How old is the Earth? And how do we know?

In 1650 the Archbishop James Ussher announced that the Earth was thousands of years old.

Almost six thousand years old in fact. He had discovered that the world had been created on the 23^rd of October 4004 BC. At lunchtime.

This was a pretty momentous conclusion. Not only had Ussher calculated the age of our world to a superhuman degree of precision, he was also lending credence to the idea that the Earth had a beginning (many cultures believed the Earth has always existed, and always will) and that it was pretty damn old, 6,000 years-worth of old.

Today, most people are comfortable with the idea that the Earth hasn’t existed in perpetuity, and most people are OK with the fact that the Earth is billions of years old. But this is a pretty recent mindset. Up until about one hundred years ago, no one really had any idea how old the Earth was.

Here’s what happened…

It might sound pretty crazy to hear that the world is only six thousand years old. After all, a lot of us discover at a young age that the dinosaurs lived many millions of years ago, so we grow up comfortable with the concept of “deep time”, the idea that the Earth is ancient almost beyond comprehension, so its easy to laugh at Ussher and dismiss him as a religious crank. We shouldn’t though.

In 1650 no one knew how old the Earth was, people simply had no idea. Some thought our world could be hundreds of years old, others thousands. At this point I don’t think anyone was considering the possibility that the world was millions to billions of years old, as science was still very much in its infancy, Galileo had only just died, Newton has just been born, and the first dinosaur would not be described by a scientist for another 74 years.

Given this context I think Ussher should be commended. Although his estimation was somewhat wide of the mark, at least he adopted an almost scientific approach to his calculation by basing it on evidence. Ussher painstakingly trawled through masses of ancient biblical texts in order to establish a family tree for the whole of humanity. He literally went through all of the “Abraham begat Isaac; and Isaac begat Jacob; and Jacob begat Britney” stuff and worked-out how many generations of humanity there had been since Adam and Eve. The Bible taught that Adam and Eve were created at the same time as the Earth, thus Ussher realised that if you could work out the age of humanity, you could work out the age of our world.

Here’s Ussher at work. You can tell he’s not quite a full scientist as his beard is too small

Unfortunately Ussher made a few errors in his assumptions. Firstly he assumed that the Earth is as old as humanity, this isn’t the case though, as humans are an extremely recent genus. If we think of the lifespan of the Earth as a 24-hour day, with the Earth created at 00:00 and today being 24:00, then our species has been on the Earth since 23:59:56. Not very long. Secondly, he assumed that religious texts were an accurate account of the history of humanity. They aren’t.

Again, its quite tempting to mock Ussher for these assumptions, particularly the second one, but I think he should be lauded, as questioning the veracity of the Bible was probably a pretty radical thing to do in the 1650s, and at least Ussher had based his calculations on evidence, not great evidence admittedly, but I can forgive him that. In many ways Ussher had approached the question of how old is the Earth as a scientist.

Since Archbishop James Ussher’s calculation a number of other enquiring scientists made their own estimations of the age of our planet. Many of these people were an odd breed of scientist called geologists.

In the late 18^th century a new science was born, the science of Geology. Most people have a vague idea of what Geology is, something to do with bearded men in inadvisably-small shorts striding up and down the countryside worrying rocks. Geology isn’t just about rocks though, it’s actually the study of the Earth, which does happen to be mostly made of rock, hence geologists fascination with them, but geologists are also into plants and animals, the weather, flowing water, sunset walks along the beach etc etc.

A geologist in action, notice the predilection for unfeasibly small shorts

One of the early titans of Geology was a Scotsman by the name of James Hutton. Hutton made lots of contributions to the science, but his greatest was the concept of Geological Time, the idea that the Earth is extremely old, not just hundreds or thousands of years old, but many millions.

Hutton reached this conclusion by observing the world around him. He realised that processes like weathering and erosion (large rocks being worn-down by the weather to produce small bits of rock) sediment transport (water moving bits of rock), deposition (those bits of rock travelling in water being “dropped-off” and collecting together) and lithification (those small bits of rock slowly transforming into large beds of rock again) were happening around him all of the time. He also realised these processes happened extremely slowly, that it would probably take thousands of years for rock strata to form, and that these processes probably occur at the same speed today as they always have (geologists call this the Principle of Uniformitarianism), so that if rocks take thousands of years to form today, they took thousands of years to form in the past too.

Hutton realised that if the “present is the key to the past” then there is literally hundreds of millions of years worth of rock all around us. On learning about Hutton’s revelation, the geologist John Playfair later wrote “the mind seemed to grow giddy by looking so far into the abyss of time“. Many geologists today also speak about the strangeness of contemplating geological time, and even claim that geologists often have a different concept of time to other, more normal people.

James Hutton, not wearing proper small geologist shorts here, but I bet he had some

At first I thought this was a bit pretentious, as the idea that the Earth was billions of years old certainly wasn’t that strange to me. But I admit, after studying geology for a few years I’ve had a couple of occasions when my mind expanded in a kind of intellectual vertigo, as I got fleeting glimpses into understanding just how in-humanly massive such a time-scale really is. Anyway, on with the story.

After the idea of deep time became more-and-more established in Victorian-era scientific circles, the race was on to try to work out exactly how old the Earth really was.

The astronomer Edmund Halley proposed a method based on sea salt. Halley knew that “salt” in the sea was actually small pieces of rock (OK, ionic minerals from rocks) that had been eroded from the land and washed into the sea, so he concluded that if you could work out how much salt was in the sea, and how much salt is washed into the sea each year, then you could work out how old the sea was and thus the age of the Earth. A number of Victorian scientists made calculations of this kind. In 1876 T. Mellard Reade calculated the Earth was 25 million years old, in 1899 John Joly reached a figure of 99.4 million years, and in 1910 George F. Becker estimated the age of the Earth to be between 50 and 70 million years.

The keen-eyed amongst you will notice that these estimates sound a little on the short-side. Unfortunately the sea salt method is based on faulty assumptions, the most important of which is that once salt is in the sea, it stays there. This isn’t true though; the sea can lose salt through evaporation, meaning it can’t be used as a clock to work out the age of the Earth.

Another method was proposed by the great William Thomson, a.k.a Lord Kelvin. Kelvin, was a superhero of science, he was a pioneer of physics and amongst many other discoveries, was the first person to realise that temperature had a bottom limit of -273°C, or zero degrees Kelvin (he got his very own scale!)

Kelvin used a heat-loss method to estimate the age of the Earth, he calculated the initial temperature of our planet when it first formed (it was hot, think entire seas worth of molten lava), estimated how much heat the Earth loses over time, and was thus able to work out the age of our world.

Lord Kelvin. He’s got a proper Victorian scientist’s beard, so you know he’s legit

Kelvin’s first published estimates dated the Earth at 400 million years old, but as he refined his calculations he continually revised the age of the Earth down, and finally concluded that it was 24 million years old. Unfortunately, the more Kelvin had worked on this theory, the further from the truth he got. However, in a demonstration of great insight, Kelvin conceded that his calculation would only be accurate if no another source of heat existed within the Earth. He didn’t really think this was the case though, so he was confident on his predicted age of 24 million years. He was wrong though, there is another heat source within the Earth, its radioactive decay.

In 1904 a scientist called Ernest Rutherford presented a paper at the Royal Institution in London (an organisation of Victorian scientists who enjoyed debating, drinking wine and eating large meals) in which he announced that the world was 700 million years old. Older than anyone else had so far claimed, at least in public. Kelvin was actually at this meeting, but apparently slept through most of Rutherford’s presentation, possibly on account of one of those large meals.

The impudent Ernest Rutherford brazenly flouted convention by not even having a beard!

Rutherford had found the cause of radiation. Marie Curie had already discovered radiation itself (energy emitted from objects, quite often rocks), but Rutherford had realised this energy came from radioactive decay, the breakdown of large unstable atoms, like uranium, into smaller stable ones, like lead. Rutherford also realised that the Earth’s crust and mantle was chock-full of such radioactively unstable elements, thus he’d found an additional heat source within the Earth that Kelvin had overlooked. Rutherford was therefore convinced the Earth was far older than Kelvin’s estimate.

Radioactive decay wasn’t just useful for embarrassing Lord Kelvin though. It also provided a way to date rocks. Rutherford realised that radioactive elements decayed into stable elements at a fixed speed. Therefore, if you could work out how much of a parent radioactive element, and how much of a daughter stable element is found in a rock, say the proportion of uranium to lead, you could work out how long it would have taken radioactive decay to produce that proportion, and thus you could calculate the age of that rock. Rutherford realised that the rocks themselves could be dated.

The race was then on to find the oldest rocks on Earth. Unfortunately the Earth has a rock cycle (although I should probably say fortunately, as it keeps life on Earth alive), which means that the Earth’s rocks are continually, if very slowly, being recycled by weathering and erosion. This means that old rocks are very rare and hard to find. The oldest found so far is 4.28 billion years old, and was discovered in Canada in 2008. So we know that Canada, and therefore the Earth, is at least this old.

But to get the most accurate date we need to find rocks that were formed at the same time as the Earth, but which haven’t been recycled in any way. And these rocks are found in space, they’re asteroids.

Like this (a carbonaceous chondrite meteorite)

Certain types of asteroids were formed at the same time as the Earth, in fact, the Earth was basically formed from asteroids smashing together and accreting into the planet we live on today. Some asteroids avoided this fate though, they weren’t incorporated into a planet and survived untouched in space, mostly in a belt of asteroids between Mars and Jupiter. Some of these asteroids have fallen to Earth as meteorites, and many have been dated using Rutherford’s radiometric technique.

These asteroids all provide a date very close to 4.54 billion years. Thus, today, we now know that the Earth, and the other rocks in our Solar System all formed at this time.

So there you go. This post could have been a lot shorter, but I enjoy this tale and couldn’t help indulging myself. The Earth is 4.54 billion years old, and we know this from the radiometric dating of asteroids.