Date sent: Sun, 28 Apr 96 09:21:02 0400 Gaia and Plate Tectonics By Lonnie B. Bird Gaia and Plate Tectonics Geologists and biologists have traditionally thought of life as having adapted to changes in the environmental conditions over time, but a new view of the earth derived from the Gaia theory has emerged. Many scientists now look at the entire earth as an organism, where living and nonliving matter evolve together maintaining an environment nearly ideal for life. The Gaia theory maintains that soon after the formation of life, and organisms began to change the environment, and as well as adapt to this environment. One example of Gaian regulation is in the earth's maintenance of a relatively constant atmospheric temperature since life begun. While the sun now gives off 30 percent more heat to the earth compared to four eons ago. The earth's global temperature has remained relatively constant. One regulator in reducing global temperature is the conversion of carbon dioxide and water in the presence of light into carbohydrates and oxygen. As the amount of light reaching the earth has increased, the rate of photosynthesis increased, thereby removing CO2 and cooling the planet. Another regulator is the storage of CO2 in calcium carbonate in the shells of limestone producing organisms. This control of global temperature has been critical in Gaian regulation of the planet. (Anderson,348) Another example of how the earth and the biosphere may have evolved is in its dependence and possible influence on plate tectonics. It's fairly obvious that plate tectonics has a great effect on the biota, but the hypothesis that the biota has altered plate tectonics is still in its infancy. Some scientists believe that plate tectonics can only occur on a planet that has a moderate surface temperature. High surface temperatures, such as those found on Venus, "favor the development of a thick, buoyant crust." ( Stolz,50 ) For plate tectonics to work, the plates must be thin enough and dense enough to break and subduct. If this is indeed true, then it also must be true that life has an influence on plate tectonics. If today's carbon dioxide levels were comparable to the levels before life became abundant, then we would have a significantly warmer surface temperature, possibly warm enough to prevent plate tectonics.(Stolz,77) The impact of life on plate tectonics, although speculative, would probably be significant. Without life on earth, the atmospheric composition would be approximately 98 percent CO2, roughly the same as is on Venus. The presence of large amounts of CO2 in the atmosphere four billion years ago was necessary to prevent the oceans from freezing through, but if this level of CO2 was present in today's atmosphere, the increased insolation would bring the atmospheric temperatures to 290° F., or 50° C., and causing the oceans to boil away and making the planet forever dry, being comparable to Venus. It has often been suggested that life originated on the earth because of a coincidence between the narrow temperature interval over which water is liquid and the temperature extremes that actually occur on the earth. The earth apparently is also exceptional in having active plate tectonics. If the carbon dioxide in the atmosphere of Venus could turn into limestone, the surface temperatures and those in the upper mantle would drop. The basalt-eclogite phase change would migrate to shallow depths, causing the lower part of the crust to become unstable. Thus there is the interesting possibility that plate tectonics may exist on the earth because limestone-generating life evolve here. (Nisbet,54) The primary reason limestone forming organisms are sometimes credited with allowing plate tectonics to occur on Earth is that they consume carbon dioxide, converting it to calcium carbonate in their shells. When the organisms die, the shells sink to the bottom the ocean and are gradually buried. Eventually some of the carbon dioxide is returned to the atmosphere through volcanic activity, but the net effect is a reduction in carbon dioxide in the atmosphere. Another hypothesis of the influence of limestone forming organisms for which there seems to be little empirical evidence is that the weight of these organisms, mostly stromatolites, depressed the oceanic crust sufficiently to cause subduction. This seems rather unlikely, however, and unless more evidence is gathered supporting this hypothesis, it will probably never be taken seriously.(Stolz,39) It's difficult, if not impossible, to separate the argument of how life effects plate tectonics from the argument of how plate tectonics affects life. One can imagine what the earth might be like if plate tectonics were not active here. Pangea was probably relatively featureless, and if the earth's crust was stationary, it would undoubtedly remain flat. In addition, weathering and erosion would quickly bring the continent to sea level, and eventually the tides would disperse the sediment across the ocean floor. (Perhaps like an Atlantis effect) The impact the absence of plate tectonics would have on life on earth would likely be tremendous. There is a good chance that life could have formed in the oceans in the absence of plate tectonics, but it might not have persisted. It is doubtful that life would ever have been able to survive on land. With weathering and erosion constantly washing vital nutrients into the sea, and without some mechanism for returning these nutrients to the land, the rock would become incapable of supporting life. The presence of water in the liquid state is also vital to plate tectonics as well as life. Water plays an important role in plate tectonics in that as magma rises at constructive boundaries, water permeates the pores of the solidifying rock. This happens at such a rate that, on the modern Earth, a volume of water roughly equivalent to the total volume of the oceans passes through the new lava about once every ten million years, which is a relatively short time, geologically speaking. In the distant past the rate of cycling known as hydrothermal circulation was probably much greater. (Westbroek,322) This process is significant to both plate tectonics and life, as the water in the rock helps the rock to melt again at a lower temperature, and it controls the chemistry of the water. These facts suggest that if plate tectonics were not active, the oceans might have become too toxic to support life. (Lovelock,134) With this information, clearly there is some relationship between life and plate tectonics. While there is the indication that a primitive plate tectonics was in effect before Gaian regulation began, nevertheless there is significant evidence that life has kept the process going. It seems that without life, and the resultant high surface temperature would cause the crust to gradually thicken and become more buoyant, thereby making subduction unlikely. Likewise, without plate tectonics the land would have become sterile and the oceans may have become toxic, and preventing the continual existence of life on Earth. Works Cited Anderson, Don L. The Earth as a Planet: Paradigms and Paradoxes . Science January, 1984): 347-354. Lovelock, James E. Gaia: A New Look at Life on Earth. Oxford: Oxford University Press, 1979. The Ages of Gaia: A Biography of Our Living Earth. New York: Bantam Books, 1988. Nisbet, E. G. Living Earth: A Short History of Life and Its Home. New York: Harper Collins Academic, 1991. Stolz, John F. Biomineralization and Gaia Scientists on Gaia. Cambridge: The MIT Press, 1991. Westbroek, Peter. Life as a Geological Force: Dynamics of the Earth. New York: W. W. Norton and Company, 1991.