. What the theory of mantle convection can give us insight into, though, is how and why some features of the planet’s topography came to be the way they are. And with time, I’m sure it will lead to a solution to that whole continental drift thing. Or at least a good movie or two. —Joe Kissell

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More Information about Mantle Convection...

This article was featured in Panta Rei (#4).

Web sites relating to mantle convection:

• Scott D. King’s Mantle Convection Homepage
• Some unanswered questions from the U.S. Geological Survey
• Convection in the Oilfield Glossary
• Walter Kiefer: Mantle Convection Research
• Mantle Convection at the University of Winnipeg
• Mantle Convection in Three Dimensions by Paul Tackley (UCLA) at NPACI

Recommended reading on mantle convection includes Mantle Convection in the Earth and Planets by Donald L. Turcotte, Gerald Schubert, and Peter Olson (1st edition, 2001); Geodynamics by Donald L. Turcotte and Gerald Schubert (1982; 2nd edition, 2001); and Dynamic Earth: Plates, Plumes and Mantle Convection by G. F. Davies (2000).

You can read more about the International Stop Continental Drift Society in Scientific Thinking LO21985.

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by Morgen Jahnke

One of the central paradoxes of scientific research and technological development is that while every new discovery brings previously unknown possibilities to light, these discoveries can also have negative effects that may not be readily apparent. For example, certain medicines may provide exciting new treatment options, but it’s only later that their side effects come to light. One of the most glaring examples of this was the thalidomide scandal in the late 1950s, when thousands of women took this drug to combat morning sickness during pregnancy, and it was later found to cause birth defects. Similarly, in the 19th century, opium was thought of as a cure-all before its highly addictive nature was fully understood.

Along the same lines, Marie Sklodowska Curie’s discovery of the element radium in 1898 at first seemed to lead the way to a variety of novel medical treatments, but as the properties of radioactive materials became better known, radium’s health benefits came to seem more limited. Once added to everything from toothpaste to face cream, radium’s reputation went from cutting edge to dangerous within a few short decades.

The Element of Surprise
Marie Curie’s eventual discovery of radium was first set into motion by the research of French physicist Henri Becquerel, who noticed that materials containing uranium produced rays that fogged photographic plates. Looking into this phenomenon further, Marie Curie found that not only uranium, but also the element thorium, caused these effects regardless of their physical state (for example, dry or wet, crushed or solid), and from this deduced that the rays were part of the elements’ atomic makeup. She coined the word “radioactivity” to describe this property of these two elements, and along with other scientists of the time, opened the way to a new understanding that the atom was not the smallest unit of matter, but that even smaller particles (notably electrons) existed within it.

Building on this information, and on her observation that two uranium-containing compounds, pitchblende and chalcolite, produced much more radiation than uranium alone, Marie Curie speculated that there were other, as yet unknown, elements in these compounds. After extensive experimentation, aided by her husband Pierre Curie, Marie Curie was able to identify two new elements in pitchblende, which she called polonium (after her native Poland), and radium (after the Latin word for “ray”). Although the process of isolating radium involved processing a ton of pitchblende in order to obtain just a fraction of a gram of radium, even with similar levels of effort, the Curies found that it was impossible to isolate polonium. Later on, when the principle of radioactive decay was developed, scientists realized that the short half-life of polonium—138 days—was the reason for this problem.

Radium Reign
With the help of industrial partners who could produce radium much more quickly in their processing facilities than it was possible to do in the lab, the Curies began to develop new uses for this marvelous material. However, the Curies never became rich because of their discovery, but as a service to the scientific community and the rest of the world, freely shared their method of obtaining radium. One of the first uses of radium was as an anti-cancer treatment, owing to its observed ability to damage tissue. The resulting treatment, known as Curietherapy in France, and radiumtherapy elsewhere, is still used in some instances to treat cancer today.

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