Earth Age: How Old is Earth?

Earth Age

Our best estimate is that Earth age is a staggering 4.5 billion years, give or take 50 million years. But how do we know this?

We can estimate our planet’s age in different ways:

  • First, we can find the oldest rock on Earth and date it.
  • Second, we can collect sample rocks from the moon and estimate their age.
  • Lastly, we can study meteorites that fall on Earth and establish a date.

So let’s try to pinpoint Earth age by looking closely at Earth rocks, moon rocks and meteorites.

Grand Canyon’s layers of rocks

Grand Canyon Layers

A real simple way to estimate Earth age is to gaze at cross-sections like at the Grand Canyon.

Younger rocks pile on top of older layers. It’s like stacking pancakes one on top of another. The coldest are on the bottom. It’s the same concept for rocks. This is the concept of relative dating.

So if you look at the sedimentary rocks at the Grand Canyon, it’s the Unkar Group at the bottom. These rocks are about 1.25 billion years old. Then beneath those, there are Precambrian basement rocks. These metamorphic rocks are dated at about 2 billion years old.

Ok, this is a pretty decent estimate. But how can we get more accurate?

Estimating the planet’s age by dating Earth rocks

Earth Age

First, we can simply find the oldest rocks on Earth and then conclude that this is how old Earth is.

But the problem is that the rock cycle is always recycling Earth’s crust. If you could speed up time, you’d see rocks continuously sinking deep down into the interior then rising back up again as magma.

So if you took the age of rocks (Acasta Gneiss) in northern Canada, Earth age is about 4.04 billions years old. Alternatively, some grains of rocks (zircon) in western Australia are about 4.36 billion years old.

So already, we can get a pretty good estimate of Earth age. But it’s not exact. Because in the primeval Earth, Earth was still a fiery cinder so it didn’t cool down to sustain rocks until a billion years later.

Pinpointing Earth age by moon rocks and meteors

Antarctica Meteors

Another way to evaluate Earth age is by dating geologic samples of moon rock. Because the moon formed around the same time the Earth did, we estimate moon rocks collected during the Apollo 11 mission to be 4.4 to 4.5 years old.

Further to this, we study meteorites that fall in Antarctica because they’re easier to find in the ice. Similar to the moon, these ancient remnants of our solar system are also 4.4 to 4.5 years old.

So if you take into account both the age of lunar samples and meteorites, then we can obtain a fairly robust estimate of Earth age. That age is a staggering 4.54 billion years old (± 0.05 billion years).

READ MORE: Carbon Dating: How Radioactive Isotopes Estimate Age

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