Yarrabubba
Key facts
- Country
- Australia
- Diameter
- 70 km
- Age
- about 2.23 billion years
- Location
- 27.17° S, 118.83° E
Very little: an area of low granite hills and red plains on a sheep station in Western Australia, where shocked rocks and a body of melted rock are the only surviving traces.
Yarrabubba, in the Murchison region of Western Australia, holds the record as the oldest known impact structure on Earth. It is so ancient and so worn down that there is almost nothing to see, but careful study of tiny crystals in its rocks has given it a remarkably precise age.
The impact. About 2.23 billion years ago an asteroid struck the ancient crust of what is now Western Australia. The crater it formed is thought to have been about 70 km across. Over more than two billion years, erosion has removed the crater entirely, leaving only the deep rocks that lay beneath it. These rocks contain shatter cones, shocked minerals and a body of rock that was melted by the impact, which is how geologists recognised the structure.
Recognition. Yarrabubba was identified as an impact structure in the early 2000s, after geologists found shatter cones and other shock features in the granite of the area. Its age, however, remained uncertain for many years.
A precise age. In 2020 a team of scientists studied tiny crystals of the minerals zircon and monazite in the impact rocks. These crystals contain uranium, which decays into lead at a known rate, so they act as natural clocks. The impact reset these clocks, and by measuring the uranium and lead the scientists found that the impact happened about 2.229 billion years ago. That made Yarrabubba about 200 million years older than Vredefort, previously the oldest known impact structure.
An icy impact? The age of Yarrabubba is intriguing because it falls near the end of a time when much of the Earth may have been covered by ice. Some of the scientists suggested that the impact might have struck an ice sheet, releasing large amounts of water vapour, a greenhouse gas, into the atmosphere. This might have helped warm the climate. The idea is interesting but unproven, and other scientists have questioned it.
Why it matters. Yarrabubba shows that very old impact structures can still be recognised and dated, even when almost nothing remains at the surface. The Earth must have been struck by many large objects in its early history, but plate tectonics, erosion and burial have erased most of them. Finding and dating the few that survive helps scientists understand the history of impacts on Earth and how they may have affected the planet's climate and environment.
The site today. Yarrabubba lies on a pastoral station in a remote part of Western Australia. The impact rocks are subtle and not easily recognised, and access requires permission from the landholder. For most people, its importance lies not in what can be seen but in what its rocks have revealed.
Did you know?
Yarrabubba is the oldest known impact structure on Earth.
Its age was measured from tiny crystals of zircon and monazite.
It is about 200 million years older than Vredefort.