We’re used to a lot of different natural things falling out of the sky. These can include snow, rain, and sometimes even frogs (yes, really). All of these relate to weather phenomena.
Far more exotic things fall from the sky that are not related to weather. Earth is pelted by about 14 tons of micrometeorites each day. And larger meteorite falls also happen daily, which are visible as fireballs that streak across the night sky.
When an asteroid collides with Earth, it can trigger even stranger debris. Tektites are glassy droplets that form by melting during a meteorite impact, and are then ejected hundreds to thousands of kilometres away from the impact site. The Australasian tektite field that formed some 790,000 years ago from an unknown impact and might cover 10–30% of Earth’s surface is the most famous example.
In a new study published in the journal Meteoritics and Planetary Science, we describe the discovery of a previously unknown 4km-diameter meteorite impact crater in the Eastern Goldfields of Western Australia.
A gold band points the way
The impact site is near the town of Ora Banda (Spanish for “gold band”), a historic gold mining district about 50km north of Kalgoorlie.
For now we’ve named the site the “Ora Banda impact structure”, given its proximity to the historic mining district. However, the region has a much longer history of First Nations culture, and we’re currently working with collaborators at the Goldfields Aboriginal Language Centre on establishing an Indigenous name for the site.
The impact site is interesting for a number of reasons. Ora Banda is one of the few impact craters on Earth whose target rocks – meaning, all rocks in the area affected by the impact – are ancient greenstones, which are metamorphosed volcanic rocks like basalt.
Greenstones are valuable to the economy of Australia because in some places they contain gold. The Ora Banda impact was accidentally discovered during exploration drilling for gold.
The ‘smoking gun’ evidence for impact
If you find a site you suspect might be an impact crater, the scientific process to confirm that’s indeed the case involves documenting what’s known as diagnostic evidence.
Diagnostic impact evidence – the “smoking gun” of an impact – is that which is found nowhere else. It can include either evidence of the space rock itself, or unique high-pressure shock wave damage in the target rocks.
The first evidence for impact we found at Ora Banda was shatter cones – distinctive conical features in rocks that record the passage of the shock wave. We found a few shatter cones in rubbly outcrops at the surface, and we also found some in the drill cores.
Aaron Cavosie
The discovery of shatter cones nailed it – we knew then this spot had to be an ancient impact site.
However, we set out to look for more evidence in order to further support our new impact hypothesis and learn more about the event. So, we went back to the cores.
Unusual rocks
The Ora Banda drill cores contained a range of different rock types. At the top was a sequence of clay-rich sediments – these washed into the crater after it formed. At the bottom were rocks that had a different story to tell: impact breccias.
Breccia is a name for any rock that’s been broken up into smaller fragments and has a matrix of smaller particles that “glue” it all together. Breccias are commonly found at impact craters, because the high-energy shock waves can cause rocks to instantly shatter.
Not surprisingly, there are different types of impact breccias, depending on what they contain.
A breccia is “monomict” if it consists of just one rock type, or “polymict” if it contains pieces of different rocks. Polymict breccias provide strong evidence of mixing, as if the rocks were thrown together in a blender. Both breccia types occur in the Ora Banda cores.
If breccia contains glassy melt particles along with other bits of rock, we call that “suevite”. The glassy bits provide key evidence for an even stranger part of the impact process.
They hint that molten material was thrown up into the sky when the meteorite smashed into Earth. While flying in the air, the molten particles turned to glass before landing back into the newly formed crater, resulting in a layer of suevite breccia.
Aaron Cavosie
But that’s not all. We found two additional types of microscopic “smoking gun” impact evidence in the breccia.
The first was shocked quartz grains, deformed in a way that’s unique to meteorite impacts. The second was meteorite residue in the glass. This happens because the meteorite vaporises and partly dissolves within the glassy melt particles.
With the discovery of shatter cones, shocked quartz, and extra-terrestrial meteorite residue, our hypothesis that the Ora Banda structure is an impact crater was confirmed.
Raining gold?
Glass and shocked minerals wasn’t all we found in the Ora Banda breccias. Some also contained small nuggets of gold.
This means that during the impact event, when all the shocked rock fragments and glass were thrown up into the sky, gold particles were also raining back down onto the surface, into the newly formed breccia deposits. That’s not something typically found in impact craters, and it shows how unique this geologic setting is.
Raiza Quintero
With Ora Banda, and the recently discovered Ilkurlka and Miralga structures, there are now 34 confirmed meteorite impact craters across Australia. They range in age from a few thousand years old, to the 2.2 billion-year-old Yarrabubba structure.
Some, like the iconic Wolfe Creek (Kandimalal) crater, are youthful and well preserved. Most others, including Ora Banda, are older and eroded to the point that a circular crater is no longer visible.
