The headline picture for an article on rock around the world seems appropriately like water everywhere.
Boffins blame meteorites for creating Earth's oldest rocks
The oldest rock formations on Earth were born when meteorites pummelled into the ground over four billion years ago, according to a Nature Geoscience paper published on Monday. A team of geologists have analysed samples of felsic rocks known for containing high concentrations of silica near the Acasta River near Great Bear …
COMMENTS
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Tuesday 14th August 2018 11:37 GMT Anonymous Coward
Re: Confused
"how they can be the earliest?"
I think what was meant was that they are (amongst) the earliest rocks for which we have evidence i.e. they still exist - any older rocks have been entirely reprocessed by erosion or complete remelting deep inside the Earth and no longer exist in any identifiable form.
The Acasta Gneisses are not quite reckoned to be the oldest rocks - at ~4.4 Gy old zircons from the Jack Hills in Australia are older, but they are believed to be the oldest exposed rocks.
What bothers me about the hypotheses is that metamorphic rocks are formed under both temperature and pressure, with pressure seeming to play a greater part - Wikipedia says that the temperatures just need to be greater than 150-200C (the original rock doesn't need to be remelted to be transformed to metamorphic rock) but the pressures need to be greater than 100 megapascals (1,000 bar). Now whilst a meteorite impact will create great pressure, it will be in the form of a brief shock wave, which will have more of a brisant shattering effect than a compressing effect, and indeed, it is these shattering effects, such as 'shatter cones' and 'shocked quartz' that are regarded as proof of an impact.
Another problem is that whilst an impact event could certainly produce temperatures high enough to remelt surface rocks it would also mix them all up in that melting but all the pics of Acasta Gneiss that I've seen show some banding, which suggests that they weren't mixed up - the stratification appears to have been preserved.
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Wednesday 15th August 2018 12:39 GMT bombastic bob
Re: Confused
wasn't the entire earth formed by a bunch of dirt, dust, and rocks colliding and collecting together to form a gravity well, and eventually the planet? So what's the point, here?
So yeah EVERYTHING was once 'a meteor'
I'm also not really happy with uranium vs lead isotopic dating. It more or less assumes that the lead content came entirely from the uranium. Unless we have a baseline of the content ratio from the exploding star that probably created it, you can only guess.
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Monday 20th August 2018 14:09 GMT deadlockvictim
Re: Pedant Alert
You and Ms. Quach are wrong.
A team has... is correct.
A team have... is incorrect.
This is primary school [2] English.
A team [1] has analysed samples of felsic rocks known for containing high concentrations of silica near the Acasta River near Great Bear Lake, the largest lake in Canada.
[1] A team of geologists.
[2] I'm not sure what it's called in the States: K12? grade school?
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Wednesday 15th August 2018 21:16 GMT Michael Wojcik
Re: OK, come on folks ...
Less than half of Lake Superior is Canadian territory, so it's the largest lake partly in Canada.
That said, Superior has more than twice the surface area of Great Bear, and I don't know precisely how much is Canadian (didn't find it in a quick search and I'm too lazy to do more). So there may yet be more Superior in Canada than there is Great Bear.
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