How do we know how old the earth is

He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil. This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law which nearly all of us would accept and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.

That is the background to the intellectual drama being played out in this series of papers. It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains. We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds. Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.

One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists.

Most notable is William Thomson, ennobled to become Lord Kelvin in , whose theories make up an entire section of this collection. He was one of the dominant physicists of his time, the Age of Steam. His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Harlow Shapley, who wrote an article in on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe.

Russell, author of the article on radioactive dating, was familiar to me for his part in developing the Hetzsprung-Russell diagram for stars, but I was surprised to discover that he was also the Russell of Russell-Saunders coupling, important in atomic structure theory.

Sollas , assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift. The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics.

The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity. Lord Kelvin and his allies used three kinds of argument.

It turns out that the world is about 4,,, years old. This article was originally published on The Conversation and republished here with permission. Read the original article. Originally published by Cosmos as How do we know how old the Earth is? The Conversation is an independent, not-for-profit media outlet that uses content sourced from the academic and research community. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science.

Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today. Finding super old rocks is conceptually straightforward, but practically difficult. The processes of plate tectonics mean that the Earth is constantly recycling its rock, breaking it down into magma in the interior before pumping it back up to the surface once more.

But old rocks do exist, says Reich, and the oldest rock we know is a tiny piece of zircon found in western Australia. The process of figuring out a rock's age often falls to the scientific techniques of radiometric dating , the most famous of which is radiocarbon dating. This method is called radiometric dating , and it involves the decay, or breakdown, of radioactive elements. Using radiometric dating techniques, it became possible to determine the actual age of a sample.

Radiometric dating requires an understanding of isotopes. Isotopes are variations of an element differentiated by the number of neutrons in their nuclei. The isotopes of unstable radioactive elements—known as parent isotopes—eventually decay into other, more stable elements—known as daughter isotopes—in a predictable manner, and in a precise amount of time called a half-life.

The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay. The age of a sample can be determined based on the ratio of parent to daughter isotopes within the sample.

One problem with this approach to dating rocks and minerals on Earth is the presence of the rock cycle. During the rock cycle, rocks are constantly changing between forms, going back and forth from igneous to metamorphic to sedimentary. This makes finding an exact age for Earth difficult, because the original rocks that formed on the planet at the earliest stages of its creation are no longer here. The oldest rocks that have been found are about 3.

To get around the difficulty presented by the rock cycle, scientists have looked elsewhere in the solar system for even older rock samples. They have examined rocks from the moon and from meteorites, neither of which have been altered by the rock cycle. The same techniques of radiometric dating have been used on those rocks. All the data from Earth and beyond has led to the estimated age of 4. The age of rocks is determined by radiometric dating, which looks at the proportion of two different isotopes in a sample.

Radioactive isotopes break down in a predictable amount of time, enabling geologists to determine the age of a sample using equipment like this thermal ionization mass spectrometer.