A geological Event: Fossil dating

A geological Event: Fossil dating

Introduction

According to Matzke et al (35), these are the methods which are used by archaeologists to determine the age of fossils. Fossils refers to the remains of the pre historic organisms which are preserved in a petrified form or as mold in rocks. They provide the evidence for of presence of animals or plants in the earlier geological periods. Fossils are formed from as a result of the replacement of materials and tissues with ground water, thus forming a replica of the original organism in stones (Puttick et al, 23). The age of fossils cannot be exactly determined but it is argued that these organisms existed at least 10,000 years ago. This is a date which marks the end of the Cenozoic Era and the Pleistocene period. There are two techniques which are used in dating fossils. These are the absolute dating methods and the relative dating techniques. These two techniques of dating fossils have their own unique differences which will be discussed in this essay.

Relative dating technique

As said by Aburto (12), relative dating technique is a method used by archaeologists to determine the age of the prehistoric organisms in comparison to other organisms, without necessarily determining their estimated age. This method of dating is used to arrange the geological events and the rocks which they leave behind and this is done in a sequential manner. Fossils are very important stuffs in when determining the relative ages of sedimentary rocks. In the history of life, different organisms have appeared, made some flourishment and then became extinct in nature (Stillinger, 96). As they disappear out of the real nature, they leave behind their own remains as fossils which occur in sedimentary rocks and thus geologists usually study the order in which such remains occur in rocks so as to determine their nature, calculate their ages and determine their identity, in a process called biostratigraphy (Stillinger, 100).

In relative dating, it is important to understand that sedimentary rocks forms an important part in which geologists identify the age of fossils (Schaller, 45). In this case, it is important to understand that various principles have been developed to offer an easy path way for geologists in their journeys to determine the age of the remains of ancient plants and animals which existed long time ago. These principles have been discussed below.

The principle of uniformity: Under this principle, the geologists observed that various modification of the earth’s surface in the present age has worked in the same way over a geological time (Aburto, 76). The principle of original horizontality: this principle states that most sedimentary rocks occurs in the form of horizontal layers. This makes it easy for fossils to be deposited in the same places thus making it possible for geologists to identify such remains (Schaller, 99).

The principle of superposition states that sedimentary rocks are normally laid down in a sequential manner, one on top of the other in relation to their ages. The oldest sedimentary rocks are found at the bottom while the youngest occur at the top (Aburto et al, 54). This arrangement allows sedimentary rocks to be viewed in as a vertical timeline on the earth’s surface. Finally, the principle of inclusion is also applied in relative dating and it states that fragments of included in one rock must be older than the other surrounding rocks.

Techniques in relative dating

1. Stratigraphy

According to Matzke et al (24), stratigraphy refers to the study of the sequential layering of sedimentary rocks. It is the oldest method of relative dating. From the principle of superposition discussed above, rocks are usually laid down in the form of a sequence, one at the top while the other one at the bottom. In determining the age of one rock, geologists determine the age of one rock, especially the one at the bottom because it is the oldest. The age of the other rocks which occurs at the top is determined in relation to the one in at the bottom and the converse is true. Stratigraphy can be natural or arbitrary (Matzke et al, 29).

1. Biostratigraphy

This is a technique of relative dating technique in which a comparison is made in fossils from different stratigraphic sequences and it is used to relatively estimate which layers are younger than the others. It is a technique which was being employed in the early Pleistocene periods (Veres, 77). It is also the common technique which was used in determining the age of fossils in major historical site especially in the African continent, for example the Olduvai George.

• Cross and relative dating

Cross dating is a method that estimates the age of artefacts and other natural features based on the components which they have in common. Comparisons are made based on similarities with other comparable materials (Schaller, 123). Relative dating is a technique whereby the age of fossils is determined by comparing them with other fossils in from other layers of rocks. They first identify the best type of dating then proceed to determine the ages of rocks.

1. Seriation

This is a technique in dating in which geologists used artefacts to determine the relative age of ancient plants and animals, occurring in sedimentary rocks (Schaller,90). Artifacts are ordered in temporal series of which are based on their similar attributes or the frequency of such attributes. They also use concepts of culture as well as use of battle curve. It is a standard method of dating in archaeology (Stillinger, 104).

1. Index fossil dating

These are fossils that portrays various time periods and in which the same time periods are used to relate the age of fossils. In this technique of relative dating, fossils remains of known age are used to estimate the age of geological stratum where they are found (Aburto et al, 80). A good example is the use of marine arthropods known as trilobites as an index fossil to determine the age of Ordovician geological formations or Cambrian.

Cross-correlation in relative dating

Correlation forms the major part in relative dating. It is the process which is used to understand the relationships between the strata at different locations on the earth’s surface. Correlation can involve the act of matching between two rocks, the dated and undated locations (Puttick et al, 85). Correlation can be done in the case where fossils at one place exist but cannot be dated using the absolute methods. The basic tool of concept which is used in correlation is the index. Correlation is also the major concept in the principle of superposition whereby geologists determines the placement and the arrangement of layers of rocks in order to determine their ages. As mentioned earlier, layers are arranged in such a way that the older layers occurs at the bottom while the recent ones at the top (Puttick et al, 25). Through correlation, the age of the daughter rocks can be calculated through making some relations with the bed rocks.

Faunal correlation is a common method of dating fossils. It is a relative technique in which there is a comparison of fossils from different stratigraphic sequences and are used to estimate the age of layers or rather the age of fossils. Faunal correlation was the common method which was used by many archaelogistics in their journeys to identify the age of fossils which were found in many pre historic sites in the African continent (Aburto et al, 142). The principle of fossil correlation is also important in understanding correlation in fossils. It states that strata containing fossil assemblages have the same age. It further explains that those fossils which are found at the bottom of strata are usually order than those at the bottom.

The absolute dating technique

This refers to the process of determining the age of ancient plants and animals in a chronological order. It is a method of assigning numerical to give rocks an actual date or a date within a certain range in a specified number of years (Cook, 81). Absolute dating is based on measurable chemical or physical associations or qualities. It is also called chronometric dating. In absolute dating, the age of rocks is usually expressed in centuries, years, months, days or even millennia, which is discrete known as interval in time. Most absolute dates for rocks are obtained using radiometric methods. This means that they use some radioactive minerals in rocks called clocks. There exists some chemical elements with different forms referred to as isotopes which undergo the process of radioactive decay. This was put forward by Agatova (51).

Types of absolute dating

1. Radio carbon dating

It is also called carbon dating or carbon-14 dating and is a type of carbon dating which involves determining the age of objects or rocks containing some organic materials by the use of properties of radiocarbon, commonly known as 14 C which is a radioactive isotope of carbon (Puttick et al, 27). A radioactive element is the one which can undergo a radioactive decay so as to release some harmful compounds in to the atmosphere. In determining the age of such elements, the half-life is determined by either calculating numerically or plotting it on a table (Tomassetti, 49).

1. Use of the half-life

This is the time taken by a radioactive element desecentegrate and reduce by half its value. It’s the time taken for the radioactivity of a specified isotope to reduce by to half of its original value. It is always possible for geologists to determine the half-life of a radioactive substance (Veres, 88). A graph of count rate against time is usually drawn and shows a decay curve for a radioactive substance. If the count rate drops from 60 to 30 within a span of 2 days, then that is the half-life.

1. Potassium argon method

This is usually abbreviated as K-Ar dating and is an example of a radiometric dating which is a technique of absolute dating used by geologists in archaeology and geochronology and it is used to determine the age of ancient organisms (Tomassetti, 20). It involves measuring of the product of a radioactive decay of an isotope of potassium and argon. It is a common method which is used by geologists.

Radioactive isotopes

According to Schaller (Schaller, 104) Isotopes are elements of the same mass number but different atomic number. Radioactive isotopes, which are also called radioisotope are those isotopes which have an unstable nuclide and when subjected to too much energy, the nucleus disintegrates spontaneously thus emitting radiations in form of alpha, beta or gamma rays. This decays enables the radioisotope to attain stability in its nucleus and they occur naturally in nature (Veres, 137). There are two types of isotopes. These includes the artificial and natural isotopes. Natural isotopes are those ones which are found in nature and they include hydrogen. Artificial isotopes are usually manufactured in nuclear laboratories through the bombardment of sub atomic particles. Radioactive isotopes are usually used in the medicine industry in the diagnosis and treatment of diseases (Agatova et al, 144). It is also applied in agriculture for food preservation. Radioactive isotopes are used in dating of ingenious rocks which are usually found near the fossils. This is referred to as geological event dating.

Conclusion

In conclusion, the essay has elaborated on both relative and absolute dating techniques of establishing the ages of fossils. It has discussed the types of both absolute and relative dating which can enable geologists to make decisions on which good method to use. It is also important to note that rocks enables geologists in determining the ages of fossils, thus it is important to understand the nature of such rocks. Sedimentary rocks are deposited bits of a long period of time. The types of rocks which contain fossils include sedimentary rocks, metamorphic rocks and igneous rocks. There are several differences between relative dating and absolute dating. Relative dating involves determining the age of a fossil by comparing it with another while absolute involves making calculations. Relative dating is only used to compare ages but absolute dating gives the age in numerical. Finally, relative dating makes use of the common sense principle but in absolute dating comes up with the precise age of an artifact.

References

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