There are various methods of dating. Below we explain the main methods of dating and technique.

METHODS OF DATING

GLACIAL VARVES

Varve analysis is one of the oldest techniques for geological dating. The different layers or laminations observes on the floors of former glacial lakes are famous as varves in Swedish. It is based on the principle that the melting of glaciers during their recession is accelerated during the summer months and retarded in the winter months.

During the melting of the dust and silt lying on the top of ice settles down and thus keeps a mark of each of the summer seasons. Eventually, mere counting of these marks can lead to the age determination of the deposits. A careful study of these deposits and their comparison has successfully used to date objects as old as 10,000 years.

DENDROCHRONOLOGY

Some kinds of forest trees have clearly defined annual rings of growth in their trunk. The thickness and shape characteristics of these rings vary in detail. They are dependent on environmental moisture and other factors during the period of their growth. By counting the number of these rings in the cross-section of a tree cut on a known date, one can easily determine the age of the given tree. Archaeological materials when accompanied by such trees are dated accurately by this method.

WEATHERING

Prehistoric materials found buried together might show striking variation in their stale of preservation. These variations often lead to fruitful conclusions in the separating of older material deposited together with new material. The common kinds of weathering are as follows:

Mechanical weathering

When an artifact undergoes rolling for a long time due to river action, or when exposes to high wind current, solar heat and extensive raining, the sharp edges and corners tend to be round.

Chemical weathering

Patination and encrustation are two kinds of weathering effects often met within prehistoric artifacts. Due to many unknown factors acting in the presence of salt-bearing soils a coating of darker color, usually red is forms on the entire surface of the object. Usually, this transformation is common in flint and other like materials and is famous as patination.

Encrustation

On the other hand, encrustation is a local effect. It results in the deposition of metallic salts as small patches on the object. The presence of such objects within a group of comparatively fresher objects can always be taken to mean that the former is older while the latter is younger.

ASSOCIATED FINDS

The presence of objects or fossils of tentatively approves age within a deposit of unknown age are sometimes uses to estimate the age. This is always of great help to know the probable age on the basis of the associated finds of known age.

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RELATIVE DATING

All these methods of dating tend to compare a given event with some other event in a time sequence. Further, it is difficult to believe that all these events occurred everywhere simultaneously. Hence, exact contemporaneity with similarly dated objects from other parts of the world can also be implied. These are famous as relative dating in prehistoric archaeology.

POTASSIUM-ARGON DATING

Other isotopic methods for measuring the age of material that is hundreds of thousands or even millions of years old is now developed. The most useful of them is the potassium-argon technique. This technique is based upon the transformation of potassium-40 (K40) into argon-40 (A40). The half-life period for this is enormously long, so long that it has to be expressed as 1.30 X 104 years.

FLUORINE DATING

The fluorine dating test is not isotopic, nor does it measure elapsed lime. It determines the relative age between two finds discovered practically in the same area. It is based on the fact that fluorine is found naturally present in the water in many regions.

And in such areas, this fluorine gradually accumulates in buried bones and teeth, and forms ‘fluor-appetite’ compounds. Comparing the amount of fluorine in human skeletal remains with that in the bones or teeth of other animals found close by, it is, therefore, possible to show whether or not they were truly contemporary.

Other scientific time-reckoning techniques make use of terrestrial magnetism, the thickness of hydration layers on obsidian artifacts and thermoluminescence for determining the age of archaeological objects like pottery. We shall, however, conclude this section by describing a geochronological method, viz., the method of dating the lateritic crusts.

LATERISATION

In tropical countries like India and Africa, the rocks exposed on the surface often undergo a change. This change helps in establishing relative dating for alluvial sites. It is the process that leads to hardpan formation in the areas of high rain-fall the intensive chemical weathering of the humid tropic leads to the breakdown of clay minerals, with the selective release of colloidal silica.

This mobile silica is penetrating downward to a relative enrichment of silica in the intermediate layers of the soil. These initial processes appear to be common to most woodland soils. These initial processes appear to be common to most woodland soil in tropical regions. This intensive weathering is famous as lateralization.

Such lateritic horizons may be several meters deep and reach the thickness of well over 10 meters. When these lateritic regions are exposed at the surface by erosion, particularly in regions of pronounced seasonal drought, irreversible crystallization follows and a durable crust is formed.

RADIOCARBON DATING

Radiocarbon Dating is one of the most common methods of dating. With the help of this dating, absolute dates for fairly recent periods can be determined by varve and tree-ring methods described above. But for dating remoter periods neither of this evidence is present, so it requires an alternative method. Prof. Willard F. Libby (1952) of the University of Chicago developed the radioactive carbon-14 (CM 14) method to determine the absolute age of prehistoric sites.

The Technique of Radiocarbon Dating

Cosmic rays have been bombarding the earth’s atmosphere ever since it was formed, and their rate of striking the outer air can be taken as constant over a long period of time. When these rays strike in an atom of nitrogen they produce an atom of carbon-14 and a proton. Carbon-14 is a radioactive isotope of ordinary carbon-12. When carbon-14 forms it combines with oxygen to produce radioactive carbon dioxide, C1402. This, along with other normal carbon dioxide is absorbed by plants and is used by them in growing up. Herbivores eat the plants and carnivores eat the herbivores while man, who is omnivorous eats the giant, herbivores, and carnivores.

Thus, the C14 originally produces in the environment is equally distributes among all living things and maintains at a constant level until their death. In other words, we all are moving radioactive objects. When an organism dies it stops taking in C14 and its present begins to decay at a constant rate

A sample of the specimen to be tested is then taken with great care in order not to contaminate it with more recent material. The sample is then weighed and oxidized to get oxide gas. The gas inserted under pressure into a Geiger counter, which determines, the radioactivity of the sample. The calculation of the age of the sample relies on the basis of the half-life value of carbon-14.

Example of Radiocarbon Dating

For example, after 5,568 years, one-half of the original C 14 atoms still remain; after 11,140 years one quarter remains and in the same way after 22,280 years one-sixteenth remains. After 50,000 years the amount left becomes so infinitely small that it cannot be determined by this method.

 

Sources: Mohammad Rafi Komol & O. Jnanendra Singh

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