In 1986 the world’s leading science journal, Nature, announced that the most ancient rock crystals on earth, according to isotope dating methods, are 4.3 billion years old and come from Jack Hills in Western Australia.
W. Compston and R.T. Pidgeon (Nature 321:766–769, 1986) obtained 140 zircon crystals from a single rock unit and subjected them to uranium/uranium concordia (U/U)1 and uranium/thorium concordia (U/Th)2 dating methods. One crystal showed a U/U date of 4.3 billion years, and the authors therefore claimed it to be the oldest rock crystal yet discovered.
A serious problem here is that all 140 crystals from the same rock unit gave statistically valid information about that rock unit.3 No statistician could ever condone a method which selected one value and discarded all the other 139. In fact, the other 139 crystals show such a confusion of information that a statistician could only conclude that no sensible dates could be extracted from the data.
A further problem is that the 4.3 billion-year-old zircon, dated according to the U/U method, was identified by the U/Th method to be undatable. An unbiased observer would be forced to admit that this contradiction prevents any conclusion as to the age of the crystal. But these authors reached their conclusion by ignoring the contradictory data! If a scientist in any other field did this he would never be allowed to publish it. Yet here we have it condoned by the top scientific journal in the world.
This is not an isolated case. I selected it because it was identified by the journal editors as a significant advance in knowledge. Another example is the work of F.A. Podosek, J. Pier, O. Nitoh, S. Zashu, and M. Ozima (Nature 334:607–609, 1988). They found what might have been the world’s oldest rock crystals, but unfortunately they were too old!
So far scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. Nevertheless, scientists have been able to determine the probable age of the Solar System and to calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in the Solar System formed at the same time and are, therefore, of the same age.
The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half lives of 700 million to more than 100 billion years to stable isotopes of other elements. These dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements.
Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years). [See Editor's Note.] These ancient rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent. An interesting feature of these ancient rocks is that they are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water, an indication that Earth history began well before these rocks were deposited.
Scientists determined the Earth's age using a technique called radiometric dating. Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie. The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus. (The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight. Some isotopes are stable, but some are unstable or radioactive.
Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay. Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years. The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures. These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay. After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
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