The oldest pieces of Earth

The story

Looking for a stone that remembers everything

Imagine you had to find out what your town was like a thousand years ago, but you only had a few scattered scraps of brick. Scientists who study the newborn Earth are in the same situation. The planet is a place in constant change: continents move, rocks sink into the mantle and melt, volcanoes bring new ones to the surface, rain and wind wear down the mountains. That is why almost nothing is left of the rocks of the first 500 million years of the Earth's life.

Among the oldest rocks known today in one piece are the Acasta gneisses, in northern Canada, about 4 billion years old (by some studies the Nuvvuagittuq belt, also in Canada, is even older, but its age is disputed). But there are even older fragments: tiny crystals, smaller than a millimetre, found in the Jack Hills in Western Australia. They are crystals of zircon, a mineral that forms from magma, and they are about 4.4 billion years old.

Zircon: a time capsule

Zircon is one of the toughest materials in nature. When it forms, a little uranium gets into its structure, but no lead. Over time the uranium, which is radioactive, slowly turns into lead, at a rate that scientists know precisely. By measuring how much uranium and how much lead there is in a crystal, they can work out how much time has passed since it formed, as with an hourglass. The crystal withstands heat, impacts and even chemical changes: it often survives even when the rock that held it is destroyed.

The zircons of Jack Hills are not found in the rock where they were born. They are grains that water carried elsewhere, billions of years ago, and that ended up trapped in sands that later turned into a rock called conglomerate. This rock is about 3 billion years old, but the grains inside it are much older. The oldest was dated in 2001 by a team of researchers led by Simon Wilde, of Curtin University in Australia, who found an age of about 4.4 billion years, only 150 million years after the Earth formed. In 2014 another team, led by John Valley of the University of Wisconsin, repeated the measurement with very precise techniques and confirmed that the crystal is 4.4 billion years old.

What they tell us

They are not just an age. Each crystal carries clues about the place where it was born. In the zircon studied by Wilde and colleagues, the amount of certain kinds of oxygen suggests that the magma it formed from had to do with rocks that had interacted with liquid water. If so, 4.4 billion years ago the Earth already had a solid crust and there was water on the surface. This changed the picture scientists had of the newborn planet: not just a hell of lava, but a world perhaps cooler than people thought.

As always in science, the conclusion is a cautious one. Some researchers question the interpretation of the oxygen and the date of some stages, and study whether the crust was thin and unstable, covered by shallow oceans. What is certain is that these grains are among the oldest things ever touched by human beings, and that they are younger only than meteorites and the rocks brought back from the Moon.

A hill and a lot of patience

Jack Hills is a chain of low hills in the Murchison region, in the desert of Western Australia. To find the crystals, geologists collect rocks, crush them and separate the heavy minerals, then look at each grain under the microscope. Out of hundreds of thousands of grains, only a small share are more than 4 billion years old. It takes a good eye, patience and instruments such as the ion mass spectrometer, which can analyse a crystal the size of a pinpoint without completely destroying it.