Ch6_Time_Scale - Understanding the Time Scale 1 Chapter Six...

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Understanding the Time Scale 1 Chapter Six: Understanding the Time Scale Knowledge of any process requires a sense of the time involved. Because our time scale as human beings is limited to centuries, early estimates of the age for the Earth and universe were limited to thousands of years, a time that seemed ancient to human experience and imagination. Even Newton and Descartes, founders of modern physics, believed the earth formed in roughly 4000 B.C. Early geologists, looking at the evidence apparent in the rocks around them, began to question these ideas. They noted that processes operating in the present could give rise to observed rocks and features of the landscape, but since these processes occurred very slowly, long times must be involved—many millions if not billions of years. 19 th century physicists disputed these claims by showing that Earth’s current heat flow was not consistent with such a long time scale. Discovery of radioactivity revolutionized this view, by permitting quantitative measurements of the ages of rocks, and a new heat source with the Earth. Only in the second half of the twentieth century was scientific understanding and instrumentation adequate for the widespread application of the dating techniques based on radioactivity. Long-lived radionuclides, such as uranium, thorium potassium and rubidium permitted the dating of ancient events, and showed that the meteorites and Earth all formed as a group some 4.55 billion years ago. Theoretical calculations of the production rate of elements in stars coupled with present day abundances of the uranium and thorium isotopes permitted calculation of the age at which stellar explosions began to distribute elements in the galaxy, and the time scale of roughly ten billion years prior to solar system formation is consistent with the age of the universe inferred from the red shift distance relationships of the Big Bang. Events in the early history of the solar system emerged late in the twentieth century from the discovery of shorter-lived radionuclides. 26 Al, a radioactive isotope of Al with a half life of less than one million years created in super novas, was present in the early solar system. Its presence indicates that the solar system formed within an interstellar cloud where many stars were forming and exploding, contributing to the dust from which the meteorites and planets were accreted. These now extinct radionuclides would have been important heat sources in the early solar system, and may have facilitated rapid planetary heating and differentiation. INTRODUCTION Knowing what happened in the evolution towards a habitable planet requires a time line. What is the appropriate time scale ? Is it thousands, millions or billions of years? When exactly did events happen? How long did they take? What is the sequence of events occurring at different places? All such questions, fundamental to the unravelling of Earth’s history, depend on time.
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