Geologic history of Earth

Geologic history of Earth, evolution of the continents, oceans, atmosphere, and biosphere. The layers of rock at Earth’s surface contain evidence of the evolutionary processes undergone by these components of the terrestrial environment during the times at which each layer was formed. By studying this rock record from the very beginning, it is thus possible to trace their development and the resultant changes through time. The Pregeologic Period:From the point at which the planet first began to form, the history of Earth spans approximately 4.6 billion years. The oldest known rocks—the faux amphibolites of the Nuvvuagittuq greenstone belt in Quebec, Canada—however, have an isotopic age of 4.28 billion years. There is in effect a stretch of approximately 300 million years for which no geologic record for rocks exists, and the evolution of this pregeologic period of time is, not surprisingly, the subject of much speculation. To understand this little-known period, the following factors have to be considered: the age of formation at 4.6 billion years ago, the processes in operation until 4.3 billion years ago, the bombardment of Earth by meteorites, and the earliest zircon crystals.It is widely accepted by both geologists and astronomers that Earth is roughly 4.6 billion years old. This age has been obtained from the isotopic analysis of many meteorites as well as of soil and rock samples from the Moon by such dating methods as rubidium–strontium and uranium–lead. It is taken to be the time when these bodies formed and, by inference, the time at which a significant part of the solar system developed. When the evolution of the isotopes of lead-207 and lead-206 is studied from several lead deposits of different age on Earth, including oceanic sediments that represent a homogenized sample of Earth’s lead, the growth curve of terrestrial lead can be calculated, and, when this is extrapolated back in time, it is found to coincide with the age of about 4.6 billion years measured on lead isotopes in meteorites. Earth and the meteorites thus have had similar lead isotope histories, and so it is concluded that over a period of about 30 million years they condensed or accreted as solid bodies from a primeval cloud of interstellar gas and dust—the so-called solar nebula from which the entire solar system is thought to have formed—at about the same time. Models developed from the comparison of lead isotopes in meteorites and the decay of hafnium-182 to tungsten-182 in Earth’s mantle, however, suggest that approximately 100 million years elapsed between the beginning of the solar system and the conclusion of the accretion process that formed Earth. These models place Earth’s age at approximately 4.5 billion years old.

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