CompuServe Messages

Dan and Soren… nice "talking" to you earlier…as my sother message was hinting at, the formation of the earth can be an emotional issue..depending on who is answering… here is some information that you might find useful. ——– It is known that the history of the Earth spans a period of more than 4 billion years, although how the Earth formed is still a matter of speculation. Some experts suggest that it originated from condensing gas and dust. Others regard it as having developed from a cloud of meteorites and meteoric dust revolving around the Sun. Gravitational forces caused the particles to come together, or accrete, with heavier particles gravitating to the center and lighter ones collecting outside. This contraction of the mass generated heat, which caused melting and the gradual development of three layers: core, mantle, and crust (see EARTH, STRUCTURE AND COMPOSITION OF). Geologic history began when the crust first formed, about 4.6 billion years ago, as determined by radiometric age-dating of the oldest rocks and of meteorites. This event must have been preceded by a considerably longer period of astronomical history during which the SOLAR SYSTEM and, before that, the universe were created (see COSMOLOGY). Geologic history is divided into four main eras. The first, the PRECAMBRIAN ERA, began 4.6 billion years ago and includes the first 4 billion years of geologic history, about 85 percent of all geologic time. The second, the PALEOZOIC ERA, lasted from 600 to 225 million years ago, roughly 10 percent of geologic time. The third, the MESOZOIC ERA, lasted from 225 to 65 million years ago, about 4 percent of geologic time. The fourth, the CENOZOIC ERA, embraces the last 65 million years, only about 1.5 percent of geologic time. Being the most recent, the Cenozoic is best known; the landscapes and life forms of modern times reached their present expression during this era. I'll send you a bibliography shortly! Rick Needham/SYSOP