John Carlos Baez

Everyone's talking about the new rocks! You know, the new *really old* rocks.

The hellishly hot span of time called the Hadean Eon began with the Earth's formation about 4.6 billion years ago, and ended shortly after the invention of rocks. 🔥 Then came the Late Heavy Bombardment, 4.1 billion years ago, when a lot of meteors and asteroids hit the inner planets like Earth. ☄️ Then came the Archean Eon, when the Earth was almost completely covered with deep oceans. 🌊

The Nuvvuagittuq Greenstone Belt in Canada seems to be the only bit of the Earth's crust dating back to the Hadean. The picture shows part of this belt.

A new study of isotopes of samarium and neodymium in some rocks from there claims they're about 4.16 billion years old! Here's a pretty good explanation:

https://www.sciencenews.org/article/earths-oldest-rocks-4-16-billion-years

The Nuvvuagittuq Greenstone Belt is on the eastern shore of Hudson Bay. The closest town is Inukjuak, 40 kilometers away, not accessible from the rest of the world by road. This is much cooler than if the only known fragments of Hadean crust were in Trenton, New Jersey.

Aug 1 at 4:48pmWeb
Show threadSunflower Björnskalle 🌻23h ago

@johncarlosbaez I have to go there.

Tell me something, maybe you know: Can the Earth ever be almost completely covered with deep oceans again, like if all the glaciers melt or would the continents have to sink as well?

Show threadJohn Carlos Baez22h ago

@apodoxus - The continents would have to sink. If the entire Antarctic melted, sea levels would rise by around 60 meters (180 feet). The other glaciers and ice caps would contribute a bit, but not enough to cover all the land, since after all there are lots of mountains over 4000 meters above sea level.

By the time the Earth's mantle cools, tectonic activity will cease, mountains will stop being formed, and the Earth will flatten out. But the oceans may evaporate before that, thanks to the ever-increasing heat from the Sun. 3.5 billion years from now the Sun should become 40% brighter than today, and the oceans will boil. I don't honestly know when tectonic activity will cease before this.

Show threadSunflower Björnskalle 🌻22h ago

@johncarlosbaez Indeed, I've been watching a lot of Alpine climbing lately. That is disappointing to hear, but I had intuited as much.

Thank you.

Show threadDavid Zaslavsky20h ago
@johncarlosbaez "shortly after the invention of rocks" is the sentence fragment I didn't know I needed in my life 👍
Show threadJohn Carlos Baez20h ago
@diazona - thanks! I've been waiting with gritted teeth for someone without a sense of humor to tell me rocks weren't "invented". This is much better.
Show threadDavid Zaslavsky16h ago

@johncarlosbaez Oh dear 😅 good point, I forgot this was the internet

Well, I suppose it's fortunate that jokes about the invention of things that were clearly not invented are among my favorites!

Show threadDavid Zaslavsky15h ago
@johncarlosbaez Plus I think "the invention of rocks" is a nice poetic way to describe the formation of a solid crust for the first time
Show threadBashStKid19h ago
@johncarlosbaez Geochemistry has improved an incredible amount over the past few decades. It truly has become the magic eye to the past.
Show threadProf Lutz19h ago
@BashStKid @johncarlosbaez  There are a lot of places in Nord du Québec not accessible from the rest of the world by road 🙂
Show threadLevel 98 🇺🇦18h ago

@johncarlosbaez The Canadians do OK, but if you want REALLY old you need to come to Australia!

4.4 GYr, apparently.

https://www.nature.com/articles/ngeo2075

Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography - Nature Geoscience

The oldest minerals on Earth are thought to have formed in the Hadean eon, but the reliability of the dates has been questioned. Atom-probe tomography of an ancient zircon confirms that the mineral formed about 4.4 billion years ago, implying that any mixing event of the silicate Earth occurred before that time.

Nature
Show threadDoug Baker - Elbows Up!16h ago
@johncarlosbaez Listened to a BBC Crowd Science podcast this week; explained that the water on earth (there's more UNDER the crust than on the surface, btw) and how it came here via comets. Fascinating.