In the Pilbara of Western Australia, some of Earth’s oldest rocks lie beneath the sky, as they have for billions of years. They are dark, weathered volcanic rocks, close to 3.5 billion years old, cut by veins and stewed by deep time.
Their survival is remarkable. Most rocks this old have moved back into Earth’s interior. These ones, still on the surface, have changed, but not enough to erase their first story.
In places, they still preserve the rounded forms of pillow basalts — lava that erupted underwater and cooled on an ancient sea floor.
The same rock record also holds some of the earliest widely accepted evidence for life on Earth. But looking closely at some surfaces, you find fine lines that fan through the rock.
These are shatter cones — the frozen signature of a meteorite shock wave, and the clearest sign that something from space once struck Earth.
When our team first reported these rocks in 2025, we suggested they were part of an ancient impact crater at the ironically named North Pole Dome. But one question remained difficult: exactly how old was the impact?
In our new study, published in Geology, we used tiny mineral clocks inside the damaged rocks to show that the impact most likely happened 3.024 billion years ago.
That makes North Pole Dome the oldest known impact structure on Earth, and the only recognized impact crater from the Archean, the period between 4 and 2.5 billion years ago.
The gift of deep time
This is a story about a scar on the early Earth. It is also about one of geology’s greatest gifts to society: the concept of deep time.
Humans have been around for some 300,000 years. But Earth is about 4.5 billion years old. Most of our planet’s story happened on timescales so vast, they’re difficult to imagine.
Rocks are the pages of that story. Some begin as lava flows, others as mud on a sea floor. Over time, Earth’s movements bury, harden, fold, heat, and sometimes lift the rocks back to the surface. A geologist’s job is to work out the order of these pages and, where possible, put dates on them.
One way to do this is stratigraphy, the study of rock layers. If two lava flows lie on top of one another, the lower one is usually older. If a vein cuts through a rock, the vein must be younger than the rock.
But ancient rocks are rarely tidy. Over billions of years, layers can tilt, fold, and erode. Geologists, therefore, use correlation. We match rocks from one place to another using their position, appearance, chemistry, magnetic signals, or nearby layers with a precise date.
Correlation is powerful, but it’s a bit like working out where a loose page belongs in a damaged book. You may know whether it comes near the start, middle, or end, but the page number itself is missing. That was the challenge at North Pole Dome; the signs of a meteorite impact were clear. But when did it happen?
Piecing the story together
Early estimates suggested an extremely ancient impact, based on where the shocked rocks sat in the local rock layers. A later Harvard-led study challenged this, arguing that the impact could have happened much later, anywhere between 2.7 and 0.4 billion years ago, a span equal to roughly half of Earth’s history.
Both interpretations depended on matching ancient rocks across a complicated landscape. In the Pilbara, that is difficult work. Linking one fine-grained black rock to another across the outback can be surprisingly hard.
So instead, we looked inside the rocks. Tiny crystals inside shocked rocks can act as clocks, recording when they formed or changed. In other words, mineral dating can sometimes recover the missing page number.
Tiny crystal clocks
The key mineral was zircon. Zircon is tiny, tough, and unusually good at keeping time. It contains uranium, which slowly decays into lead. By measuring uranium and lead in a zircon crystal, we can estimate when that crystal formed, or when something strongly altered it.
In one shatter cone, we found several types of zircon. Some preserved ages older than 3.4 billion years. These likely reflect the ancient rocks that were hit.
But another group looked very different. These zircons had skeletal shapes, like tiny frozen lightning bolts. These can form when crystals grow or recrystallize very quickly under unusual conditions. Similar zircon textures have been found in impact rocks from the Moon. The best-preserved of these skeletal zircons gave an age of 3 billion years.
On its own, that still wasn’t enough. Skeletal zircon can form in more than one way, so we needed another clock. We found it in apatite, a phosphate mineral that also contains tiny amounts of uranium.
Apatite can grow when hot fluids move through broken rock — exactly the kind of system an impact creates, as heat and fractures drive water through a crater. The apatite gave the same age as the modified zircons.
Two clocks, in different minerals and different rocks, pointed to the same event about 3.02 billion years ago.
A rare moment from Earth’s violent youth
Other minerals told us what happened later. Muscovite, a shiny silver mineral in a vein that cut across the shatter cone, gave an age of about 1.66 billion years. The vein’s shape told us it must have formed long after the impact, when the rocks were disturbed again by some natural process.
But those events don’t date the impact — they are later chapters in the same damaged book.
The story of dating this crater shows Earth’s oldest history is not gone. It’s just hard to read. Unlike the Moon, Earth constantly destroys its ancient surface through erosion, burial, heating, and plate tectonics.
Most craters from the early Earth have vanished. At North Pole Dome, one survived. Its rocks preserve the trace of a space impact from 3.024 billion years ago — a rare page from the violent youth of our planet, with the date still written in the stone.
Chris Kirkland, Professor of Geochronology, Curtin University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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