Here is a question ........ How old is the Earth? Well almost everyone has a birth certificate, but, our Planet Earth don't have one.
The age of Earth is determined by the radiometric age-dating of the ever changing crust of Earth, rock samples from Earth's neighbors, such as moon and visiting meteorites. By using this method scientists have calculated that the age of Earth is 4.54 billion years with an error of 50 million years!
What is radiometric / radioisotope dating :
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| Alpha decay is one of the types of radioactive decay. |
In the 20th century, research showed that the isotopes of the radioactive elements decay into non - radioactive elements at a predictable rate. As a result, by examining the current amount of the element we can calculate the initial concentration the radioactive element. Using this we can determine the time required for the element to decay, thus, allowing us to determine the age of the rock.
Examples of end products :
- Argon from decay of Potassium-40.
- Lead from the decay of Uranium and Thorium.
Early calculations of the Age Of Earth :
In 1862, physicist Sir William Thomson, 1st Baron Kelvin publishes his estimation for the age of Earth. He assumed Earth to be completely molten and calculated the time it would require Earth to reach the then temperature gradient. According to his calculations, he estimated the age of Earth to be between 20 million to 400 million years!! He didn't considered heat produced due to radioactive decay (a then unknown process) and other sources.However, there were some scientists who could not digest this. The Darwin's theory of evolution suggested that the time required for life to evolve are much more than above mentioned period. Still during the 19th century most of the scientist estimated the age of Earth to be in some million years.....which we know today is not true.
Modern Calculation :
In an effort to find the age of Earth, scientist turned to the rocks on the surface of the Earth. However, due to geological activities, the Earth's crust is constantly revamped. As a result, the first rocks on Earth would have been recycled and melted long ago.
The oldest rocks on Earth are found in north-western Canada, which are found to be around 4.03 billion year old. However, rocks more than 3.5 billion year old are found in all the continents.
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| A fist-size sample of the Acasta Gneisses, rocks in northwest Canada that are the oldest known rocks on Earth. (Image credit: Mike Beauregard) |
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| A 4.4 billion year old zircon crystal from Australia is the oldest piece of Earth yet found. The source rocks for the small shards have not yet been identified. (Image credit: John Valley, University of Wisconsin.) |
In an effort to refine the age of Earth, scientist shifted their focus on the bodies in solar system.
Unlike Earth, the Moon does not have a atmosphere and geological activities. Hence, the first rocks formed on the surface of Moon are still present there. The samples returned from the Apollo and the Luna missions revealed the age to be between 4.4 billion to 4.5 billion years.
Scientists have also studied the meteorites that have fallen on Earth. These meteorites have arrived from various different sources. Some of the meteorites, have arrived from the planets like Mars due to violent collisions while some are the remains from the early solar system.
One such is the Barringer Crater formed by a meteorite around fifty thousand years ago in Arizona. It was named after the nearby Canyon Diablo. The Canyon Diablo meteorite samples showed a spread from 4.53 to 4.58 billion year old. Scientists comprehend this range to be the time periods for the solar nebulae to collapse into solar disk which formed the Sun and the planets.
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| Barringer Crater, Arizona where the Canyon Diablo meteorite was found. |
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| The fragment of the Canyon Diablo meteorite. (Image source : Geoffrey Notkin, Aerolite Meteorites of Tucson) |
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