Since the planet Earth doesn't have a birth certificate to record its formation, scientists have spent hundreds of years struggling to determine the age of the planet. By dating the rocks in the ever-changing crust, as well as 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?
Several attempts to scientifically date the planet have occurred over the past 400 years. Scientists 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 science progressed, these methods were proven to be unreliable; for instance, the rise and fall of the ocean was shown to be an ever-changing process rather than a gradually declining one.
In an 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.
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 rates that can be easily predicted. By examining the existing elements, scientists can calculate the initial quantity, 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 Gneisses in northwestern Canada near the Great Slave Lake, which are 4.03 billion years old. 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 to 3.5 billion years. Samples in Western Australia run 3.4 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.
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 roughly 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, does not suffer from the resurfacing problems that cover Earth's landscape. As such, rocks from early lunar history should be present on the moon. Samples returned from the Apollo and Luna missions revealed ages between 4.4 and 4.5 billion years, helping to constrain the age of Earth.
In addition to the large bodies of the solar system, scientists have also studied smaller rocky visitors to that fell 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.
More than 70 meteorites have fallen to Earth to have their ages calculated by radiometric dating. The oldest of these have ages between 4.4 and 4.5 billion years.
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. In 1953, Clair Cameron Patterson measured ratios of lead isotopes in samples that put tight constraints on Earth's age.
The Canyon Diablo meteorite is important because it represents a class of meteorites with components that allow for more precise dating. Samples of the meteorite show a spread from 4.53 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.
By using not only the rocks on Earth but also information gathered about the system that surrounds it, scientists have been able to place the age of the Earth at approximately 4.54 billion years. For comparison, the Milky Way galaxy that contains the solar system is approximately 13.2 billion years old, while the universe itself has been dated to 13.8 billion years.