Plate Tectonics Gets Squishy
Two reports on plate tectonics this week make it seem less like “hard” science. Over 30 years ago, plate tectonics theory surprised many by going mainstream. In recent years, however, observations have complicated matters.
In the July 8 issue of Nature,1 Norman H. Sleep evaluates a paper in the same issue2 that tackles the problem of hotspots. Regarding “inadequacies in understanding the relative motions between plates,” he comments, “In case you think this has been sorted out to decimal places in the past 30 years, it hasn’t.” (For background, see 04/02/2004 and 11/04/2003 headlines.) Sleep praises the efforts of Steinbeck et al. to understand hotspots and fluid motions in the mantle, particularly how the Hawaiian chain could make a sudden turn. But he ends, “I expect that debate will continue on the relative fixity of hotspots, the rigidity of tectonic plates and mantle dynamics.”
The Himalayas have been a poster child of plate tectonics theory. Richard A. Kerr in the July 9 issue of Science3 discusses new satellite measurements around the Tibetan plateau that cast a common assumption into question. It has long been taught that Mt. Everest and its range were thrust upward to their lofty heights by India crashing into the Asian continent. New synthetic aperture radar measurements of the Tibetan plateau from the InSAR satellite, however, show much slower movement along faults than expected – like 0 to 7mm per year instead of 30, in one instance, and a factor of 10 lower in another. Interference diagrams, on the other hand, show the entire region deforming. Instead of a rigid mass moving between faults “like a watermelon seed between two fingers,” the Tibetan plateau seems to act like a fluid, as if “India were colliding with a water bed.” Kerr remarks, “For almost 40 years, scientists have recognized that Earth’s ocean floors jostle and slide past one another like enormous rigid plates. But how well continents fit into this plate-tectonic scheme has been less clear. Now, satellite measurements of the Tibetan Plateau suggest that when continents go head-to-head in mountain building, they can behave more like unbaked pizzas.” Another scientist concluded from the new data, “Continental tectonics is not plate tectonics.” This part of the continent, Kerr says, rather than standing up and fighting, is trying to escape.
1Norman H. Sleep, “Earth science: Kinks and circuits,” Nature 430, 151 – 153 (08 July 2004); doi:10.1038/430151a.
2Steinberger, Sutherland and O’Connell, “Prediction of Emperor-Hawaii seamount locations from a revised model of global plate motion and mantle flow,” Nature 430, 167 – 173 (08 July 2004); doi:10.1038/nature02660.
3Richard A. Kerr, “Hammered by India, Puttylike Tibet Shows Limits of Plate Tectonics,” Science, Vol 305, Issue 5681, 161, 9 July 2004, [DOI: 10.1126/science.305.5681.161a].
4Wright, Parsons, England, and Fielding, “InSAR Observations of Low Slip Rates on the Major Faults of Western Tibet,” Science, Vol 305, Issue 5681, 236-239, 9 July 2004, [DOI: 10.1126/science.1096388]. 1
Now I’m getting hungry for pizza and watermelon. Geological fads are like panaceas that cure all the symptoms until the MRI arrives. Data have a way of spoiling the fun of storytelling. Some unscrupulous theorists like Charles Lyell fudged data to make it fit their mental pictures of how the world should work.
When plate tectonics theory became popular in the 1960s, some holdouts complained it was being foisted on them like a new religion. For example, even as late as 1983, in a popular geology book sold in western National Parks, Donald L. Baars had asked whether the theory was “Geophysics or Metaphysics?”—
The concept of the New Global Tectonics may be liked to a new religion; since hard facts are lacking, if one is not a “believer” one is considered an “atheist” with regard to the many theories and interpretations of the “clergy”—the oceanographers and geophysicists. Many of the concepts are plausible and exciting, and sometimes they fit the hard geologic facts. Many times, however, they are contradictory and totally incongruent with known geologic facts, at which time the facts are ignored. With enough “faith,” every known earth event is compatible with the religion, especially with respect to oceanography. On land, however, where outcrops and fossils abound, it is often extremely difficult to be a “follower.” The entire doctrine may in time be proven true, it may be completely disproven by geologists, or a compromise may be reached. I prefer to think the last possibility is likely. … [He describes some examples of contradictions.]
It would require another book to argue fully the pros and cons of plate tectonics theory. It is obvious at this point that I have not been totally converted to the religion. That is a matter for individual preference. You are free to believe as you wish, but please, don’t send missionaries!
—Donald L. Baars, The Colorado Plateau: A Geologic History (Univ. of New Mexico Press, 1972, 1983, pp. 217-218, p. 219).
No one doubts that various fluids and solids are moving various whichaways down under our feet, at various speeds and in various directions. But as with many things in science, the phenomena are too complex to reduce to simple models. What explains one province may not explain another. A neat global diagram of rigid plates floating on convecting mantle currents makes a nice flannelgraph in Monday School, but what was the Historical Geophysics? We’ll have to wait and see what happens to this religion. The lesson is: don’t take the national park diagrams on blind faith.