Age of the Earth
The question “how old is the earth” has not been precisely answered. Because so far scientists have not found a way to find out the exact age of the Earth directly from the rocks because most of the Earth’s oldest rocks have been destroyed and recycled by the action of plate tectonics movement. So far, none of the earth’s primordial rocks have been found. However, the geologists have become able to determine the plausible age of the Solar System and thus calculated the accurate age of the earth on the basis of the assumption that the Earth and the rest of the celestial solid bodies in the Solar System came into existence at the same time and are, therefore, of the same age.
How to determine the age of Earth?
The age of the earth as determined by current methods of radiometric dating is about 4600 million years. Before the discovery of radiometric dating, several attempts were made to determine the age of the earth. The important one’s are as follows.
- From the history of organic evolution.
- From the rate of sedimentation.
- From salinity of seawater.
- From the rate of Cooling.
From History of Organic Evolution:
As soon as the earth cooled and solidified, life speared on it. Initially, the organisms had a simple body structure. The complexity increased with the passage of time. The man which has evolved quite recently possesses the most complex structure. From the history of organic evolution, biologists have determined the age of the earth be about 1000 million years.
From Rate of Sedimentation:
The formation of sedimentary rocks started with the solidification of the earth. Hence efforts were made to calculate the age of the earth from the rate of sedimentation in the ocean. The method involved was to determine the total thickness of sedimentary rocks that had been deposited during the earth’s history and then to divide it by the rate of sedimentation. This method gave an age of around 500 million years. This method had many difficulties some of which are as follows.
- Different sediments accumulate at different rates under varying conditions.
- A rate of sedimentation determined for recent sediments can not necessarily be applied to the past.
From Salinity of Sea Water:
It is assumed that the oceans had originally had fresh water. If the amount of salt carried to the ocean each year by rivers is known, it can be calculated how much time it took to accumulate the salt now present in the ocean. This method gave the age of the earth as about 120 million years. this estimate was not accurate as the rate of accumulation of salts could never be uniform all the history of the earth.
From the rate of Cooling:
In 1897, the British physicist Lord Kelvin calculated the age of the earth from the temperature difference between the initial molten planet and its present state. He assumed that the rate of heat loss was constant throughout history. This method gave an age of 20 to 40 million years which was more than 100 times lower than current estimates.
Kelvin also made estimates based on assumptions concerning the origin of the sun’s heat. The age of the earth calculated by this method gave similar results, that is the figures ranged from 20-40 million years. Kelvin’s estimates for the sun and the earth were wrong because he did not take into account the fierce heat generated by the decay of radioactive elements.
The methods of radiometric dating are based on the radioactive decay of certain isotopes that have very long half-lives. The radioactive isotope is often referred to as the “parent” and the elements resulting from the decay of the parent are called the “daughter products”. The age of rocks and minerals that contain radioactive isotopes is determined by measuring the accumulation of the daughter products in them. This procedure is called “Radiometric Dating”. Radiometric dating is very reliable because radioactive decay proceeds at a constant rate and it remains unaffected by any physical or chemical agents.
A commonly used term in radiometric dating is “Half-Life”. it is a common way of expressing the rate of radioactive decay. Half-life may be defined as the time required for a given amount of radioactive substance to decay to one-half of its initial value. For example, the half-life period of Uranium-238 is 4500 million years. This means that in 4500 million years one gram of Uranium-238 would be reduced to half a gram, the rest having been transformed into the lead-206.
Of the many radioactive isotopes that exist in nature, only three have proved useful in providing radiometric ages for ancient rocks. These three isotopes are given in the following table. The methods which are commonly used for dating rocks include Potassium-Argon, Rubidium-Strontium, and Uranium-Lead. The basic equation for calculating the age of a rock sample is as follows.
Age of a rock = 3.323 T log10 [1+Nd/Np]
Where Nd is the number of atoms of the daughter product. Np is the number of atoms of the parent radioactive substance present today and T is the half-life of the radioactive substance.
|Radioactive Parent||Half-life in million years||Daughter Product||Minerals/Rocks commonly dated|
|Uranium-238||4500 m.y||Lead-206||Zircon, Sphene|
|Potassium-40||11900 m.y||Argon-40||Muscovite, Biotite, Hornblende, Glauconite, Volcanic rock.|
|Rubidium-87||47000 m.y||Strontium-87||Muscovite, Giotite, Microcline, Metamorphic rocks.|
In practice mass spectrometers are used to measure precisely and accurately the quantities of parent and daughter radioactive substance in rocks. in radioactive dating, it is assumed that none of the daughter products has been leached out from the rocks and that none of the daughter products has been leeched out from the rock and that none has been introduced from outside.
Radiometric dating methods give a figure of some 3500 million years for the oldest rocks of the earth’s crust. The radioactive analysis of meteorites has yielded dates of up to 46 million years which probably correspond to the age of the earth.
Common Radiometric Elements
The method of half-life and ratio of the radioactivity of some elements like Potassium, Rubidium, Uranium, and Radiocarbon is also used to determine the age of the earth. For example, if the half-life of an element is 10 million years, and it has decayed up to 50 % meaning that it has passed 5 million years yet. It means that the age of that element is 5 million years. The age of the earth is also equal to the age of that element. So the Earth is also 5 million years old.
1. Potassium-Argon Method:
Potassium-40 has a half-life of 11900 million years. It is commonly found in many rocks. It decays into argon-40; an inert gas found in the sedimentary rocks. The Potassium-Argon method can be used to date rocks that are millions of years old, including meteorites(age 4600 million years). However, rocks younger than 0.1 million years are difficult to date because they contain very little radiogenic argon. The minerals that are suitable for K-Ar dating include biotite, muscovite, hornblende, and nepheline. The potassium felspars, such as orthoclase and microcline are unsuitable for K-Ar dating because they are likely to lose argon readily at atmospheric temperatures.
2. Rubidium-Strontium Method:
Rubidium-87 has a half-life of 50,000 million years. The Rb-Sr method can be applied to rocks of almost any geological age. However, rocks younger than 20-30 million years may create analytical difficulties because of the minute amount of radiogenic Sr-87 present. The Rb-Sr method can be used to date such common rock-forming minerals as muscovite, biotite, and all types of potassium felspars including orthoclase and microcline.
3. Uranium-Lead Method:
The half-life of Uranium-238 is 4500 million years. uranium and Thorium frequently occur in the same mineral and it is, therefore, possible to make two independent age determinations on one mineral sample.
The U-Pb method can be applied over the greater part of the geological age range, although their usefulness decreases sharply for rocks younger than 100-200 million years. This method is less commonly used than the K-Ar and Rb-Sr methods but may be used to date rare minerals in which uranium and thorium are major constituents. it is frequently applied to minerals such as zircon and sphene.
It should be noted that the U-Pb methods are of much greater value than the Th-Pb method. The latter is now only rarely used.
4. Radiocarbon Dating:
There are three isotopes of Carbon: C-12, C-13, and C-14. Of these C-14 is radioactive. Since it has a half-life of only 5730 years. It can be used to date events in recent geological history. Although C-14 is only useful in dating the last small fraction of geological time (75000 years). It has become an important means for dating the Ice Age and archaeological material.
Radiocarbon (C-14) is produced in the atmosphere by cosmic ray bombardment of Nitrogen-14. The C-14 combines with oxygen to form carbon dioxide which is absorbed by living organisms. A constant ratio of C-14 to C-12 is established in each organism during its life. After death, no more carbon dioxide is absorbed and the C-14 decays steadily by emitting beta particles to N-14.
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