The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
We have discovered the oldest meteorite impact crater on Earth, in the very heart of the Pilbara region of Western Australia. The crater formed more than 3.5 billion years ago, making it the oldest known by more than a billion years. Our discovery is published today in Nature Communications.
Curiously enough, the crater was exactly where we had hoped it would be, and its discovery supports a theory about the birth of Earth’s first continents.
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The very first rocks
The oldest rocks on Earth formed more than 3 billion years ago, and are found in the cores of most modern continents. However, geologists still cannot agree how or why they formed.
Nonetheless, there is agreement that these early continents were critical for many chemical and biological processes on Earth.
Many geologists think these ancient rocks formed above hot plumes that rose from above Earth’s molten metallic core, rather like wax in a lava lamp. Others maintain they formed by plate tectonic processes similar to modern Earth, where rocks collide and push each other over and under.
Although these two scenarios are very different, both are driven by the loss of heat from within the interior of our planet.
We think rather differently.
A few years ago, we published a paper suggesting that the energy required to make continents in the Pilbara came from outside Earth, in the form of one or more collisions with meteorites many kilometres in diameter.
As the impacts blasted up enormous volumes of material and melted the rocks around them, the mantle below produced thick “blobs” of volcanic material that evolved into continental crust.
Our evidence then lay in the chemical composition of tiny crystals of the mineral zircon, about the size of sand grains. But to persuade other geologists, we needed more convincing evidence, preferably something people could see without needing a microscope.
So, in May 2021, we began the long drive north from Perth for two weeks of fieldwork in the Pilbara, where we would meet up with our partners from the Geological Survey of Western Australia (GSWA) to hunt for the crater. But where to start?
On the hunt for shatter cones in a typical Pilbara landscape with our trusted GSWA vehicles.
Chris Kirkland, Curtin University
A serendipitous beginning
Our first target was an unusual layer of rocks known as the Antarctic Creek Member, which crops out on the flanks of a dome some 20 kilometres in diameter. The Antarctic Creek Member is only 20 metres or so in thickness, and mostly comprises sedimentary rocks that are sandwiched between several kilometres of dark, basaltic lava.
However, it also contains spherules– droplets formed from molten rock thrown up during an impact. But these drops could have travelled across the globe from a giant impact anywhere on Earth, most likely from a crater that has now been destroyed.
After consulting the GSWA maps and aerial photography, we located an area in the centre of the Pilbara along a dusty track to begin our search. We parked the offroad vehicles and headed our separate ways across the outcrops, more in hope than expectation, agreeing to meet an hour later to discuss what we’d found and grab a bite to eat.
On the hunt for shatter cones in a typical Pilbara landscape with our trusted GSWA vehicles.
Chris Kirkland, Curtin University
A serendipitous beginning
Our first target was an unusual layer of rocks known as the Antarctic Creek Member, which crops out on the flanks of a dome some 20 kilometres in diameter. The Antarctic Creek Member is only 20 metres or so in thickness, and mostly comprises sedimentary rocks that are sandwiched between several kilometres of dark, basaltic lava.
However, it also contains spherules– droplets formed from molten rock thrown up during an impact. But these drops could have travelled across the globe from a giant impact anywhere on Earth, most likely from a crater that has now been destroyed.
After consulting the GSWA maps and aerial photography, we located an area in the centre of the Pilbara along a dusty track to begin our search. We parked the offroad vehicles and headed our separate ways across the outcrops, more in hope than expectation, agreeing to meet an hour later to discuss what we’d found and grab a bite to eat.
