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SYFY WIRE Bad Astronomy


By Phil Plait

Uluru is a gigantic sandstone monolith in the Australian Outback. And I do mean gigantic; it’s 3 kilometers (10,000 feet) across its widest point, and juts up an imposing 350 meters into the sky.

Well, imposing from the ground. From space it looks decidedly different.

The image above is from Proba-1, a European Space Agency satellite designed as a testbed for new tech. Proba-1 is less than a meter on a side and when it launched in 2001 had a lot of cutting-edge tech on it (paving the way for the later generation Proba-V sat). It has a high-resolution camera onboard capable of seeing objects just 10 meters across, which is pretty good for a camera 600 kilometers above the Earth.

Uluru is amazing. Australia’s northwest desert used to be a shallow sea. The seabed was made of iron-rich sand, and Uluru was part of a huge alluvial fan, sediment deposited from streams or rivers that spread out into a fan shape. This built up many layers of sand, deposited over the seasons. Eventually the sea dried up, leaving behind all those layers, now compressed into rock, and colored red as the iron oxidized, forming rust.

About 400 million years ago, mountain ranges in that area were thrust up due to tectonic forces. This uplifted the layers of sandstone, tilting them from horizontal to very nearly vertical. Most of those uplifted blocks have eroded away, but Uluru still remains. The scoring, the lines you see across the top are actually the layers of sandstone deposited by those prehistoric rivers! The layers have different strengths, and erode at different rates, causing the striations. If the layers hadn’t been tilted upward, we’d only see one layer, the top one.

Geology is weird.

But miserly—it uses the same processes all over the world. As I read about the geologic history of Uluru I had to laugh: It’s very similar to what happened outside my window! The western part of the U.S. was also once a shallow sea with iron-rich sand (the remains of the ancestral Rockies, mountains that built up and eroded away long, long ago). The sea dried up, the current Rockies pushed their way skyward, and uplift exposed the layered sandstone. The city of Boulder’s west end is graced by the Flatirons, immense and iconic sheets of reddish rock cracked and pushed to dizzying angles by the powerful forces under the Earth’s surface.

You can see examples of this uplift all over town; I took this picture while biking north of Boulder:

You can see the layers of rock, pushed up at an angle. Keep tilting until they’re vertical, and multiply the size by a gajillion, and you get Uluru.

The Proba-1 image is grayscale, but other images from space show its color. I’ve been to Australia twice, but never to this area. It’s a long way from anywhere, but if I go back, I may have to put this on the agenda. To see such a thing from up close must be truly magnificent.

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