How old is the Earth??

Well, since this is a blog about all aspects of geology I figure it is best to start at the beginning and ask a question that has been hotly debated by humans throughout our history: How old is the Earth? The age of the Earth that is globally accepted by geologists is 4.6 billion years.

4.6 billion is a ridiculously large number, so before, we delve into how we came up with it in the first place lets get some perspective on how much time 4.6 billion years actually is. Most of us can't envision much more than 1000 years accurately so trying to understand the incomprehensibly large amount of time 4.6 billion years represents is just that: incomprehensible. There are many ways of putting it into perspective but one that works for me is to imagine the age of the Earth is represented by distance. Imagine a path 100 metres long. If we were to let the length of this pathway represent the entire age of the Earth each year would be:

100m / 4,600,000,000 years

= 2.17 x 10^-8 m/year

= 21.7 nanometres/year

Each year is 21 nanometres which is about the width of 10 atoms lying side by side, or the size of a virus or 2000x less than the width of a human hair...so really, really small! Therefore, when we think about this in terms of our 100m pathway we can start to understand how much time 4.6 billion years actually represents.

Of course, this scenario assumes that the age of the Earth really is 4.6 billion years old and begs the question how did we come up with this number in the first place? Time for a bit of history.

One of the first attempts to date the Earth, which was largely accepted at the time, was performed in 1650 by Archbishop James Ussher. Ussher calculated the age of the Earth by adding up the ages of people in the Bible and correlating the dates of events in the Bible with historical records from other cultures. In this way he found that the  Earth was created on the evening of October 22nd, 4004 BC.

Ussher's age for the Earth was accepted until Lord Kelvin (the namesake of the temperature unit "Kelvin") proposed one of the earliest scientific methods for dating the Earth in 1846. Lord Kelvin, a physicist, suggested that the rate of cooling of the Earth could be used to predict when the Earth formed. He assumed the Earth was molten at the time of formation and by simply calculating the time it would take for the Earth to reach its present temperature given a steady rate of cooling he was able to calculate an age. Kelvin's calculations resulted in an age for the Earth between 20-30 million years ago. Many geologists disputed Kelvin's results, but were unable to offer a satisfactory rebuttal until the assumption that the Earth had begun in a molten state was challenged and because, in my opinion, they were afraid to challenge a man with such an epic beard. Ironically, the major flaw in Kelvin's calculations was that he had not accounted for heat produced by radioactive decay, which would later become the very tool used to date the Earth.

Lord Kelvin and his beard

The next step in dating the Earth did not come about until the concept of radioactive decay and radioactive dating was created. Some elements have very long half lives which enables geologists to use them to date rocks. For example, by calculating the amount of a certain daughter product present and the initial amount of the parent element we can calculate the time it took for the parent to decay into the daughter giving us an age for a rock. Radioactive dating relies on dating isotopes trapped in rocks to find a date. Some of the oldest rocks that have been found thus far come from Northern Canada in a geological unit called the Acasta gneiss. The rocks from the Acasta gneiss have been dated at 3.96 billion years using uranium-lead dating. Grains that are even older of the mineral zircon have been discovered in metamorphosed sandstones in Australia that date at around 4 billion years old. There is even an argument currently raging in the geologic community about the new oldest rocks that have been found in the Northern Canada. I personally have seen these rocks and I can tell you they don't look like anything special. However, there is still a problem: none of these rocks actually tell us how old the Earth is as the crust did not form immediately and may have taken many millions of years to form. So then how did we come up with this crazy 4.6 billion number? Primordial lead ratios.

The Acasta gneiss

Yes, I realize primordial lead ratios sounds like gibberish a geologist might spit out after a few too many drinks in the field, however, they are the key to our 4.6 billion year old date for the Earth. About a third of the lead on today's Earth is a product of the radioactive decay of uranium and two thirds is from the original formation of the planet. But, some meteorites contain no lead produced by radioactive decay so they have lead isotope ratios that have not changed since they were formed. These are the primordial lead ratios. The reason this matters is because it allows us to extrapolate from our current lead ratios, which have been changed by the addition of radiogenic lead, back to the original lead ratios at the time of the formation of the Earth. Since we know the rate that radiogenic lead is produced by the decay of uranium we can calculate how long it has taken to get from our primordial lead ratio to our current lead ratio. The answer is 4.6 billion years!!! Phew.

Meteorite

So there you have it. The progression of the age of the Earth up to our currently accepted value of 4.6 billion years and how we got there. Thanks for reading.

Matt

Reference: Prothero and Dott, Evolution of the Earth , 7th Edition, 2004.