Zircon crystals are the closest thing we have to a time capsule from Earth in its early years. Some found in Australia are as old as four billion years old, which is especially interesting because the Earth is just over four and a half billion years old.
Scientists are studying these crystals to try to understand what the Earth was like when the oldest of these zircon crystals formed.
Originally, scientists thought these crystals formed as a byproduct of plate tectonics (giant chunks of the Earth colliding with each other). Current evidence suggests that plate tectonics the way we understand them today weren't in play back then.
Now, researchers believe that zircon formed in impact craters, which are indentations in the Earth created by asteroids crashing into it. The asteroid hits Earth at such a high speed that it melts the surface beneath it. That creates a perfect condition for zircon formation.
To test this idea, scientists studied zircon from the Sudbury impact crater in Canada: one of the best preserved and the second‑oldest impact crater on Earth. They found that the compositions of those crystals were indistinguishable from the four billion‑year‑old ones, implying that it's likely the older crystals formed in an impact crater as well.
Barthelmy, David. "Zircon Mineral Data." Mineral Database. September 5, 2012. Accessed November 16, 2016.
Trinity College Dublin. "Origin of Earth's oldest crystals." ScienceDaily. (accessed November 16, 2016).