In northwestern Australia, there's an administrative region called Pilbara.
Even though on a map, it looks kind of long and narrow, it's big. The area of Pilbara is just shy of that of California and Nevada put together. (I suspect that I'm like many non-Australians in consistently forgetting just how big Australia is. It's the sixth largest country in the world, and is almost the same size as the continental United States. Flying from Sydney to Perth is comparable to flying from Atlanta to Los Angeles.)
Pilbara is also extremely hot and dry, and very sparsely populated, with only a bit over sixty thousand residents total, most of whom live in the western third of the region. The northeastern quadrant is part of the aptly-named Great Sandy Desert, one of the most inhospitable places on Earth. There are only a few Indigenous tribes that somehow eke out a living there, most notably the Martu, but by and large it's uninhabited.
What brings up the topic, though, is that Pilbara is interesting for another reason than its hostile climate.
It is the home to some of the oldest rocks on Earth.
The Pilbara Craton -- a craton is a contiguous piece of continental crust -- is estimated to be around three and a half billion years old. For reference, the Earth's crust only solidified 4.4 billion years ago. Since that time, plate tectonics took over, and as I've described before, tectonic processes excel at recycling crust. At collisional margins such as trenches and convergent zones, usually one piece slides under the other and is melted as it sinks. Even in places where two thick, cold continental plates run into each other -- examples are the Alps and the Himalayas -- the rocks are deformed, buried, or eroded.
The result is we have very few really old rocks left. The only ones even on the same time scale as Pilbara are the Barberton Greenstone Belt of South Africa and the Canadian Shield (and even the latter has been heavily metamorphosed since its formation).
This makes Pilbara a great place to research if you're interested in the conditions of the Precambrian Earth -- as long as you can tolerate lots of sand, temperatures that often exceed 36 C, and a fun kind of grass called Triodia that has leaf margins made of silica.
Better known as glass.
Frolicking in a field of Triodia is like running through a meadow made of Exacto knives.
Be that as it may, geologists and paleontologists have begun a thorough study of this fascinating if forbidding chunk of rock. The most recent reconstructions suggest that both Pilbara and the aforementioned Barberton Greenstone were once part of an equatorial supercontinent called Vaalbara (which preceded the supercontinent most people think of -- Pangaea -- by a good three billion years). And those might be the only chunks of that enormous piece of land left intact.
There are two other reasons Pilbara is remarkable.
It contains numerous fossilized stromatolites, which are layered sedimentary structures formed by cyanobacteria, thought to be the earliest photosynthetic life forms. There are still stromatolites forming today -- probably not coincidentally, in shallow bays in Western Australia.
As such, the Pilbara stromatolite fossils are the oldest certain traces of life on Earth, dating to 3.48 billion years ago.
The other reason is that it's also home to a massive impact crater dating to 3.47 billion years ago. Shortly after those earliest, tentative life forms were living and thriving in the warm shallow ocean waters, a huge meteorite struck near what is now the town of Marble Bar, forming a crater and shatter cone between 16 and 45 kilometers in diameter (because of erosion, it's hard even for the geologists to determine where its edges lie). The resulting Miralga Impact Structure blew tremendous amounts of molten debris up into the air, and some of it landed on that chunk of Vaalbara that would eventually end up in South Africa -- only to be recovered by geologists almost three and a half billion years later.
So there's a place in Australia that gives new meaning to the phrase "old as the hills." Given its remoteness and inhospitable climate, I'm unlikely ever to visit there, but there's something appealing about the idea. Walking on rock that is an intact remnant of a continent from over three billion years ago is kind of awe-inspiring. Even if all the other rock is still here somewhere -- melted and reformed and eroded multiple times -- the idea that this chunk of the Earth has somehow lasted that long more or less intact is mighty impressive.
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