The record for the oldest solid piece of earth has been shattered by a tiny zircon crystal found on a sheep ranch in the Jack Hills region of Western Australia, about 800 km north of Perth (see the full story here: http://www.smh.com.au/technology/sci-tech/gem-found-on-australian-sheep-ranch-is-the-oldest-known-piece-of-earth-scientists-find-20140224-hvdkd.html).
Scientists have measured the crystal’s age using two different techniques. Their results show that the crystal first solidified from molten rock 4.4 billion years ago, only 160 million years after the initial formation of the Earth, and around the time of the formation of the moon (http://www.history.com/shows/the-universe/videos/creation-of-the-moon).
The crystal is a surviving part of the earth’s early crust, which solidified from a planet-wide sea of lava. Most of the early crust has been recycled by plate tectonics, making surviving fragments of the oldest crust quite old. Zircon crystals are able to resist breakdown, and are often the only part left of what was once a large mass of rock.
The rock layer containing the zircon crystal was buried underground for hundreds of millions of years. However, around 3 billion years ago, the forces of erosion exposed it once again. The crystal was washed downstream by water until it was buried by sediments. These sediments underwent lithification, a process that cemented them into solid rock. Here, the zircon waited, buried in their silent tomb for 3 billion years before scientists uncovered it in the Jack Hills.
The discovery has changed our conceptions of early Earth:
Scientists had previously thought that the earth would be much too hot for the formation of solid materials so early in its history. In fact, the geologic time period that the crystal dates to is known as the “Hadean”, after the ancient Greek god of the underworld, because conditions were thought to be extremely harsh. The discovery of a zircon crystal from the early Hadean implies that conditions weren’t quite as harsh as scientists had thought.
This has implications for the origin of life – the fact that solid crust existed 4.4 billion years ago implies that the planet could have supported life in that early period of its existence.
This isn’t the first time an age record has been broken by an Australian crystal. Since Australia has been spared much of the tectonic stresses acting on other continents, it is not unusual for extremely ancient rocks to be well preserved here.
It’s important to note that although the Jack Hills zircon is the oldest crystal, it is not the oldest rock. Rocks are made up of many crystals cemented or fused together.
The record for the oldest intact rock unit is held by the Acasta Gneiss, which is found in northwestern Canada and has an age of about 4 billion years. To learn more about the Acasta Gneiss, check out: http://www.sciencebuzz.org/blog/acasta-gneiss-earths-oldest-surface-rock.