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5.3. Geological time: historical ideas about the age of the Earth

Age of the earth

Humanity has always wondered how old the Earth could be. To try to calculate it they were based on the observation of nature and the sky together with the religious beliefs of the time.

We now know that the Earth has an age of approximately 4.6 billion years, but to reach this date, it has been necessary to make numerous attempts based on the scientific knowledge of each era.

Some of those who tried to calculate the age of the Earth were the following:

  • 17th century: Archbishop James Usser, based on the ages of the Jewish Patriarchs that appear in the Old Testament, estimated the age of the Earth as 4004 years before Christ, specifically on October 23 at 9 in the morning. Some creationist theories still hold that the age of the Earth is several millennia.
  • In 1774, Buffon (Georges Louis Leclerc, Earl of Buffon), created a globe of small dimensions similar in composition to the Earth, and starting from that molten mass, he calculated an age of 75,000 years by measuring its rate of cooling. He was judged by the church, as his calculations were far from the 6000-year-old Earth that the church proclaimed. His calculation was incorrect because it was unknown that most of the internal heat of the Earth comes from the decay of radioactive isotopes.
  • At the beginning of the XIX century, the procedure of the salinity of the sea was used . Starting from the fact that, at first, the oceans were made up of fresh water, calculating the amount of salts that the rivers contribute to the sea until reaching the current salinity, the estimated age was about 5 million years. (They did not count on the fact that salinity does not increase indefinitely, but that the precipitation of these salts occurs, forming sedimentary rocks).
  • In the middle of the 19th century, two currents emerged, although the first was accepted:
    • In 1862, William Thomson, (Lord Kelvin) , based on the time it would take for the planet to cool down assuming that it had formed as a molten rocky ball, calculated that the age of the Earth was between 24 and 90 ma
    • Charles Lyell, believed that more time was needed for the phenomena of erosion and sedimentation to occur. Observing the thickness of the strata, he calculated that about 2000 my were necessary, quite accurate calculations, since the oldest rocks had not yet been discovered. Darwin also saw a long period of time necessary for species to evolve, proposing an age of 300 m
  • In 1896, Becquerel discovered radioactivity, but it was in 1903 that the Curies discovered that radioactive elements gave off heat, the source of energy that heated the interior of the Earth. This was the basis for accurately calculating the age of the Earth.

  • In the middle of the 20th century, the age of the Earth was already between 1600 and 3000 million years, although at present it is estimated at 4600 ma, and it is not believed that this estimate will vary much in the coming years.

It is difficult to get an approximate idea of ​​how much this age represents. In history you talk about centuries, 100 years, a relatively long amount for a person, but that is practically nothing compared to the age of the Earth. Here we can look at a geological clock to get an idea of ​​what 4.6 billion years mean compared to 12 hours on a clock.

Geological time: historical ideas about the age of the Earth

It also talks about the different ideas that have been had about the age of the Earth throughout history in:


         

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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.