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Time Fossils and the Scientific Process Life through Time Evolutionary Milestones Extinctions Fossil Activities Fossils Glossary Further Research Link to Dinosaur Walk Link to Prehistoric Life |
 Measuring the age of the EarthThis segment deals with the concepts of absolute and relative ages of rocks, the development of the relative geological time scale in the nineteenth century, and some of the methods that have been developed during the twentieth century to determine absolute ages. There are two different ways in which geologists may refer to the age of rocks, and hence to the age of the Earth: the absolute age, measured in years (usually millions of years); and the relative age, expressed in terms of named episodes in Earth history. This division of Earth history into named episodes is called the relative geological time scale. When this time scale was established in Europe in the nineteenth century, geologists were not able to determine the absolute age of rocks; they knew only that rocks deposited during one particular episode in the time scale were younger or older than other rocks deposited during an earlier or later episode respectively. This method of dating is analogous to the historian's method of referring to a particular event as having occurred during the reign of a certain king, queen or emperor, or during a particular dynasty (for example, the Ming dynasty). With the development early in the twentieth century of reliable methods for determining the absolute age of rocks, it was possible to attach absolute ages to the episodes in the relative time scale. Nevertheless, even today geologists still usually refer to the age of rocks in terms of the relative time scale; for example, a geologist is more likely to say that a rock is of Silurian age than that it is 430 million years old.
Development of the geological time scaleThe relative geological time scale was established during the nineteenth century in Europe, where geologists observed that rock sequences in certain areas were characterised by distinctive assemblages of fossils. These sequences were given names based either on the areas where they were first recognised (for example, Devonian after the English county of Devonshire), or on the distinctive nature of the rocks (for example, Carboniferous after the extensive coal deposits). The same name was also used for the interval of Earth time during which the sequence and its characteristic fossils were deposited. It soon became clear that similar sequences with the same assemblage of fossils could be recognised in widely separated areas, and the same name was applied to them wherever they occurred. Such sequences were overlain or underlain by others with different assemblages of fossils. Where one sequence overlay another, it was known that the lower one was older because of the principle of superposition, established in the seventeenth century by the Danish anatomist Nicolaus Steno (1631-1686) This principle states that, in an undisturbed sequence of layered rocks, the oldest layers lie at the bottom and successively higher layers are progressively younger.For convenience, the geological time scale divides Earth history into hierarchical intervals in the same way that historical time is divided into centuries, decades and years. The most widely used of these time intervals are Periods, which are grouped into larger time units called Eras, and divided into smaller time units called Epochs. In the following table, geological convention is followed in placing the oldest time intervals at the bottom.
The interval of Earth history prior to the Cambrian, and predating the appearance of the first abundant fossils, is called the Precambrian; as yet division of Precambrian time into periods is provisional.
Absolute age determinationIn order to determine the absolute age of a rock, we must find some form of clock within the rock and be able to read it. To be of use, the clock must have been set to zero when the rock was formed, and it must have run at a uniform rate, without interruption, to the present day.The problem of finding a suitable clock was solved in 1896 when the French scientist Henri Becquerel discovered radioactivity. Here was a process which proceeded at a uniform rate under all imaginable conditions of temperature and pressure encountered on or near the surface of the Earth. Geologists quickly realised the potential of this discovery for dating rocks, and a decade later the first attempts at measuring the age of minerals using radioactivity were made. The method of determining the age of rocks using radioactivity is called radiometric dating. This method relies on the radioactive decay of an unstable type of atom (parent isotope) within the rock into another type of stable atom (daughter isotope). In a certain period of time, called the half-life, half of the parent isotopes will have decayed into daughter isotopes; in an additional, equivalent period of time, half of the remaining parent isotopes will have decayed, and so on. The length of the half-life, which can be measured, varies for different isotopes. By measuring the ratio of parent to daughter isotopes, the time that has elapsed since decay began can be calculated; this is equivalent to the age of the rock. After almost a century, geologists have found many different radioactive isotopes of elements to use for dating. Of these, the most widely known is carbon-14 (14C), which decays into nitrogen-14 (14N); however, this process proceeds at such a rapid rate in geological terms (half-life 5,730 years) that it can only be used to date objects younger than about 70,000 years. It is like a highly accurate stopwatch, useful for dating the very recent past, during the period of human written history and back into the last Ice Age. For dating older rocks that are millions or even thousands of millions of years old, other radioactive isotopes such as potassium-40 (40K, half life 8,400 million years) or uranium 238 (238U, half-life 4,510 million years) must be used. Another method for determining the absolute age of rocks by radioactive decay is called fission track dating. Certain minerals in rocks contain small amounts of uranium which decay radioactively by the splitting apart of the atomic nucleus (nuclear fission). The two fission fragments produced are highly energetic and highly charged, and they produce a linear trail of radiation damage in the surrounding crystals of the rock. This trail is known as a fission track. Fission tracks can be enlarged by chemical etching until they can be observed and measured under a microscope. The number of tracks is proportional to the time since they started to accumulate, and to the amount of uranium in the rock. The amount of uranium present can be determined by irradiating the rock with neutrons to produce a second set of fission tracks. The ratio of the original tracks to the new ones gives a measure of geological age. A recently developed method for determining absolute age is the amino-acid racemisation method, used to date bones and shells that are up to several hundred thousand years old. The method relies on the fact that molecules of amino-acids, the building blocks of proteins, occur in two different forms that are mirror images of each other. These two forms are referred to as left-handed and right-handed. In living organisms, only left-handed amino acid molecules are present, but once the organism dies they slowly convert to their right handed form. Simultaneously, the right handed forms produced slowly convert back to left handed forms, until an equilibrium is reached (half left handed and half right handed), at which point the ratio remains constant. The time taken to reach equilibrium is known, so by determining the ratio of right handed to left handed forms it is possible to estimate the time elapsed since the organism died.
References:Eicher, D. L. 1968. Geologic time. Prentice-Hall, New Jersey.Geyh, M. A. & Schleicher, H. 1990. Absolute age determination. Physical and chemical dating methods and their application. Springer-Verlag, Berlin. Gleadow, A. J. W. 1996. Fission track thermochronology of southeastern Australia: unique perspectives on the evolution of our continental margins and mountains. Proceedings of the Royal Socity of Victoria 108: 6-15. Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G. & Smith, D. G. 1989. A geologic time scale 1989. Cambridge University Press, Cambridge. Berry, W. B. N. 1987. Growth of a prehistoric time scale. Based on organic evolution. Blackwell Scientific Publications, Palo Alto, California. Fortey, R. 1982. Fossils. The key to the past. Heinemann and the British Museum (Natural History), London.
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