age of earth

                                            Age of earth

                                                                                                                                  :- Many philosophers have speculated on the age of earth & their estimate vary from one another in a great range.

The age of the earth has been determined from

 a) Evidence from the evolutionary changes of animals. 

 b) Evidence from the rate of formation of sedimentary rocks i. e. average rate of deposition, thickness of sediment in oceans.

c) Evidence from salinity of sea water.

d) Evidence from rate of cooling of the earth.

e) Evidence from disintegration of radioactive elements. This is modern method of determining age of earth.

The earth is an continuous state of change. The total amount of changes as for produced & the rate of changes are calculated.Then it is possible to determine the age of earth by formula.

      Age of earth =         Total amount of changes   /     Rate of changes.

Since the beginning of 19^th century the attempts were made to solve the problem of age of earth. On the scientific  basis  following are the methods.

1)    Physical method.                     2) Geological method.

 3) Astronomical method.           4)  Chemical method.  

1) Physical Method: - Hord Kelvin, the great English physicist regarded that earth is loosing heat & getting cooler the age of earth determined 20 – 40 M. yr.

2) Geological Method:-

 a) Sodium deposit in sea. Total salt of sea 3.5 % by wt. 75 % of salt is Na. Total amount of Na in it 16 x 10¹⁵. The annual increment through being river is of order of 16x107 tones. 10 million yrs.

 Many serious objections are:-

1) Annual rate of addition of Na may not be uniform.

 2)  All Na may not be in dissolved state.

b) Sedimentation: - Varred sediments, homglass method. Sir Geikie determined the age of earth by calculating the thickness of geological formation are dividing the result by assuming rate of the deposition of sediment. But this is an unreliable method because of rate of sedimentation extremely variable.

c) Biological Evolution Method: - This method is still vaguer & often to criticism that other organisms have evolved from simplest to animal cules to present state.  

3) Astronomical Method: - According to Jeffery all planets are of same age this method is based on ocentric the orbit of planet mercury.

4) Chemical Method: - This method is most reliable since they are based upon the changes & unaffected by the temp. & pressure, radio active etc. The radioactive elements have remarkable properties of self disintegration ultimately resulting in new stable element.

The principle radioactive isotopes are U²³⁸ ,   U²³⁵ ,   Th²³²  ,    Pb ⁸⁷ , K⁴⁰   Pb²⁰⁶   Ar⁴⁰  Sr⁸⁷

Half life is the time taken by radioactive element to decay it’s half the amount.

1)    Helium Method: - U²³⁸ -------------à  Ra²²⁶  + 3He .            One gram of                                                          uranium gives one ccs of the He in 8.8

2)    Lead Method: - Uranium – lead ratio is more accurate than uranium – Helium.

                                    U²³⁸ -----------à Ra²²⁶  + 3 He⁴ ------------à Pb²⁰⁶  + 8 He ⁴

One gram of uranium gives 1 c c lead in 7600 mg.

3)    Alkali Method :-                           |---------- Ca ⁴⁰

                                                  K ⁴⁰    ---à|

                                                                    |----------   Ar ⁴⁰ 

Age of earth is 4550 million yrs.

4)    Radioactive Method: - Radioactive dating is the determination of age of radioactive carbon. This method has got great over other method.

 Radioactive C ¹⁴ is formed is the atmosphere by collision of cosmic rays (neutron) with atoms of Nitrogen N ¹⁴. In turn each C ¹⁴ nucleus losses a β particle. Α is transformed back into N¹⁴ the rate of decay is such that the half life is 5570 yrs.