Researchers from Trinity College have established where our planet’s oldest rocks (zircon crystals) came from: loads and loads of asteroids.
Let’s start at the beginning. Earth’s oldest rocks are more than 4bn years old. Earth itself is 4.5bn years old.
The rocks are tiny, about the width of a human hair, and are ‘naturally occurring’. Their importance lies in their age. As they are from a time when Earth was very young, finding out about them allows us to learn about our planet’s nascent time.
Scanning electron microscope image of a tiny zircon crystal which formed in the Sudbury impact crater – analysis has shown that the oldest zircons on Earth could have formed in a similar setting, via Gavin Kenny
The problem is, we don’t know where on Earth they came from. Well, we didn’t know, until Gavin Kenny from Trinity College Dublin got a team together and, with the help of the Irish Research Council and Science Foundation Ireland, got studying.
The original theory was that our tectonic plates rubbing together created these rocks, however, according to Kenny, that can’t be, as the crystals predate the collision of tectonic plates on Earth. So, what other possibility was there?
The answer had to be asteroids. Kenny and his team collected thousands of zircons from the 2bn-year-old Sudbury impact crater in Ontario, Canada – the best preserved large impact crater on Earth.
Larger-scale evidence for the impact at Sudbury in the form of these distinctive features known as shatter cones – the shock wave from the impact migrated from the bottom to the top of the picture, producing these pyramidal forms, via Gavin Kenny
They then took a closer look at the zircon crystals and realised their composition was identical to older examples found elsewhere, meaning it’s possible they were deposited on impact. “What we found was quite surprising,” said Kenny, whose paper appears in Geology.
“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.”
Scanning electron microscope image of a zircon crystal which was hit by the impact at Sudbury – the parallel lines running from the top left to bottom right of the crystal are known as planar fractures and were produced by the force of the impact, via Gavin Kenny
Main asteroid image via Shutterstock
