Geologic time rock dating methods

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Long before scientists had developed the technology necessary to assign ages in terms of number of years before the present, they were able to develop a 'relative' geologic time scale.They had no way of knowing the ages of individual rock layers in years (radiometric dates), but they could often tell the correct sequence of their formation by using relative dating principles and fossils.These rocks, of which only a few were returned by the Apollo missions, have been dated by two methods at between 4.4-4.5 billion years in age.The majority of the 70 well-dated meteorites have ages of 4.4-4.6 billion years.Early geologists used the relative positions of rock layers as clues to begin to unravel the complex history of our planet.However, it was not until this century that nuclear age technology was developed that uses measurements of radioactivity in certain types of rocks to give us ages in numbers of years.The time scale on the right above shows the subdivisions of geologic time in a form that will fit on a single page.

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The written record of human history, measured in decades and centuries, is but a blink of an eye when compared with this vast span of time.Geologists studied the rates of processes they could observe first hand, such as filling of lakes and ponds by sediment, to estimate the time it took to deposit sedimentary rock layers.They quickly realized that millions of years were necessary to accumulate the rock layers we see today.The Phanerozoic Eon began 570 million years ago and continues today.This time scale, from the Decade of North American Geology, is widely used in North America.

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