From the book——
RARE EARTH!
by Peter D. Ward and Donald Brownlee
PLATE TECTONICS #2
Why Is Plate Tectonics Important to Life?
The rate of continental growth is of major importance to life and its ecosystems. The majority of Earth's biodiversity is today found on continents, and there is no reason to believe that this relationship has changed over the last 300 million years. As continents have grown through time, they have affected global climate, including the planet's overall albedo (its reflectivity to sunlight), the occurrence of glaciation events, oceanic circulation patterns, and the amount of nutrients reaching the sea. All of these factors have biological consequences and affect global biodiversity.
The Surprising Importance of Plate Tectonics
In the last chapter we proposed that diversity (roughly the number and relative abundance of species on the planet at any given time) is a major hedge, or defense, against planetary extinction or sterilization of life: High levels of diversity can counter the loss of body plans during mass extinctions. Plate tectonics can augment diversity by increasing the number and degree of separation of habitats (which promotes speciation). For example, as continents break apart, the seaways forming between them create barriers to dispersal. This in turn reduces gene flow and enhances the formation of new species through geographic isolation. Plate tectonics also increases the nutrients available to the biosphere, which may (or may not) also promote increased biotic diversity.
Plate tectonics promotes environmental complexity—and thus increased biotic diversity—on a global scale. A world with mountainous continents, oceans, and myriad islands such as those produced by plate tectonic forces is far more complex, and offers more evolutionary challenges, than would either totally land or ocean-dominated planets without plate tectonics. James Valentine and Eldredge Moores first pointed out this relationship in a series of classic papers in the 1970s. They showed that changes in the position and configuration of the continents and oceans would have far-reaching effects on organisms, causing both increased diversification and extinction. Changes in continental position would affect ocean currents, temperature, seasonal rainfall patterns and fluctuations, the distribution of nutrients, and patterns of biological productivity. Such varying conditions would cause organisms to migrate out of the new environments—and would thus promote speciation. The deep sea would be least affected by such changes, but the deep sea is the area on Earth today with the fewest species. Over two-thirds of all animal species live on land, and the majority of marine species live in the shallow-water regions that would be most affected by plate tectonic movements.
The most diverse marine faunas on Earth today are found in the tropics, where communities are packed with vast numbers of highly specialized species. In higher latitudes the number of species lessens, and in Arctic regions there may be only a tenth as many species as in equivalent water depths or habitats found in the tropics. Not only are there fewer species in the higher latitudes, but the composition of species is also different there. Physiological adaptation constrains most species to fairly narrow temperature limits. Animal species adapted to warm, tropical conditions cannot survive in the cold, nor can the cold-adapted species tolerate the warmth of the tropics. Given that temperature conditions change rapidly with latitude, it's not surprising that north-south coastlines of continents show a continuously changing mix of species. North-south coastlines promote diversity because of the latitudinal temperature gradients. East-west coastlines, on the other hand, often show similar species.
As continental positions change through time, the relative abundance of north-south and east-west coastlines can change. Also, the larger the continents, the lower the environmental heterogeneity. If many or all of the continents were welded together into "supercontinents," biodiversity would be expected to be lower than if there were many smaller and separated continental masses. On one large continent, groups of land animals have fewer barriers to dispersal—and thus less opportunity to form new species. Clearly, continental size and position should affect biodiversity, and this appears to have been the case in Earth history.
What Would Happen If Plate Tectonics Ceased?
The fossil record suggests that there are more species of animals and plants alive on Earth today than at any time in the past, estimates vary between about 3 and 30 million species. This great diversity has come about through many physical and evolutionary factors. We contend that the effects of plate tectonics are among the most important. But once created, does high biodiversity require the continued presence of plate tectonics? We can examine this question with a thought experiment.
An End to Volcanism
Imagine that all volcanism on Earth's surface suddenly ceases. This will stop the many dozens of volcanic eruptions that occur on the continents each year (usually causing great media fanfare and little damage). But the cessation of volcanism will have a far more profound effect. If all volcanism stops, so does sea floor spreading—and thus plate tectonics as well. And if plate tectonics stops, Earth eventually (through erosion) loses most or all of the continents where most terrestrial life exists. In addition, CO., is removed from the atmosphere via weathering, causing our planet to freeze. Of all of the attributes that make Earth rare, plate tectonics may be one of the most profound and—in terms of the evolution and maintenance of animal life—one of the most important.
Only cessation of the flow of heat up from Earth's interior or thickening of the crust would stop volcanism. It is this heat that causes convective motion of the interior, the subterranean motor of plate tectonics. To stop plate tectonics would require eliminating these great lithic boiling pots, and that cannot be done unless all heat emanating from the Earth's interior is stopped (which would require that all the radioactive minerals locked away there decay to stable daughter products) or the composition of Earth's crust or upper mantle changes such that movement can no longer occur. This could happen if the crust became too thick or the mantle too viscous to allow movement. None of these conditions is likely to occur on Earth in the foreseeable future, but there is speculation that just such events occurred on both Venus and Mars in the past.
If Earth's tectonic plates did suddenly stop moving, subduction would no longer occur at the contacts between colliding plates. Mountains—and mountain chains—would cease to rise. Erosion would begin to eat away at their height. Eventually, the world's mountains would be reduced to sea level. How long would it take? The problem is a bit more complicated than simply measuring average erosion rates and calculating the number of years required for the mountains to disappear. This is because of the principle of isostacy. Mountains (and continents) are a bit like icebergs: If you cut off the top, the bottom rises up relative to sea level, causing the entire iceberg (or mountain) to rise. Eventually, however, even this isostatic rebound effect would be overcome by the extent of the erosion.
What would sea level be in a world without further plate tectonics? All of the sediment produced by the simultaneous erosion of the world's mountains would have to go somewhere—and that somewhere is the ocean. The eroding continental mass carried into the oceans by river and wind transport would displace seawater and cause the level of the sea to rise. Calculations by David Montgomery, a geomorphologist at the University of Washington, suggest that the entire Earth might become covered by a global ocean— much shallower than the oceans of today, of course, but global in extent nevertheless. Our planet would have returned to its state of 4 billion years ago: a globe covered completely (or nearly so) by ocean. And with the continents awash, Earth would witness a mass extinction more catastrophic than any in the past. All land life would die off under the lapping waves. Paradoxically, the increase of ocean area would probably also be accompanied by extinctions in the sea. Ocean life depends on nutrients, and most nutrients come from the land as runoff from rivers and streams. With the disappearance of land, the total amount of nutrients (though initially higher as so much new sediment entered the ocean system) would eventually lessen, and with fewer resources, there would be fewer marine animals and plants.
How long before such a water world would be achieved? Tens of millions of years would be required for the mountains and continents to erode to sea level. Yet mass extinction would ensue long before that. Planetary calamity for complex life would occur shortly after the cessation of plate movement, for plate tectonics is not only the reason we have mountains, it turns out to control our planet's climatic thermostat as well.
[THERE WAS ONCE AN EARTH THAT WAS INDEED COVERED BY WATER; AFTER A GREAT HEAVENLY BATTLE, BETWEEN THE FORCES OF THE ONE KNOWN IN THE BIBLE AS SATAN THE DEVIL, AND HIS HELPERS KNOWN NOW AS DEMONS. THEY WANTED THE THRONE OF HEAVEN. A BATTLE TOOK PLACE, SO MIGHTY AND POWERFUL THAT THE HUMAN MIND CAN NOT COMPREHEND. SATAN WAS ALLOWED BY GOD TO DESTROY THIS EARTH AND SO IT BECAME COVERED BY WATER. AND GENESIS 1:1-2 BRINGS US ON THE SCENE WHEN GOD WOULD BRING LIFE BACK TO THE EARTH WE KNOW AS THE BLUE PLANET - Keith Hunt]
Loss of Planetary Temperature Control
The temperature of Earth must remain in a range suitable for the existence of liquid water if animal life is to be maintained. The range of temperature that Earth experiences is the result of many factors. One is the existence of the atmosphere. The average temperature of the Moon is — 18°C, for example, well below the freezing point of water, simply because it has no appreciable atmosphere. If Earth did not have its cloaking atmosphere, including such insulating gases as water vapor and carbon dioxide (producing the much-discussed Greenhouse Effect), its temperature would be about the same as that of the Moon. Yet the Earth, thanks to the greenhouse gases, has an average global temperature of 15°C (33°C warmer than the Moon). Greenhouse gases are keys to the presence of fresh water on this planet and thus are keys to the presence of animal life—and many scientist now believe that the balance of greenhouse gases in Earth's atmosphere is directly related to existence of plate tectonics.
Greenhouse gases are those with three or more atoms, such as water vapor (H20; three atoms), ozone (O,), carbon dioxide (CO2; three atoms), and methane (CH4; five atoms). All can capture outgoing infrared energy from Earth's surface and, in so doing, warm the planet. Their role in keeping Earth's temperature within the critical levels necessary not only for allowing the presence of liquid water (O°C to 100°C) but also for maintaining animals (about 2°C to about 45°C) has been beautifully summarized by Columbia University geologist Wally Broecker in How to Build a Habitable Planet. Broecker describes the following scenario. Imagine that the sun's energy was diminished for a period of time brief by geological standards but long enough for the oceans to freeze. If the sun then resumed its normal output of today, Earth would remain frozen. Once frozen, water reflects much of the light that hits it, and even the current volume of greenhouse gas would be insufficient to reheat the planet to a temperature at which the water would thaw. This condition is called a Global Icehouse, and it is one way a planet can lose its animal life. They freeze to death.
Now, say we reversed this situation and allowed the sun's energy to increase for a geologically short period of time, but long enough so that all of Earth's oceans boiled away, filling the atmosphere with steam. If we then reduced the sun's energy to its present-day levels, the oceans might not recondense, and the planet would stay hot. Once in the atmosphere, the steam would keep the planet hot through its properties as a greenhouse gas, even when solar radiation hitting the planet had decreased. This situation is called a Runaway Greenhouse.
The Earth's greenhouse gases are rare compounds of our planet's atmosphere. It turns out that the major constituents of our atmosphere, nitrogen and oxygen, play little role in the greenhouse warming, because they do not absorb infrared radiation. Carbon dioxide and water vapor, on the other hand, do, even though they make up only a tiny fraction of the gas volume of the atmosphere (carbon dioxide constitutes only 0.035% of the atmosphere). Plate tectonics plays an important part—perhaps the most important part— in maintaining levels of greenhouse gases, and these in turn maintain the temperatures necessary for animal life.
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 re-gions 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 supporting) 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 thermostatic 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 = CaC03 + 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 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 O°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 in-deed, and it may be that only the plate tectonic thermostat makes this fine-tuning possible.
Plate Tectonics and the Magnetic Field
Outer space is not a particularly friendly place. One of its hazards is cosmic rays, which are elementary particles—electrons, protons, helium nuclei, and heavier nuclei—traveling at velocities approaching the speed of light. They come from many sources, including the sun and cosmic rays from distant supernovae, the explosions of stars. These catastrophic events send great numbers of particles hurtling through space.
In The Search for Life in the Universe, D. Goldsmith and T. Owen speculate that without some sort of protection, life on Earth's surface would be extinguished within several generations by cosmic rays hitting our planet's surface. However, the vast majority of cosmic rays are deflected by Earth's magnetic field. The innermost layer of our planet, its core, is made up mainly of iron, which in the outermost region of the core is in a liquid state. As Earth spins, it creates convective movement in this liquid that produces a giant magnetic field surrounding the entire planet. What produces the convection cells in the core is loss of heat. Heat must be exported out of the core, and this liberation of heat appears to be greatly influenced by Earth's plate tectonic regime. Joseph Kirschvink of Cal Tech has suggested that without plate tectonics, there would not be enough temperature difference across the core region to produce the convective cells necessary to generate Earth's magnetic field, no plate tectonics, no magnetic field. The magnetic field also reduces "sputtering" of the atmosphere, a process whereby the atmosphere is gradually lost into space. No magnetic field, perhaps no animal life. Plate tectonics to the rescue again.
Why Does Earth (But Not Mars or Venus) Have Plate tectonics?
Why is there plate tectonics on Earth? The recipe for plate tectonics seems simple enough at first glance. You need a planet differentiated into a thin, solid crust sitting atop an underlying region that is hot, fluid, and mobile. You need this underlying region to be undergoing convection, and for that you need heat emanating from even deeper in the planet. And you are likely to need water—oceans of water: Much new research suggests that without water you cannot have plate tectonics (though perhaps it is simply that without water you cannot get continents).
As in so much else in planetary geology, there is still a great deal we don't know about why our planet (and, more important, any planet) develops and then maintains plate tectonics. Because ours is still the only planet we know that has plate tectonics, we have nothing with which to compare it.
Much of the data pertaining to plate tectonics lies so deep that we are unlikely ever to sample it directly.
As an illustration of the degree of uncertainty about what we might call planetary plate tectonics, which we can define as the theoretical (as opposed to the Earth-based actual) study of plate tectonics, we cannot be certain whether plate tectonics would operate if Earth were 20% larger or smaller, or if it had a crust with more iron and nickel than it does, or if its surface had only 10% of the present-day volume of water. The best current work on these types of questions is being done by planetary geologists V. Solomatov and L, Moresi, who are using computational models to study how convection (the driving force of plate tectonics) works. Yet the abstract of their 1997 paper on the subject concluded, "The nature of the mobility of lithosphere plates on Earth has yet to be explained." We know the plates move, and we know convection moves them. The physics behind the convection is well understood, but its application to subduction is still an enigma.
When we asked about the physical condition necessary to produce plate tectonics on a planet, Solomatov responded, "It is a very interesting problem and we've just started exploring the physical conditions required for plate tectonics to occur on a planet. So far, we have been moving to the conclusion that water might be the factor which is crucial for plate tectonics: no water, no plate tectonics." Without water, the lithosphere (which is the plate of plate tectonics, the rigid surface region composed of the crust and uppermost part of the mantle) is strong and cannot break and descend back into the mantle— the process known as subduction that occurs along the linear subduction zones described earlier in this chapter. According to Solomatov, subduction is a major requirement for plate tectonics. Apparently, subduction zones operate only when the crust is "weak," or able to bend and break, which allows it to descend into the regions where the mantle convection cells sink downward. All of this work is being done with mathematical modeling. Solomatov and his colleagues are using computers to arrive at these generalizations—not trips to the center of the Earth with Jules Verne's heroes.
Even in the absence of water, plumes of hot magma may rise to a planet's surface. But this new material must ultimately go somewhere, and if subduction is not operating, the plates will not move, for the new crustal material must ultimately duck down into the mantle, along the linear subduction zones. Without subduction zones, there is no plate tectonics, even if mantle convection cells are operating inside a planet.
Venus and Mars both lack subduction zones and thus lack plate tectonics. Although both might have the internal mantle convection necessary to move surface plates, the surface itself is composed of "strong" rock (Solomatov's term) that cannot move. Because of its thickness and strength, the crust on these planets is now immobile. The lack of water on both of these plates may be the reason why this is so. Because both of these planets may in the past have had liquid water and crustal composition similar enough to that of Earth, we may find that Venus and Mars once did have plate tectonics—and perhaps lost it when they lost their liquid water. Venus and Mars may be experiencing what Solomatov and Moresi describe as a "stagnant lid regime": The viscosity difference between the convecting mantle and the solid surface is so great that little or no movement of the crust can occur. Yet heat continues to flow upward, and in the case of Venus, this heat caused the entire surface of the planet to melt about a billion years ago (the planetary "resurfacing" we alluded to at the start of this chapter). On Earth this great viscosity difference does not occur. Earth has a "small viscosity contrast regime," according to the technical scientific papers describing all of this, and the result is the very actively moving crust so important for mountain formation, nutrient cycling, and life.
Yet perhaps we have this story reversed. Perhaps Mars and Venus had water but lost it because they had no plate tectonics—and thus no planetary thermostat.
[COULD BE THESE INNER PLANETS WERE VERY DIFFERENT IN THE PAST; BUT WHEN THAT WAR IN HEAVEN CAME ALONG, GOD ALLOWED A SMASHING OF THESE INNER PLANETS, INCLUDING EARTH, AND THE END OF THE DINOSAUR AGE - Keith Hunt]
How (and When) Did Plate Tectonics Start on Earth?
The time of onset of plate tectonics is controversial. Many sources believe it began 1 to 1 billion years after Earth's formation, whereas others view plate tectonics as being far more ancient, its inception dating back over 4 billion years. Much of the controversy involves the rate of heat flow from the early Earth and how this would have affected the composition and rigidity of the planet's surface.
By the time the crust had solidified, more than half of the heat that could result from planetary accretion, core formation, and decay of radioactive isotopes (such as uranium-235) had already been lost from Earth. During the 1 billion years of the Archaean era, heat flow slowed. Some workers believe that the early crust was still too hot and thin to act as the rigid plate necessary for plate tectonics, according to this hypothesis, plate tectonics may not have commenced until 2.5 billion years ago. There is evidence in much older rocks, however, of fault lines and movements consistent with plate tectonics.
[REMEMBER THESE WRITERS ARE IN THE COMPANY OF THOSE WHO BELIEVE THE EARTH VERY SLOWLY FORMED OVER BILLIONS OF YEARS. THERE IS AS YOU SEE, NO TALK OF A QUICK FORMATION, WHAT WE GOD BELIEVERS WOULD SAY “GOD SPOKE AND IT WAS DONE.” THE TRUTH IS WE ARE NOT TOLD IN THE BIBLE HOW QUICKLY OR SLOWLY THE EARTH WAS FORMED - Keith Hunt]
The rate at which plate tectonics built continental surfaces on Earth was not constant. If we plot the size of the continents through time relative to the present area, we do see not a linear increase but a logistic curve—a curve that started slowly, picked up speed in its middle, and then slowed near the end. We have spoken in another context of the "Cambrian Explosion." Here, Earth underwent a "continental explosion" that resulted in a rapid formation of land area. Many lines of evidence suggest that by far the greatest growth took place rather rapidly, during a period between about 2 and 3 billion years ago. This rapid growth completely changed Earth from a planet dominated by oceans to one dominated (at least in terms of its global temperatures and chemistry) by continents.
[AGAIN, WE ARE NOT TOLD BY THE CREATOR, IN HIS HOLY WORD, THE BIBLE, HOW LONG HE HAD DECIDED TO WORK THE FORMATION OF THIS PLANET CALLED EARTH - Keith Hunt]
Could Plate Tectonics Actually
Have Inhibited the Formation
of Animal Life on Earth?
In this chapter we have contended that plate tectonics facilitated the rise and then the maintenance of animal life on Earth. But might not the opposite actually be true? Could it be that plate tectonics actually retarded the rise of animals? This is the contention of two NASA scientists, H. Hartman and C. McKay, who hypothesized that plate tectonics slowed the rate of oxygenation of the Earth atmosphere. In a 1995 article, Hartman and McKay proposed that plate tectonics slowed the rise of oxygenation on Earth and, by inference, on any planet.
As we have detailed in an earlier chapter, animal life did not arise on Earth until less than a billion years ago, whereas life on this planet antedates the first animals by about 3 billion years. One of the most puzzling aspects of life's history on Earth is this singular gap between the first life and the first animal life. Many factors were surely involved, but there is irrefutable evidence that oxygen is a necessary ingredient for animal life (at least on Earth), and there is much evidence that sufficient concentrations of oxygen were not present in the oceans and atmosphere until less than 1 billion years ago. Many scientists suspect that the long time it took for Earth to acquire an oxygen atmosphere accounts for some, or even all, of the delay between the origin of the first life and the origin of animal life on Earth. Hartman and McKay make the novel suggestion that this delay was partly due to the existence of plate tectonics on Earth.
[YOU SEE THE DIFFERING IDEAS BETWEEN PhD SCIENTISTS ON WHEN LIFE CAME ON EARTH, OR THE TIME BETWEEN MICRO-LIFE AND ANIMAL LIFE. YES THERE WAS AN AGE BEFORE GENESIS 1. HOW THAT AGE WAS BROUGHT INTO BEING THE BIBLE DOES NOT TELL US. WHAT GENESIS 1 IS TELLING US IS WHEN GOD DECIDED TO BRING LIFE BACK TO A WATER COVERED EARTH - Keith Hunt]
It is universally agreed that the rise of oxygen on Earth was due to the release of free oxygen as a by-product of photosynthesis. The earliest photo-synthetic organisms used an enzymatic pathway called Photosystem 1, however, this system does not release free oxygen. The later-evolved Photosystem 2 does. This latter system may not have evolved until 2.7 to 2.5 billion years ago. Eventually, photosynthesizing organisms such as photosynthetic bacteria and single-celled plants floating in the early seas would have released vast volumes of oxygen. There was probably some source of inorganically produced free oxygen on the early Earth as well. It may be, for example, that ultraviolet rays hitting water vapor in the upper atmosphere created free oxygen, at least in small volumes. However, a net accumulation could not take place until various reducing compounds (which bind the newly released oxygen and keep it from accumulating as a dissolved gas in the oceans or as a gas in the atmosphere) were used up. For example, the amount of iron in the crust of a planet has a major effect, for all of it on the surface in contact with the atmosphere must be oxidized before free oxygen can accumulate Such reducing compounds emanate from volcanoes, and it can be argued that planets with a higher rate of volcanicity have more reducing compounds in their oceans and atmospheres. Another important source of reducing compounds is organic compounds, produced either through the death and rotting of organisms or through the inorganic formation of organic compounds, such as amino acids. Great volumes of such material are found in the oceans on Earth, but it is usually buried in sediments. In the absence of plate tectonics, argue Hartman and McKay, such sediments become buried in sedimentary basins and are never brought back into contact with the oceans and atmosphere: thus they are removed from active participation in oceanic and atmospheric chemistry. Because they are taken out of the system oxygen can accumulate faster than in the case where reducing compounds are constantly being reintroduced into the atmosphere—a case where the dead don't stay buried.
Hartman and McKay make the intriguing point that Mars may have seen the evolution of complex life within 100 million years of the formation of that planet (assuming, of course, that life originated there at all). Their argument is as follows: The rapid removal of reductants on Mars through burial in deep and undisturbed sediment would have allowed oxygenation to occur much more quickly than on Earth (see Figure 9.4), where plate tectonics constantly recycles sediments via subduction, plate collision and mountain building. All of these processes can cause previously buried sediments to be brought back up to the surface, where their reductants would once more bind whatever atmospheric oxygen was available. Hartman and McKay also point out that volcanicity on a planet like Mars that does not exhibit plate tectonics is much lower than on Earth. Thus the amount of reducing compounds (such as hydrogen sulfide) entering the atmosphere-ocean systems on Mars from volcanic sources would also have been much lower.
Could it be, then, that Earth hosted the evolution and then the maintenance of animal life in spite of plate tectonics? And that plate tectonics actually discourages the attainment of animal life on a given planet because its presence slows the accumulation of the necessary oxygen-rich atmosphere?
We cannot fault the arguments of Hartman and McKay concerning the role of reductants in retarding oxygenation. However, we can point out that plate tectonics would surely increase the rate of biologically produced oxygen on any world, because it enhances biological productivity by recycling nutrients such as nitrates and phosphates. The net productivity on plate tectonic worlds should thus be expected to be far higher than on non-plate-tectonic worlds, so the rate of oxygenation through photosynthesis should also be much higher on a plate tectonic world, perhaps offsetting the retardant effect of the recycling of sediment-sequestered reductants.
Most Crucial Element of the Rare Earth Hypothesis?
Plate tectonics plays at least three crucial roles in maintaining animal life:
It promotes biological productivity, it promotes diversity (the hedge against mass extinction), and it helps maintain equable temperatures, a necessary requirement for animal life.
It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water. How rare is plate tectonics? We know that of all the planets and moons in our solar system, plate tectonics is found only on Earth. But might it not be even rarer than that? One possibility is that Earth has plate tectonics because of another uncommon attribute of our planet: the presence of a large companion moon, the subject of the next chapter.
………………………….
AND THAT IS QUITE THE CHAPTER ON THE GREAT IMPORTANCE OF HAVING A RELATIVELY LARGE AND NEAR TO EARTH, MOON - Keith Hunt
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