It is obvious that geologists can tell us, this rock (layer) is younger than this rock (layer). This is because they can apply fundamental geology principles of stratigraphy when it comes to finding out which rock (layer) is younger and which is older. However when it comes to telling the numerical (actual) ages of rocks or layers, geologists should apply the radiometric technique known as Radiometric Dating.
Radiometric dating got attention since the discovery of the field of radioactivity since the early of 20th century that gave geologists and other scientists the new way of estimating the numerical age of various materials.
What is Radiometric Dating?
Radiometric Dating is the technique of determining the absolute (actual) ages of various materials such as fossils, rocks, archeological objects like artifacts and pottery by applying the principles of radioactivity.
The basic principle of Radiometric dating is based on natural radioactivity as worked and inverted by Ernest Rutherford in 1902. However the use of radiometric dating was first published in 1907 by Bertram Boltwood. [1]
When a crystal of minerals forms it starts with some amount of radioactive elements. As they exist as parent radioelements which are unstable in nature while their daughter isotopes stay locked within the crystal.
As the time passes the parent isotopes increase the instability due to chemical and physical changes acts on them, until they enter into the disintegration cycle which is associated by emission of radiations such as alpha, beta or gamma rays in order to gain the stability.
After breakdown to form stable daughter radioisotopes, the process of decaying continues with time until no extra daughter type elements can be present in a given mineral crystal.
If the decay rates of radioelements after every half decaying time of the existing amount of radioelements known as Half-Life is known, then it is possible to determine the age of the rock sample by calculating the ratios of parent to daughter radioelements.
Types of Radiometric Dating
Radiometric dating can be done either in one of the following methods
1.Radiocarbon (C) Dating
This method utilizes the carbon atom which exists in three isotopes which are carbon 12, 13 and 14, at which carbon 12 and 13 are stable while carbon 14 is in an unstable state. This means that the unstable carbon 14 breaks down to stable states by passing to intermediate nitrogen (N) atoms. This method is sometimes known as Carbon -14 dating
Since the half life of carbon 14, is known as 5730 years, then the actual age of a given rock or fossil sample can be determined using the same principle method as that of radioactivity.
This method is commonly applied in dating of animal and plant fossils as they contain large amounts of carbon atoms throughout their life,with carbon 12 in stable form that form a reference of carbon dating decay, hence it is mostly utilized by archeologists.
Since living organisms continually take up carbon from the environment which includes the isotope 14C. When the organism dies the 14C in it radioactively decays at a rate determined by its half-life of 5730 years. By measuring the proportion of 14C present in a sample of once-living tissue the time elapsed since it died and stopped taking up 14C from the atmosphere can be determined. [7]
The best history regarding using carbon - 14 method in dating fossils is on discovery of the fossil skull known as "Zinjanthropus" in Bed I at Olduvai Gorge, Tanzania in July 1959. The name given by archeologist Professor Louis Leakey, as an early hominin, dates back to the Lower Pleistocene, about 1.75 million years ago. This skull is that of a robust Australopithecine, at one time the Zinjanthropus skull was nicknamed ‘Nutcracker Man’ because of the large size of the teeth. [2]
This method also is useful in dating materials in recent ages such as not older than 50,000 years, however modern analytical techniques are capable of dating to 70,000 years or more. [6]
2. Potassium - Argon (K - Ar) Dating
This is an age-dating technique most commonly used by geologists as potassium (K) is commonly found in rocks and minerals. Examples of K - minerals are Orthoclase, Muscovite, Alunite which are commonly found in altered acid volcanics.
Potassium acts as the parent isotope, while argon forms as the daughter isotope at which argon is an inert gas that does not combine with other elements.
Radioactive 40K constitutes only 0.01167% of the K in rocks. It decays in two different ways,
The first one is by Beta (B) - particle emission to 40Ca20 with decay rate (dca) 4.962× 1010 yr–1
40K19 ===> 40Ca20 + B-
The second way is by electron capture to 40Ar18 with decay rate (dAr) 0.581 × 1010 yr–1.
40K19 + e ===> 40Ar20
The combined decay constant d(Ca + Ar) is equal to 5.543 ×1010 yr–1.
When taking the ratios of the decay rates related to electron capture (dAr) to combined rates, such as dAr/d(Ar + Ca) = 0.1048. The age (years) of the rock sample can be determined by using the growth rate equation as described below. [5]
D = P0 (edt - 1), where D and P0 = Daughter and Parent isotopes respectively.
40Ar = dAr/d(Ar + Ca) 40K (edt - 1)
40Ar = 0.1048 40K (edt - 1)
(40Ar/40K)/0.1048 = (edt - 1)
9.54 (40Ar/40K)= (edt - 1)
edt = (1 + 9.54 (40Ar/40K))
By applying ln on both sides, insert combined decay constant (d) = 5.543 ×1010 yr–1 and making the t, subject of the expression above, simplifies as,
t = 1.804 × 109 In(1 + 9.54 (40Ar/40K))
3. Uranium - Lead (U - Pb) Dating
Since Uranium (U) isotopes decay to stable end-product isotopes of lead(Pb). However the decay is a multi-stage process but which had a single decay constant.
The decay of 238U to 206Pb can be written as 206Pb/238U = ed238t - 1
Also the decay of 235U to 207Pb can be written as 207Pb/235U = ed235t - 1
Where d235 = 9.8485 ×1010 yr-1 and d238 = 1.55125× 1010 yr-1 are decay constants for 235U and 238U respectively
When the graph of the 206Pb/238U ratio against 207Pb/235U ratio is plotted, a curve is called Concordia curve (Figure 1).
How to plot a point on the Concordia curve, let us say time (t) was 4.5 Ga, put this in Pb/U ratios.
206Pb/238U = ed238t - 1
t = 4.5 Ga, d238 = 1.55125× 1010 yr-1
206Pb/238U = ed238t - 1 = Approx.1.0
Also 207Pb/235U = ed235t - 1
t = 4.5 Ga, d235 = 9.8485 ×1010 yr-1
207Pb/235U = ed235t - 1 = Approx. 83.0
Then plot as (83, 1) in (x, y) plane , see figure 1
The point where lead loss on Concordia line is determined and extrapolated out in the direction of the line as discordia line until it intersects the Concordia curve at this point the age of the rock sample can be determined by comparison through the standard tables of Pb /U ratios. See figure 1
Figure 1. Hypothetical concordia - discordia graph, uses U/Pb isotopic ratios to find age of rock
4.Thermal Luminescence Dating
This method uses heat (thermal) to measure the amount of radioactivity accumulated by a rock or a stone since it was last heated.
The method is basically based on Luminescence produced when the body is heated. Through measuring the intensity of Luminescence produced the time taken from when it was last heated can be determined.
Sediment at the surface is exposed to sunlight that causes a release of the stored energy (‘bleaching’), so the build-up of the energy to produce luminescence only starts once the material is buried. Measurement of the amount of luminescence produced by heating or optically stimulating a sample can therefore be used to determine how long the sediment has been buried.
This method is used for dating of archeological objects such as pottery.
Limitations
It is suitable for dating materials around ages about 100,000 years old
It only works for materials bleached when deposited, such as aeolian and fluvial sediment that has been deposited slowly with an accuracy of about 10%.
5. Paleomagnetic Dating
Since small magnetic grains in rocks orient themselves to be parallel to the direction of the magnetic field pointing towards the North Pole. This is because magnetic materials acquire the polarity of the ambient magnetic field as it cools through the Curie point, a temperature above which the magnetic dipoles in the material are mobile and free to reorient themselves.
Since the rock may contain a complex of different mineral grains of which each has its own Curie point temperature, this alternation may create a new mineral that will record the ambient magnetic field at the time of their formation known as remnant magnetism.
The remnant magnetism in the rock sample is measured to determine the orientation of the Earth's magnetic field relative to the sample at the time of the formation of the rock. [3]
The acquired primary polarity is stored on the rocks (sedimentary or volcanics) and can be measured.
Figure 2. Magnetic Polarity of the Earth's magnetic field through part of the Cenozoic (Haq et al, 1988)
Significance
This method can be used worldwide , such as in paleomagnetic studies for oceanic floor that lead to discoveries of plate tectonic theory and continental drift.
Conclusion, radioactive isotopes opened new way of obtaining numeric ages of various materials, however they are not useful in dating sedimentary rocks, since most of the radioactive grains they contain are derived from older igneous rocks, so when using this method the age would indicate the age of igneous rock. This is with exception of sedimentary rocks that contain potassium minerals such as glauconite.
Thanks for reading this article.
References
1. Boltwood B., (1907). "The Ultimate Disintegration Products of the Radio-active Elements. Part II. The disintegration products of uranium". American Journal of Science. 4. 23 (134): 77–88.
2. Concise Oxford Dictionary of Archaeology.
3. Hailwood, E.A. (1989) Magnetostratigraphy. Special Report 19, Geological Society of London.
4. Haq, B.U., Hardenbol, J., & Vail, P.R. (1988). Mesozoic and Cenozoic chronostratigraphy and cycles of sea level change. In sea level changes, an Integrated approach. Special Publication 42, Society of Economic Paleontologists and Mineralogists, Tulsa, OK; 71–108.
5. Lowrie W., (2007). Fundamental of geophysics (2nd edition), Swiss Federal Institute of Technology, Zürich, Cambridge University Press.
6. Lutgens, F.K. & Tarbuck, E.J. (2006) Essentials of Geology (9th edition). Pearson Prentice Hall, Upper Saddle River, NJ.
7. Nichols G. (2009), Sedimentology and stratigraphy, (2nd edition), Wiley Blackwell.
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