By dating the rocks in Earth’s ever-changing crust, as well as the rocks in Earth’s neighbors, such as the moon and visiting meteorites, scientists have calculated that Earth is 4.54 billion years old, with an error range of 50 million years.
How old are your rocks?
Scientists have made several attempts to date the planet over the past 400 years. They’ve attempted to predict the age based on changing sea levels, the time it took for Earth or the sun to cool to present temperatures, and the salinity of the ocean. As the dating technology progressed, these methods proved unreliable; for instance, the rise and fall of the ocean was shown to be an ever-changing process rather than a gradually declining one.
And in another effort to calculate the age of the planet, scientists turned to the rocks that cover its surface. However, because plate tectonics constantly changes and revamps the crust, the first rocks have long since been recycled, melted down and reformed into new outcrops.
Scientists also must battle an issue called the Great Unconformity, which is where sedimentary layers of rock appear to be missing (at the Grand Canyon, for example, there’s 1.2 billion years of rock that can’t be found). There are multiple explanations for this uncomformity; in early 2019, one study suggested that a global ice age caused glaciers to grind into the rock, causing it to disintegrate. Plate tectonics then threw the crushed rock back into the interior of the Earth, removing the old evidence and turning it into new rock.
In the early 20th century, scientists refined the process of radiometric dating. Earlier research had shown that isotopes of some radioactive elements decay into other elements at a predictable rate. By examining the existing elements, scientists can calculate the initial quantity of a radioactive element, and thus how long it took for the elements to decay, allowing them to determine the age of the rock.
The oldest rocks on Earth found to date are the Acasta Gneiss in northwestern Canada near the Great Slave Lake, which are 4.03 billion years old. But rocks older than 3.5 billion years can be found on all continents. Greenland boasts the Isua supracrustal rocks (3.7 to 3.8 billion years old), while rocks in Swaziland are 3.4 billion to 3.5 billion years. Samples in Western Australia run 3.4 billion to 3.6 billion years old.
Research groups in Australia found the oldest mineral grains on Earth. These tiny zirconium silicate crystals have ages that reach 4.3 billion years, making them the oldest materials found on Earth so far. Their source rocks have not yet been found.
The rocks and zircons set a lower limit on the age of Earth of 4.3 billion years, because the planet itself must be older than anything that lies on its surface.
When life arose is still under debate, especially because some early fossils can appear as natural rock forms. Some of the earliest forms of life have been found in Western Australia, as announced in a 2018 study; the researchers found tiny filaments in 3.4-billion-year-old rocks that could be fossils. Other studies suggest that life originated even earlier. Hematite tubes in volcanic rock in Quebec could have included microbes between 3.77 and 4.29 billion years ago. Researchers looking at rocks in southwestern Greenland also saw cone-like structures that could have surrounded microbial colonies some 3.7 billion years ago.
Meet the neighbors
In an effort to further refine the age of Earth, scientists began to look outward. The material that formed the solar system was a cloud of dust and gas that surrounded the young sun. Gravitational interactions coalesced this material into the planets and moons at about the same time. By studying other bodies in the solar system, scientists are able to find out more about the early history of the planet.
The nearest body to Earth, the moon, doesn’t experience the resurfacing processes that occur across Earth’s landscape. As such, rocks from early lunar history still sit on the surface of the moon. Samples returned from the Apollo and Luna missions revealed ages between 4.4 billion and 4.5 billion years, helping to constrain the age of Earth. How the moon formed is a matter of debate; while the dominant theory suggests a Mars-size object crashed into Earth and the fragments eventually coalesced into the moon, other theories suggest that the moon formed before Earth. [Related: How Was Earth Formed?]
In addition to the large bodies of the solar system, scientists have studied smaller rocky visitors that have fallen to Earth. Meteorites spring from a variety of sources. Some are cast off from other planets after violent collisions, while others are leftover chunks from the early solar system that never grew large enough to form a cohesive body.
Although no rocks have been deliberately returned from Mars, samples exist in the form of meteorites that fell to Earth long ago, allowing scientists to make approximations about the age of rocks on the Red Planet. Some of these samples have been dated to 4.5 billion years old, supporting other calculations of the date of early planetary formation.
Fifty thousand years ago, a rock hurled down from space to form Meteor Crater in Arizona. Shards of that asteroid have been collected from the crater rim and named for the nearby Canyon Diablo. The Canyon Diablo meteorite is important because it represents a class of meteorites with components that allow for more precise dating.
In 1953, Clair Cameron Patterson, a renowned geochemist at the California Institute of Technology, measured ratios of lead isotopes in samples of the meteorite that put tight constraints on Earth’s age. Samples of the meteorite show a spread from 4.53 billion to 4.58 billion years. Scientists interpret this range as the time it took for the solar system to evolve, a gradual event that took place over approximately 50 million years.