Why are these rocky outcrops red? Have a guess!

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Ann
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Why are these rocky outcrops red? Have a guess!

Post by Ann » Tue May 04, 2021 7:57 pm

Okay, Sherlock Holmes of Starship Asterisk*, why are these mountainous outcrops, Kata Tjuṯa, a group of large, domed rock formations or bornhardts located about 360 km (220 mi) southwest of Alice Springs, in the southern part of the Northern Territory, central Australia, so red?

Hint: The answer is slightly astronomical. This guy may have something to do with it, or at least his name.

I made it easy for you, didn't I? :wink:

Ann
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Chris Peterson
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Re: Why are these rocky outcrops red? Have a guess!

Post by Chris Peterson » Tue May 04, 2021 8:31 pm

Ann wrote:
Tue May 04, 2021 7:57 pm
Okay, Sherlock Holmes of Starship Asterisk*, why are these mountainous outcrops, Kata Tjuṯa, a group of large, domed rock formations or bornhardts located about 360 km (220 mi) southwest of Alice Springs, in the southern part of the Northern Territory, central Australia, so red?

Hint: The answer is slightly astronomical. This guy may have something to do with it, or at least his name.

I made it easy for you, didn't I? :wink:

Ann
Lots of sandstone or other clastic material that originated as granite is red, usually because of quartz crystals that are coated with hematite. Depending on the grain size, hematite can appear either red or gray. Both colors are found in Martian rocks.

Also, these rocks are not nearly as red as they appear in this image, which was taken when the Sun was low on the horizon, thus illuminating it with reddened light. At noon they're more of the pinkish color we usually associate with sandstone and granitic conglomerates.
Chris

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Ann
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Re: Why are these rocky outcrops red? Have a guess!

Post by Ann » Wed May 05, 2021 5:11 am

Chris Peterson wrote:
Tue May 04, 2021 8:31 pm
Ann wrote:
Tue May 04, 2021 7:57 pm
Okay, Sherlock Holmes of Starship Asterisk*, why are these mountainous outcrops, Kata Tjuṯa, a group of large, domed rock formations or bornhardts located about 360 km (220 mi) southwest of Alice Springs, in the southern part of the Northern Territory, central Australia, so red?

Hint: The answer is slightly astronomical. This guy may have something to do with it, or at least his name.

I made it easy for you, didn't I? :wink:

Ann
Lots of sandstone or other clastic material that originated as granite is red, usually because of quartz crystals that are coated with hematite. Depending on the grain size, hematite can appear either red or gray. Both colors are found in Martian rocks.

Also, these rocks are not nearly as red as they appear in this image, which was taken when the Sun was low on the horizon, thus illuminating it with reddened light. At noon they're more of the pinkish color we usually associate with sandstone and granitic conglomerates.
You are certainly right that the rocks were illuminated by reddened sunlight in this image. (And I didn't pick the "very reddest" image of the Google images of these rocks, either.)

However...
Wikipedia wrote:

The actual fresh rock that makes up the Kata Tjuṯa and Uluṟu is medium to dark grey with green or pink hues in some laminae. The bright orange-red hue, for which the structures are noted, is due to a patina over finely divided feldspar coated in iron oxide.
Iron oxide sounds like Mars and the stuff that makes Mars red, doesn't it?

Anyway, thanks a lot for the input on sandstone and other "clastic material" (I'll look it up some day, maybe). :wink:

But I mean it, thanks! :D

Ann
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Chris Peterson
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Re: Why are these rocky outcrops red? Have a guess!

Post by Chris Peterson » Wed May 05, 2021 12:56 pm

Ann wrote:
Wed May 05, 2021 5:11 am

You are certainly right that the rocks were illuminated by reddened sunlight in this image. (And I didn't pick the "very reddest" image of the Google images of these rocks, either.)

However...
Wikipedia wrote:

The actual fresh rock that makes up the Kata Tjuṯa and Uluṟu is medium to dark grey with green or pink hues in some laminae. The bright orange-red hue, for which the structures are noted, is due to a patina over finely divided feldspar coated in iron oxide.
Iron oxide sounds like Mars and the stuff that makes Mars red, doesn't it?

Anyway, thanks a lot for the input on sandstone and other "clastic material" (I'll look it up some day, maybe). :wink:
"Iron oxide" isn't just one thing, it's a family of compounds. The iron oxides that result in red minerals (like hematite) are not typically the same ones that we associate with corroded metallic iron (i.e. "rust").
Chris

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