Part II Eart h : the dynamic planet

I want to know how God created this world. I am not interested in this or that phenomenon, in the spectrum of this or that element. I want to know his thoughts, the rest are details. Overview Earth is part of the solar system and it cannot be completely understood in isolation. The chemistry of meteorites and the Sun provide constraints on the composition of the planets. The properties of the planets provide ideas for and tests of theories of planetary formation and evolution. The Earth is often assumed to have been formed by the slow accumu lation of plan-etesimals-small cold bodies present in early solar system history. In particular, types of stony meteorite called chondrites have been adopted as the probable primary material accreted by the Earth. This material, however, has to be extensively processed before it is suitable. Study of the Moon, Mars and meterorites demonstrates that melting and basaltic volcan-ism is ubiquitous, even on very small bodies. Planets form hot, or become hot, and begin to differentiate at a very early stage in their evolution , probably during accretion. Although primitive objects have survived in space for the age of the solar system, there is no evidence for the survival of primitive material once it has been in a planet. One would hardly expect large portions of the Earth to have escaped this planetary differentiation. and to be 'primordial' and undegassed. The present internal structure of the Earth was mainly established 4.57 billion years ago. This is not a central dogma of current geochemical models but the use of high-precision short-lived isotope data promises to change this. A large amount of gravitational energy is released as particles fall onto an accreting Earth, enough to raise the temperature by tens of thousands of degrees and to evaporate the Earth back into space as fast as it forms. Melting and vaporization are likely once the proto-Earth has achieved a given size. The mechanism of accretion and its time scale determine the fraction of the heat that is retained , and therefore the temperature and heat content of the growing Earth. The 'initial' temperature of the Earth was high. A rapidly growing planet retains more of the gravi-tational energy of accretion, particularly if there are large impacts. The magma-ocean concept was developed to explain the petrology and geochemistry of the Moon. It proved fruitful to apply this to the Earth,