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9.01 Earth Formation and Evolution D. J. Stevenson, California Institute of Technology, Pasadena, CA, USA © 2007 Elsevier B.V. All rights reserved. 9.01.1 Introduction 9.01.1.1 How Should We Think of Earth and Earth Evolution? Evolutionary science is for the most part based on observation and indirect inference. It is not experimental science, even though experiments can certainly play a role in our understanding of processes. We can never hope to have the resources to build our own planet and observe how it evolves; we cannot even hope (at least in the foreseeable future) to observe an ensemble of Earth-like planets elsewhere in the universe and at diverse stages of their evolution (though there is certainly much discussion about detection of such planets; e.g., Seager (2003)). There are two central ideas that govern our thinking about Earth and its history. One is ‘provenance’: the nature and origin of the material that went into making Earth. This is our cosmic heritage, one that we presumably share with neighboring terrestrial planets, and (to some uncertain extent) we share with the meteorites and the abundances of elements in the Sun. The other is ‘process’: Earth is an engine and its current structure is a consequence of those ongoing processes, expressed in the form it takes now. The most obvious and important of these processes is plate tectonics and the inextricably entwined process of mantle convection. However, this central evolutionary process cannot be separated from the nature of the atmosphere and ocean, the geochemical evolution of various parts of Earth expressed in the rock record, and life. Figure 1 shows conceptually the ideas of Earth evolution, expressed as a curve in some multidimensional space that is here simplified by focusing on two variables (‘this’ and ‘that’), the identities of which are not important. They could be physical variables such as temperature, or chemical variables (composition of a particular reservoir) or isotopic tracers. The figure intends to convey the idea that we have an initial condition, an evolutionary path, and a present state. The initial condition is dictated not only by provenance but also by the physics of the formation process. By analogy, we would say that the apples from an apple tree owe much of their nature not only to the genetics of apples (the process of their formation) but also, to some extent, the soil and climate in which the tree grew. We are informed of this initial condition by astronomy, which tells us about how planets form in other solar systems, by geochemistry (a memory within Earth of the materials and conditions of Earth formation), and by physical modeling: simulations and analysis of what may have occurred. In Figure 2, another important idea is conveyed: for many purposes, we should think of time logarithmically. This is in striking contrast to the way many geoscientists think of time, because they focus (naturally enough) on where the record is best. As a result, far more geological investigations are carried out for the Phanerozoic (10^8 years of Earth history) than the entire period before this. More importantly, the processes that govern early history are very energetic and fast. As a consequence, more could have happened in the first millions to hundreds of millions of years than throughout all of subsequent geologic time. Table 1 develops this idea further by identifying some of the important timescales of relevance to Earth history and prehistory (here taken to mean the important events that took place even before Earth formed). From this emerges the subdivision of geologic time into the accretion phase (the aggregation of bodies to make Earth), lasting a hundred million years at most, an early evolution in which the high-energy consequences of the accretion (the stored heat) and possibly later impacts still play a role, perhaps lasting as long as half a billion years, and the rest of geologic time in which the energetics of Earth is strongly affected by the long-lived radioactive heat sources. In this overview chapter, an attempt is made to identify the main themes of Earth history, viewed geophysically, and to provide a context for appreciating the more detailed following chapters. At the end, some of the outstanding issues are revisited, reminding us that this is very much a living science in which there are many things not known or understood. 9.01.1.2 History and Themes Hopkins (1839) in his ‘Preliminary observations on the refrigeration of the globe’ illustrates well the prevailing view of that time when Earth started hot and was cooling over time. This hot beginning now seems natural to us as a consequence of the gravitational energy of Earth formation, and it has been a consistently popular view even when the justifications for its advocacy were imperfectly developed. Famously, Lord Kelvin (Figure 3) took the hot initial Earth and applied conduction theory to the outermost region to estimate the age of Earth at 100 My or less. Burchfield (1975) in his book, Lord Kelvin and the Age of the Earth, documents Kelvin’s various estimates and the conflict with the Victorian geologists of the time who believed that Kelvin’s estimates could not be sufficient to explain the features we see. Kelvin’s confidence was bolstered by the similar estimate he obtained for the age of the Sun. Indeed, astrophysicists refer to the ‘Kelvin time’ as a characteristic cooling timescale for a body, defined as the heat content divided by luminosity. We now know that Kelvin was wrong about the Sun because he was unaware of the additional (and dominant) energy source provided by fusion of hydrogen to helium. Ironically, Kelvin could have obtained a correct order of magnitude estimate for Earth’s age had he evaluated Earth’s Kelvin time. For a plausible estimate of mean internal temperature of 2000 K (a number that would have seemed perfectly reasonable to Kelvin), a heat capacity of 700 J kg⁻¹, a mass of 6 × 10²⁴ kg and an energy output of 4 × 10¹³ W, he could have obtained: Kelvin: \( \frac{(2 \times 10^{24}) (700) (2000) 4 \times 10^{13}}{2 \times 10^{17}} = 7 \text{ Ga} \) It should not have been unreasonable for him to suppose that this was physically sensible since at that time the fluidity of Earth’s interior was still in doubt and the concept of efficient convective transport already existed. The subsequent discovery of fission and long-lived radioactive heat sources was ‘not’ the reason he got the wrong answer. Indeed, we now think that Earth could have a significant part of its heat outflow and dynamics even if those radioactive heat sources did not exist. Plate tectonics and mantle convection is a central theme as the primary controlling principle of most of Earth evolution. Mobility of Earth was proposed by Wegener (1912) and the connection to deep-seated motions was also suggested long ago, for example, Bull (1921). Ключевые слова: e, r, o