The oldest Earth rock was not found on the Earth.
It’s difficult to find extremely old rocks because of weather, erosion, eruptions, and the movement of tectonic plates. There are some ancient outcroppings in Canada, Australia, Greenland, Antartica, and a few countries in the south of Africa, and their origin has been dated as far back as around four billion years ago.
Apparently the title of oldest rock is hotly contested, with different research teams and methods pushing the record backwards and forwards. So! The rock I’m writing about today may not be the oldest rock tomorrow – but it is a very curious and interesting rock anyway. This rock, embedded within a football-sized boulder called Big Bertha, is from the Earth. But it was not found on the Earth.
Big Bertha is a breccia – rock that’s an accumulation of different rocks cemented together by geological processes. Big Bertha is notable because it contains significant quantities of zircon, granite, and quartz. These are all very common materials on the Earth, but granite and quartz are also very uncommon materials where the rock was found: on the Moon.
This curious rock was collected by astronaut Alan Shepard in 1971 as he was driving about the lunar surface. When it got back to our planet scientists were puzzled by its composition because it was so unlike other lunar rocks. They concluded that parts of Big Bertha were actually a meteorite. And, what is more, they concluded that this meteorite came from Earth. And when they dated it, they concluded that it came from Earth more than four billion years ago!
So, this lunar rock is in fact extremely well travelled, going from Earth to Moon and back to the Earth, and taking just four billion years to do so. It’s the oldest Earth rock, but it’s still younger than the oldest rock on Earth. That rock is older than the Sun, but that’s a story for another time.