Did You Know?
Sir William Flinders Petrie, an Egyptologist, used the seriation technique of relative dating for dating the artifacts discovered at the graveyards near the Nile river in Egypt.
Dating methods are classified into relative and absolute dating. Relative dating is a science which deals with the comparative study of events from the past. The absolute age of these events need not inevitably be known.
Grand Canyon is a gorge located in Northern Arizona, USA. It is composed of rocks and sediments deposited over millions of years. The layers are horizontally placed. Thus, it follows the principle of horizontality. While some of the layers are uplifted, most of the landform is left undisturbed by nature.
It is the evidence of Earth's history over such a long span of time. It is a perfect example of superposition (layers deposited one above the other) and lateral continuity (undisturbed and covering large distances).
Relative Dating Techniques in Geology
Geology refers to the study of the rocks and sediments that to a great extent compose the Earth.
This oldest relative dating technique in the branch of geology, as the name suggests, focuses mainly on the strata. It concentrates mainly on the placement of the strata as well as its chronological sequence. The principle of superposition is the core principle used in this method.
Strata is the layered arrangement or soil or rocks which lie parallel, one above the other. Each layer has a unique layer and consists of different sediments or material. The principle of superposition states that the layer which lies at the bottom is older than the one on top of it.
In stratigraphic relative dating, the succession of layers can be seen as the timeline of its formation or deposition. However, this is mainly applicable to an undisturbed arrangement of rocks. Most of the rock arrangements are disturbed by natural forces, such as wind and water, which result in unconformity in the sequence of rocks.
Layers get deposited above one another, over time, and fossils get trapped in these layers. When we find two fossils in the same strata of soil, we assume that both fossils were deposited during the same time period.
When they die, their remains get fossilized and are used by scientists to determine the era in which they lived. These fossils are then used as standards to determine the age of other fossils. They are called 'Index fossils'. An example can be fossils of some species of monkeys found alongside fossils of human species.
This technique of relative dating mainly works on the principle of chemical changes taking place in the fossils. When remains of living beings get buried into sediments and turn to fossils, the bacteria present in the soil breakdown the proteins and fats from the bones. Most of the nitrogen contained in these fossils gets depleted progressively.
Ground water percolates into these rocks and deposits its component elements such as fluorine, uranium, etc. The amount of fluorine in the fossils thus increases.
If two fossils belong to the same strata, then they are assumed to have the same amount of nitrogen and fluorine. In case of a difference in the fluorine content, they are considered to be from different eras.
Relative Dating Technique in Anthropology
Anthropology is the study of humans in all eras. It is an in-depth analysis in all the possible ways, taking into account all the related complexities. In anthropology, the study of humans living in the prehistoric era is done by collecting the data of human fossils found during excavations or research.
Most of the soft tissues of the human body get decomposed with only the hard tissues left for research. These hard tissues include the teeth and the bones. This technique begins with the identification of the bones.
If the skull is found, then the technique proceeds with recording its dimensions. Further on, this data is compared with the standard data to establish the age of the fossil.
Relative Dating Techniques in Archeology
Archeology refers to the study of history of mankind by excavating ancient sites. The methods used for relative dating in archeology are similar to the ones used in geology. The term used for the relative dating technique in archeology is 'Typology'.
This method is mainly used for dating the sites and objects which have archeological importance. It refers to categorization of objects based on their physical features. The result is expressed in terms of classes, which are also termed as types.
Objects having similar features are classified under one category. Likewise, dissimilar objects are classified under another. This method helps the researchers estimate the time period during which the site existed or a particular object was used.
This method is mainly used for establishing the chronological sequence in which certain artifacts existed. This technique makes it possible to understand the changes that have been modified over time. Seriation is further classified into evolutionary seriation, frequency seriation, contextual seriation to list a few.
Man-made objects or artifacts are used for relative dating. They can also be called diagnostic objects because of the fact that they indicate a time period during which they were developed and used.
Many artifacts such as ceramics, vessels, or bottles used in the prehistoric era are discovered at excavation sites. From the make of these artifacts, it is possible to identify the time period during which they were made.
Calibrated Relative Dating Techniques
Amino Acid Racemization
Proteins are a vital nutrient in living beings. Their physical structure depends on proteins. These proteins are in turn composed of amino acids. Proteins termed as enantiomers are either D-right or L-left which indicates that they are rotating in either left or right direction. After the living creature dies, the L form of acids convert to the D form.
This process is called racemization. In order to understand the time at which the creature died, it is necessary to determine the racemization rate. This method depends on various factors such as the temperature, pressure, composition of amino acids, amount of water, to list a few.
Some elements such as carbon and potassium undergo radioactive decay. The estimation of extent of the decay acts as a pointer in finding the age of fossils or rocks.
During the process of radioactive decay, unstable isotopes tend to change their number of protons, neutrons, or sometimes both. Sediments or rocks contain minerals which may be isotopes, which can possibly undergo this decay process.
Every magnet has two poles: south and the north. Earth is also considered to be a huge magnet. With a north and south pole, it has a wide magnetic field. Rocks consist of minerals which can act as magnets when exposed to magnetic field. When exposed to the Earth's magnetic field, the minerals from rocks align themselves parallel to it.
Geomagnetic polarity timescale is used as a standard to understand the change in Earth's polarity over the years. This technique helps ascertain the latitude of rocks at the time of their formation.
From all the mentioned techniques, it is crystal clear that relative dating is dependent on the order of events in entirety, rather than the age or time period of events.