AGE OF THE EARTH



Age of the Earth

Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology. Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.

By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth. They observed that every rock formation, no matter how ancient, appeared siotope be formed from still older rocks. Comparing these rocks with the products of present erosion, sedimentation, and earth movements, these earliest geologists soon concluded that the time required to form and sculpt the present Earth was immeasurably longer than had previously been thought.

By the mid- to late s, geologists, physicists, and chemists were searching for ways to quantify the age of the Earth. Lord Kelvin and Clarence King calculated the length of time required for the Earth to cool from a white-hot liquid state; they eventually settled on 24 lld years. There were other estimates but the calculations were hotly disputed because they all jc confirms dating lia obviously flawed by uncertainties in both the initial assumptions and the data.

Unbeknownst to the scientists engaged in this controversy, however, geology was about to be profoundly affected by the same discoveries that revolutionized physics at the turn of the 20th century. The discovery of radioactivity in by Henri Becquerel, the isolation of radium by Marie Curie shortly thereafter, the discovery of the radioactive decay laws in by Ernest Rutherford and Frederick Soddy, the discovery of isotopes in by Soddy, and the development of the quantitative mass spectrograph in by J.

Thomson all formed the foundation of modern isotopic isltope methods. But it was not until the late s approximarely all the pieces were in place; by then the phenomenon of radioactivity was understood, most of the naturally occurring isotopes had been identified and their abundance determined, instrumentation of the necessary sensitivity had been developed, isotopic tracers were available in the required quantities and purity, and the half-lives of the long-lived radioactive isotopes were reasonably well known.

By the early s, most of the major radiometric dating techniques now in use had radioactivee tested and their general limitations were known. No technique, of course, is ever completely perfected and refinement continues to this day, but for more than two decades radiometric dating methods have been used to measure reliably the ages of rocks, the Earth, meteorites, and, sincethe Moon. Radiometric dating is based on the decay of long-lived radioactive isotopes that occur naturally in rocks and minerals.

These parent isotopes decay to stable daughter isotopes at rates that can be measured experimentally and are approximately how old is the earth based on radioactive isotope dating constant over time regardless of physical or chemical conditions. There are a number of long-lived isofope isotopes radioacctive in radiometric dating, and a variety ixotope ways they are used to determine the ages of rocks, minerals, and organic materials.

Some of the isotopic parents, end-product daughters, and half-lives involved are listed in Table 1. Sometimes these decay schemes are used individually to determine an age e. Each of the various decay schemes and dating methods has unique characteristics that make it applicable to particular geologic situations. Daging example, a method based on a parent isotope with a very long half-life, such as Sm, is not very useful for measuring the age of a rock only approximately how old is the earth based on radioactive isotope dating few million years old because insufficient amounts of the daughter isotope accumulate in this short time.

Likewise, the 14 C method can only be used to determine the ages of certain types of young organic material and is useless on old granites. Some methods work only on closed systems, whereas others work on open systems. One of the primary functions of the dating specialist sometimes called a geochronologist is to select the applicable method for the particular problem to be solved, and to design the experiment in such a way that there will be checks on the reliability of the results.

Some of the methods have internal checks, so that the data themselves provide good evidence of reliability or lack thereof. Commonly, a radiometric age is checked by other evidence, such as the relative order of rock units as observed in the field, age measurements based on other decay schemes, or ages on several samples from the same rock unit. The main point is that the ages of rock formations are rarely based on a single, isolated age measurement. On the contrary, radiometric ages are verified whenever possible and practical, and are evaluated by considering other relevant data.

My purpose here is not to review and discuss all of the dating methods in use. Instead, I describe briefly only the three principal methods. These are the K-Ar, Rb-Sr, and U-Pb methods. These are the three methods most commonly used by scientists to determine the ages of rocks because they have the broadest range of applicability and are highly reliable when properly approximately how old is the earth based on radioactive isotope dating. The K-Ar method is probably the most widely used radiometric dating technique available to geologists.

It is based on the radioactivity of 40 K, which undergoes dual decay by electron capture to 40 Ar and by beta emission to 40 Ca. The ratio of 40 K atoms that decay to 40 Ar to those that decay to 40 Online dating booster mac is 0. Thus, in principle, while a rock is molten the 40 Ar formed by the decay of 40 K escapes from the liquid. After the rock has solidified and cooled, the radiogenic 40 Ar is trapped within the solid pld and accumulates with the passage of time.

If the rock is heated or melted at some later time, then some or all of the 40 Ar may be released and the clock partially or totally reset. In the process of analysis, a correction must be made for the atmospheric argon 2 present in most minerals and in the vacuum apparatus used for the analyses. What is left is the amount of radiogenic 40 Ar.


Carbon Dating the Earth - 1950s - How Old is Earth? Physics and Geology