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The age of rocks is usually determined by radioactive (or radiometric) dating. Some elements are radioactive and gradually convert from one isotope to another. For example, uranium 238 (238U) will gradually convert to lead (206Pb). It will do this at a constant rate. The rate is described as a half-life of the isotope in question. This is the period of time it takes for half the atoms of 238U to convert to lead. For this particular element, the half-life is 4.47 billion years and uranium/lead dating is useful for rocks between 1 million and 4.5 billion years (as luck would have it!). Other elements are also used (potassium/argon for example). The choice of element depends on how widespread it is - if it isn't found in many rocks then it's not very useful. And how long the half-life is - if the half life is shorter than the age of most rocks then its equally unuseful. Radiometric dating can only be performed on igneous rocks. The uranium/lead elements are most often used because igneous rocks often contain uranium and the half life is so long. The oldest rock found is around 4.54 billion years old. This is consistent with the age of rocks from the moon and from meteorites, which all point to an age around 4.54-4.56 billion years. Separate studies of the sun's mass and luminosity suggest that the solar system itself can't be much older than these rocks. All of this is well-known scientific information. The arguments against it lack scientific credibility, and most of them are posted by so-called Young Earth Christians. The vast majority of the scientific community accepts as valid the information provided by radiometric dating. The claims made about "assumptions" in radiometric dating of billion-year old rock are actually stepping stones that those who work in the field have worked hard to eliminate as impediments to the accuracy of their measurements. Making a "claim" about any aspect of radiometric dating without scientific proof that something is wrong, as has been done, does not invalidate the work - or the mearurements. The earth is about four and a half billion years old.

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9y ago
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12y ago

The age of the Earth is around 4.54 billion years (4.54 × 109 years ± 1%). This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples. The Sun, in comparison, is about 4.57 billion years old, about 30 million years older.

Our galaxy, the Milky Way, appears to be one of the oldest galaxies in the cosmos - engineering itself around 11-13billion years ago.

Our solar system was built from the remnants of a giant star's death - a supernova. Most of the remaining hydrogen was pulled back together after the explosion, which helped create our sun. When our sun came into life, it sent a shockwave of solar wind through our "neighborhood" which blasted the remaining materials from the dead star out into space and those materials are what gave rise to Earth and the rest of the planets we see in our solar system. That's why the rockier planets are closest to the sun and the gas giants are at the edge of the solar system - the blastwave sent the lighter materials further out than the heavier ones like carbon, oxygen, iron, etc.

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9y ago

A little history of the study of the age of the earth may help in understanding how geologists came to this answer.

  • Early in the 19th century, Charles Lyell examined the great volcano of Etna and studied the historical records of frequent eruptions. He noticed that each time it erupted, a new layer of lava would be added, causing the mountain to grow at a measurable rate. By knowing the height of the volcano, its approximate rate of growth and the frequency of eruptions, Lyall determined that the volcano must be several hundred thousand years old. Under the first lava flows, he found fossil shells that, wherever found, were among the youngest marine fossils, meaning that a hundred thousand years was geologically short compared to the immense age of the earth. Lyell had not estimated the age of the earth, but he had shown it must be millions of years old.
  • In 1862, Lord Kelvin, regarded by his contemporaries as the greatest physicist of his day, calculated the time it would take the world to cool down from its molten state: between 20 and 400 million years. With more accurate data on the melting point of rocks, he eventually settled on a figure not much higher than 20 million years. However, with the discovery of radioactivity, it was soon realised that the uranium present in the earth could have prolonged its cooling almost indefinitely. At least, the Law of Thermodynamics had set an absolute minimum on the age of the earth.
  • Samuel Haughton, an Irish geologist, calculated that sediments were deposited on the ocean floor at the rate of "one foot in 8,616 years". He then calculated a minimum duration of around 2000 million years for the maximum age of sedimentary rocks. Unwilling to accept such a long period, he scaled it back, by a factor of 10, to just 200 million years.
  • In the early years of the 20th century, Lord Rutherford established the age of a rock as 500 million years, by measuring the relative amounts of radium and helium present. Since helium is a gas, it could not have been present in the rock before the rock solidified, thus all helium present was the byproduct of radioactive decay. However, it was soon realised that the loss of helium during treatment and analysis was creating falsely short age estimates.
  • It was established that lead was the stable element resulting from radioactive decay of uranium. Since lead is not a gas, Dr Holmes decided to determine the age of rocks by using the ratios of uranium and lead. Using rocks from the Devonian age, he calculated that the Devonian age was at least 370 million years ago - quite close to present-day values. In the 1920s, Rutherford used the uranium-lead ratios to arrive at an age of 3400 million years for the earth.
  • In 1953, Fritz Houtermans used material from a meteorite to calculate the age of the earth as 4500 million years, plus or minus 300 million. In the same year, Patterson independently produced figures of 4510 and 4560 million years, using a basalt and a granite sample. It was the earth they were dating, not the meteor, but the meteor helped establish the amount of primeval lead in rocks of the solar system. In 1956, Patterson proved that the earth and meteorites had a common ancestry, thus validating the results.
  • Soon afterwards, Holmes dated the earth to 4,500 million years, plus or minus 100 million years, from terrestrial evidence alone.
  • Present estimates put the age of the earth at 4.54 billion years, plus or minus 45 million.

None of this analysis involves "circular" reasoning, and several independent methods have been used over many decades, all converging on the same answer. And that answer is not 6000 years.
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9y ago

The details are quite complicated, but as I understand it, the main evidence is radioactive decay. The exact mix of isotopes of different elements in a compound can provide evidence of how old it is (sometimes this might mean, when a substance solidified), since the mix of isotopes will change over time, in case some of the isotopes are radioactive.

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6y ago

The best estimate for Earth's age is based on radiometric dating of fragments from the Canyon Diablo iron meteorite.

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12y ago

RadioActive Dating Moon Rocks

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Q: How do scientists estimate the age of earth to be 4.6 billion years old?
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