Earth s evolution

In order to understand the Earth s character as a planet, it also is helpful to have an understanding of how the elements in our solar system were formed. Chapter 2 starts with the Big Bang theory and continues with how very small grains eventually came together and accreted to form the beginnings of what would eventually become the Earth and other planets, about 4.5 X 10 years ago (4.5 Gyr). The initial processes of the Earth s evolution involved heat... 

Deamley, R. Ancient mountain ranges — a clue to Earth s evolution. Discovery 27, 18 (1966). 

The range of possibilities to be considered for the nature of the earliest atmosphere provides such a broad spectrum of consequences for thermal and magmatic evolution that it is better to consider the atmospheres first before discussing other aspects of Earth s evolution. Therefore, in this section a brief explanation of the different kinds of early atmospheres and their likely effects on the Earth are given in cursory terms. More comprehensive information on atmospheric components is found elsewhere in Volume 4 of this treatise. 

What is the most difficult research job in all of science It is the task of deducing the history of Earth s evolution in its first two billion years, during the Hadean eon from 4.5 to 4.0 Ga (IGa = 10 years before present) and the Archean eon from 4.0 to 2.5 Ga. Detectives of ancient rocks are faced with a lack of clues to investigate. Most of Earth s surface is covered by water. The cores of the continents are probably underlain by rocks older than 2.5 Ga but they are hidden by a veneer of younger sediments. Even in localities where Hadean-Archean rocks are readily available, alteration by subsequent geologic events has obscured the record of conditions under which they were formed. It is revealing to compare and contrast the detailed information available in a textbook of Earth s modem ecologies with the sparse collection of facts accepted by disciplinary consensus for the Hadean-Archean eras. 

As discussed in 19.10, Pu has been formed in natural uranium reactors at a later stage of the earth s evolution. Many thousands of tons of plutonium has been synthesized in commercial and military reactors the annual global production rate in nuclear power reactors in the year 2000 was 1000 tons/y, contained in the spent fuel elements. The nuclear reactions and chemical separation processes are presented in Chapters 19 and 21. The build-up of heavier elements and isotopes by n-irradiation of Pu in nuclear reactors is illustrated in Figures 16.2 and 16.3. The accumulated amount of higher actinides within the European commimity is many tons for Np, Pu and Am, and himdreds of kg of Cm the amounts in the United States and Russia are of the same magnitude. 

Hann (1897) first addresses climate changes with a separate section in his handbook in the sense of periodic variations ( the distribution of climatic elements is not [an] absolute constant ) but also in the sense of continuous changes over the earth s evolution. A first systematic description of climate changes is found in Hann (1908). Bruckner (1890) first identified climatic cycles by analyzing meteorological records (the so-called Bruckner cycle of 35 years). 

One way in which Earth s evolution as a planet can be understood is by measuring the amounts of certain isotopes in rocks. One quantity recently measured is the ratio of Xe to °Xe in some minerals. In what way do these two isotopes differ from one another In what respects are they the same ... 

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