Evolution of the earth

Stages of the evolution of the earth are listed in Table 15.3. The evolution of the sun and the planets from solar nebula began about 4.6 10 y ago. Materials of this age are not found on the earth, because most primordial solids on the earth went through one or several metamorphoses. However, the material of meteorites which have been formed simultaneously with the earth make it possible to date the age of the earth at 4.5 10 y. The oldest minerals on the earth have an age of the order of 4.3 10 y and suffered metamorphoses about 3.8 10 y ago. The age of the oldest rock for- 

3- 10 y First stages of the earth s crust, formation of the oldest minerals found on the earth, formation of hydrosphere and atmosphere 

Several proposals have been made about the origin of the matter from which the earth and the solar system have been formed. These proposals are mainly based on the isotopie eomposition. The supernova hypothesis explains the presence of heavy nuelei in the solar system by a supernova explosion some time before the evolution of the solar system. This hypothesis is supported by the isotopie analysis of meteorites whieh shows an anomaly in the Xe eontent. This anomaly is attributed to the decay of (ti/2 = 1.57 10 y) which must have been present during the evolution of the solar system. 

In any case, the primordial radioactivity on the earth was appreciably higher than at present. The ratios of the aetivities at the time of the birth of the earth to those at present are listed in Table 15.4 for some long-lived radionuclides that represent the main radioactive inventory on the earth. The relatively high activity of about 2 10 y ago is the reason for the operation of the natural nuclear reactors at Oklo at that time (seetion 11.8). 

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Ronov, A. B. (1964). Common tendancies in the chemical evolution of the Earth s crust, ocean, and atmosphere, Geochem. 8,715-743. 

Archaea Period of time in the evolution of the Earth 3.8-2.5 billion years ago. 

Source. After Drever, J. I., et al. (1988). Chemical Cycles in the Evolution of the Earth, John Wiley Sons, p. 36. 

Onuma et al. 1970). By comparison with other terrestrial rocks, the range of observed 8 0-valnes is very narrow. For instance, terrestrial plagioclase exhibits an O-isotope variation which is at least ten times greater than that for all lunar rocks (Taylor 1968). This difference may be attributed to the much greater role of low-temperature processes in the evolution of the Earth s crust and to the presence of water on the Earth. 

Halliday, A. N., Rehkamper, M., Lee, D.-C. and Yi, W. (1996) Early evolution of the Earth and Moon new constraints from Hf-W isotope geochemistry. Earth and Planetary Science Letters, 142, 75-89. 

Abe Y, Matsui T (1986) Early evolution of the Earth Accretion, atmosphere formation, and thermal history. J Geophys Res 91(B13) E291-E302... 

It is fair to say that the Arctic system constitutes a unique and important environment with a central role in the dynamics and evolution of the earth system (Vorosmarty et al., 2001). 

Suspensions, and to some degree emulsions and foams, play crucial roles in the evolution of the earth s rocks, rivers, streams, lakes, oceans, and soils. Table 9.1 lists some examples. In many cases their role is somewhat disguised in that these colloidal dispersions are the precursors to the ultimate products, the latter having very different final appearances, such as many rocks, sediments, and soils. 

In recent years innumerable publications have dealt with the natural carbon cycle and its alteration by human activities. Some summary works of interest in this chapter are Atmospheric Carbon Dioxide and the Global Carbon Cycle (ed. Trabalka, 1985), The Carbon Cycle and Atmospheric CO2 Natural Variations, Archean to Present (eds. Sundquist and Broecker, 1985), Chemical Cycles in the Evolution of the Earth (eds. Gregor, Garrels, Mackenzie, and Maynard, 1988), History of the Earth s Atmosphere (Budyko, Ronov, and Yanshin, 1985), and The Chemical Evolution of the Atmosphere and Oceans (Holland, 1984). The interested reader is referred to these volumes for further discussion of material presented here. 

Because sedimentary carbonates represent primarily chemical and biochemical precipitates from seawater, and because they make up 20% of the common sedimentary rock record, these rock types have been particularly good sources of chemical and mineralogical data for interpretation of the secular and cyclic evolution of the Earth s surface environment. This carbonate rock record as a function of geological age is now explored as are age trends in other rock types and sediment properties. With this information as background material, we can then discuss what these relationships tell us about the history of carbonates and the exogenic system throughout geologic dme. 

A major increase in attention to the "CO2 problem" during this decade also stimulated considerable activity. Much of this activity was devoted to modeling current systems, such as those described in Chapter 9. While a major portion of this activity focused on the current environment and human impact, the "CO2 problem" also sparked interest in trying to understand long-term processes, such as the role of sedimentary carbonates in the evolution of the Earth s atmosphere. 

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