Measuring Geological Time

The dating of geological samples found on the surface of the Moon indicates the present age of the Earth to be about 4.7 billion years old, the amount of time which has elapsed since the formation of the solar system.

The oldest examples of geological materials found on the Earth are extraterrestrial rocks, in the form of meteorites, which are dated at about 4.6 billion years old.

The oldest terrestrial rocks are volcanic samples, which are dated at about 3.8 billion years old.

Overall, the dating of geological materials is identified with processes of crystallization and recrystallization, including partial or overlapping heating and cooling, as well as related geological, structural, and petrological formations.

The method for calculating the chronology of a single geological sample, such as a rock or mineral, consists of measuring the accumulation of the products of radioactive decay within the sample.

The rate of radioactive decay is the number of atoms which are decaying per unit of time, and is proportional to the total number of radioactive atoms present.

For example, it is known that one-half of a uranium sample will decay to lead over a period of 1.5 billion years. By measuring the ratio of uranium to lead at a given time, the age of the sample may be calculated to within an accuracy of about 2 percent or better.

The uranium to lead sequence is one of several radioactive decay processes which are used to measure the ages of rocks and minerals.

In addition, the dating of geological materials includes the timing and duration of events within a single mountainous region, measurement of the rate of ocean-floor spreading, the rise and fall of sea level, the rate of polar wandering, the frequency and rate of polar reversal, the age of glacial and interglacial periods, and the age and evolution of the universe.

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