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Rocks and Minerals

How do scientists tell how old a rock is?




There are several different ways a rock can be dated, depending on the type of rock, its composition and how old it may be. Over the years geologists have develped ways of "guessing" how old a rock is so that they can decide the best dating method. Rocks that are very hard are commonly older than rocks that are soft, but degree of hardness is not a trait that is reliable. In general, dating methods can be classified as a) relative, b) approximate, and c) absolute. The absolute methods give the rock ages in years before the present, commonly in millions of years. Some absolute dating methods are suited for giving ages in hundreds or thousands of years before the present.

a) Relative dating methods. Stratigraphic order. In a group of sedimentary or volcanic rocks that has not been greatly disturbed by folding or faulting, the lower layers are generally considered to be the older ones. This is called the "law of superposition". Intrusive igneous rocks, like granite, are commonly given relative ages (older or younger) based on whether one rock body cuts or intrudes another. If intrusive igneous rocks cut sedimentary or volcanic rocks, the igneous rocks are the younger rocks.

b) Approximate methods. Fossils. Many sedimentary rocks, like siltstone or limestone, contain fossils, the remains, usually the hard parts, of dead animals or plants that were buried by deposited sediment. After many years of studying these remains, paleontologists (who study animal fossils) and paleobotanists (who study plant fossils) have worked out how the animals and plants have changed over very long periods of time. A classification of time periods was developed from these changes in life forms. Today it is common for geologists to get an approximate age of rocks from fossils in the rocks. This system works well for sedimentary rocks deposited during the last 500 million years. Rocks older than that are more difficult or impossible to date using fossils.

c) Absolute methods. Absolute or radiometric methods of dating rocks gives ages in number of years before the present and are based on the atomic breakdown of certain unstable elements. Research has shown that these elements break down into other more stable elements and release energy at a certain rate, called the "half-life". The methods involve measuring the generally small quantities of the radioactive elements, or parent elements, and the products, or daughter elements. There are about ten of these radiometric methods. Another method, called the fission-track method, measures the damage to glass or mica (a mineral) that is caused by particles released during uranium decay. Although these methods have the advantage of obtaining the age of a rock in years, the analytical work is meticulous and the results need to be evaluated for consistency because of the possibility of laboratory error and the possibility that secondary changes (changes in the rock that occur between the time of formation of the rock and the time the rock is sampled) can cause erroneous results.

James G. Evans, United States Gelogicla Survay

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    Last Modified: 2004-12-10