The oldest rocks on Earth–zircon crystals–are over four billion years old and barely the width of a human hair. Previously thought to have been created by tectonic plate shifts, a new study reveals that these delicate crystals may instead have been formed by violent asteroid impacts that struck our planet when it was just a wee 500 million years old.
As the oldest material we have from the early Earth, zircon crystals can provide geologists with key clues as to how our planet developed–such as when water first appeared, how climate has varied, and where and when early life formed. Which has left scientists rather eager to determine exactly how zircon crystals formed.
Around ten years ago, a team of scientists in the US proposed that zircon crystals were formed when tectonic plates shifted and slammed into one another, but recently that theory has been debunked as evidence emerged that plate tectonics was not occurring during that period in Earth's history.
So researchers at Trinity College Dublin decided to explore another, far newer theory; that the tiny crystals formed when the early Earth was subject to frequent strikes by asteroids some several kilometers in diameter. In order to do so, they studied the much younger–two billion-year-old–Sudbury impact crater in Ontario, Canada.
According to a press release from Trinity College, the Sudbury site is best preserved large impact crater on Earth and the planet's second oldest confirmed crater. The team collected thousands of zircon crystals from the site and analyzed them at the Swedish Museum of Natural History in Stockholm, where they were able to determine that the crystals are "indistinguishable" from their four billion-year-old counterparts ... thus cementing a solid theory that the oldest crystals known to man may indeed have formed in impact craters around the globe.
PhD Researcher in Trinity's School of Natural Sciences, Gavin Kenny, is first author of the article, and explained in a statement:
There's a lot we still don't fully understand about these little guys but it looks like we may now be able to form a more coherent story of Earth's early years–one which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times.