Jan
18
2009

How old is the Earth?

The Earth as seen by the astronauts of Apollo 17. PD

The Earth as seen by the astronauts of Apollo 17 - NASA photo PD

None of us were around when the Earth was formed, but scientists are in agreement that the age of the earth is 4.54 billion years, give or take 1 percent.

To determine the age of the earth, scientists use a process known as radiometric dating. It is based on the fact that uranium, a naturally occurring element, decays at an extremely slow rate — depending on the type (known as isotope) of uranium, it has a half-life (meaning half of it turns into lead) of 700 million to 100 billion years. By measuring ratios of different types of lead and uranium, scientists can determine how old a sample of mineral is.

To arrive at the age of the Earth, scientists make various assumptions, one of them being that the Earth and the rest of our solar system were formed at the same time. Because of geologic changes, the age of most minerals found on the Earth is much less than the age of the Earth itself, so to get more accurate figures scientists have determined the ages of meteorites. Hundreds have been analyzed, and they consistently yield an age of around 4.5 billion years, so the assumption is that the Earth is about the same age.

The “gold standard” of meteorites used in dating the Earth is the Canyon Diablo meteorite, which fell around 30,000 years ago in what is now Arizona. It provides an especially large sample of ancient mineral and has been extensively analyzed.

Since geologic changes can form a new “birth date” for minerals, even the oldest rocks found on the Earth are newer than the Earth itself. But some of them come pretty close (in geological time, that is). According to the U.S. Geological Survey, some minerals found in northern Canada have been dated at 4.03 billion years, and some in western Greenland have been dated at 3.7 billion to 3.8 billion years. Also, some zircon grains found in Australia have lead/uranium ratios consistent with an age of 4.4 billion years. The oldest rocks from the moon have similar ages.

These numbers are consistent with the estimated age of our galaxy, the Milky Way, of 11 billion to 13 billion years.

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