Plate Tectonics as Global Thermostat

Over and over again we come back to to a common theme: the importance of liquid water. For animal life based on DNA to exist and evolve, water must be present and abundant on a planet's surface. Even on the water-rich Earth today, slight differences in water content obviously affect life. In desert regions there is little life; in rainforests at the same latitude, life teems in abundance. For complex life to be attained (and then maintained), a planet's water supply (1) must be large enough to sustain a sizable ocean on the planet's surface, (2) must have migrated to the surface from the planet's interior, (3) must not be lost to space, and (4) must exist largely in liquid form. Plate tectonics plays a role in all four of these criteria.

Earth is about one-half of 1% water by weight. Much of this water arrived among the planetesimals that took part in Earth's formation and accretion. Other volumes of it were dumped here by incoming comets after Earth accreted. The relative importance of these two processes is largely unknown at this time.

Once liquid water is established on the surface of a planet, its maintenance becomes the primary requirement for attaining (and then support ing) animal life. The maintenance of liquid water is controlled largely by global temperatures, which are a by-product of the greenhouse gas volumes of a planet's atmosphere. The temperature of Earth's (and of any planet's surface) is a function of several factors. The first is related to the energy coming from its sun. The second is a function of how much of that energy is absorbed by the planet (some might be reflected into space, and this relationship is dictated by a planet's reflectivity, or albedo). The third is related to the volume of "greenhouse gases" maintained in a planet's atmosphere. Greenhouse gases have a residence time in any atmosphere and are eventually broken down or undergo a change in phase. If their supplies are not constantly replenished, the planet in question (such as Earth) will grow colder gradually until the freezing temperature of water is reached, at which point it will grow colder rapidly (as we have noted, when a planet starts accumulating ice, its albedo increases, boosting its rate of cooling). Greenhouse gases are thus vastly important in maintaining a planet's ther-mostatic reading. Both plate tectonic and non-plate-tectonic planets regularly produce greenhouse gases, because the most important source of these planetary insulators is volcanic eruption, which occurs on most or all planets. On Earth, volcanoes daily exhale vast volumes of carbon dioxide from deep within. Even so-called "dormant" volcanoes are venting carbon dioxide into the atmosphere. On any planet with volcanism there is usually an abundance of greenhouse gases—too much in some cases, and this is where plate tectonics becomes crucial.

Greenhouse gas compositions, and thus planetary temperatures, are byproducts of complex interactions among a planet's interior, surface, and atmospheric chemistry. One of the most important by-products of plate tectonics is the recycling of mineral and chemical compounds locked up in any planet's sedimentary rock cover. On non-plate-tectonic worlds, vast quantities of sedimentary material are produced by erosion. These materials and minerals become sequestered and eventually buried and lithified through sedimentation and the formation of sedimentary rocks, and in most cases, they are re-exhumed only through some process leading to mountain building. Yet, as we have seen, mountain building on non-plate-tectonic worlds is largely confined to the formation of large volcanoes over hot spots. With plate tectonics, however, the motion (and collision) of plates, the formation of mountain chains, and the process of subduction all lead to a recycling of many materials. This recycling plays a large role in maintaining Earth's global temperature values in a range that allows the existence of liquid water. One of the most important of the recycling processes is putting CO2 back into the atmosphere. As limestone is subducted deep into the mantle, it metamorphoses and, in the process, returns CO2 into the atmosphere. This is clearly an important aspect of global warming.

The most important element in reducing atmospheric carbon dioxide (which leads to global cooling) is the weathering of minerals known as silicates, such as feldspar and mica (granite has many such minerals within it). The presence or absence of plate tectonics on a given planet greatly affects the rates and efficiency of this "global thermostat." The basic chemical reaction is CaSiO3 + CO2 = CaCO3 + SiO2. When the first two chemicals in this equation combine, limestone is produced and carbon dioxide removed from the system. The feedback mechanism at work here was first pointed out in a landmark 1981 paper by J. Walker, P. Hays, and J. Kasting. (James Kast-ing has told us that he first had this insight in the middle of his Ph.D. exam!) The mechanism is related to the rates of weathering—that is, the physical or chemical breakdown of rocks and minerals. Although weathered entails the reduction in size of rocks (big boulders weather into sand and clay over time), a very important chemical aspect is also involved (see Figure 9.3). Weathering can cause the actual mineral constituents of the rocks being weathered to change. Weathering of rocks that contain silicate minerals (such as granite) plays a crucial part in regulating the planetary thermostat. Walker and his colleagues pointed out that as a planet warms, the rate of chemical weathering on its surface increases. As the rate of weathering increases, more silicate material is made available for reaction with the atmosphere, and more carbon dioxide is removed, thus causing cooling. Yet as the planet cools, the rate of weathering decreases, and the CO2 content of the atmosphere begins to rise, causing warming to occur. In this fashion the Earth's temperature oscillates between warmer and cooler as a result of the carbonate-silicate weathering and

Release of volcanic CO2

Release of volcanic CO2

Figure 9.3 The CO2-rock weathering cycle. This remarkable cycle has controlled the amount of atmospheric carbon dioxide, a greenhouse gas, to regulate Earth's surface temperature for billions of years. Because this process requires both surface water and plate tectonics, it is not known to occur elsewhere.

Figure 9.3 The CO2-rock weathering cycle. This remarkable cycle has controlled the amount of atmospheric carbon dioxide, a greenhouse gas, to regulate Earth's surface temperature for billions of years. Because this process requires both surface water and plate tectonics, it is not known to occur elsewhere.

precipitation cycles. Without plate tectonics, this system does not work efficiently. It also works less efficiently on planets without land surfaces—and much less efficiently on planets without vascular plants such as the higher plants common on Earth today.

Calcium is an important ingredient in this process, and it has two main sources on a planet's surface. It is found in igneous rocks and (more important) in the sedimentary rocks called limestone. Calcium reacts with carbon dioxide to form limestone, the material that marine animals use to build their shells (and that we humans use to build our cement and concrete). Calcium thus draws CO2 out of the atmosphere. When CO2 begins to increase in the atmosphere, more limestone formation occurs, but only if there is a steady source of new calcium available. The calcium content is steadily made available by plate tectonics, for the formation of new mountains brings new sources of calcium back into the system by exhuming (in magmas) ancient limestone, eroding it, and thus releasing its calcium to react with more CO2.

The planetary thermostat requires a balance between the amount of CO2 being pumped into the atmosphere through volcanic action and the amount being taken out through the formation of limestone. On non-plate-tectonic worlds, buried limestone stays buried, thus removing calcium from the system and producing increases in carbon dioxide. On Earth, at least, plate tectonics plays an integral part in maintaining a stable global temperature by recycling limestone into the system.

Although most accounts of habitability of planets refer to the range between 0°C and 100°C, required temperature range is really much narrower if animals are to survive. As we have seen, life such as bacteria can withstand a range of temperatures that may approach 200°C in high-pressure environments. But animals are much more fragile. Animal life on Earth—and perhaps anywhere in the Universe—depends on the narrowest of temperature ranges within the wider range that permits liquid water to exist. Extended periods of anything above 40°C or much below 5°C will stymie animal life. The planetary thermostat must be set to a narrow range of temperatures indeed, and it may be that only the plate tectonic thermostat makes this fine-tuning possible.

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