Comments on: Peering down onto an ancient Australian impact

By: Lonar crater summer getaway | Go Trek

Lonar crater summer getaway | Go Trek — Sun, 25 Mar 2012 01:10:22 +0000

[...] Peering down onto an ancient Australian impact (blogs.discovermagazine.com) [...]

By: Matt B.

Matt B. — Fri, 09 Mar 2012 19:21:55 +0000

“continental drift wiping out really old craters”

It would be really amazing to find a crater half gone at the edge of a subduction zone.

By: Joseph G

Joseph G — Thu, 12 Jan 2012 22:29:41 +0000

@24 MTU: Yep. We’re also on a very old and eroded continent which is mostly desert or semi-arid so I’d be surprised if there weren’t a lot that have since been eroded away entirely or buried and lost.
I recall seeing some kind of documentary on the geological history of the Earth, and the geologist traveled to Australia, saying that the oldest exposed rocks on Earth were there. Something like 4 billion years old!

By: Joseph G

Joseph G — Thu, 12 Jan 2012 22:26:57 +0000

@23 Wzrd1: @20, Joseph G, actually, tank penetrators VAPORIZE, some to the point of becoming plasma,
Weird. I wouldn’t have thought there was enough energy to vaporize something that size (especially made of tungsten or DU)!
But there ya go. Things aren’t really intuitive at these kinds of nutty energies

By: Wzrd1

Wzrd1 — Thu, 12 Jan 2012 18:50:00 +0000

Lugosi has a point, ocean bottom remodeling from tectonic activity does tend to erase any impact craters under the ocean. And most impactors that WOULD reach the ocean would tend to not create a crater, due to size.
Meanwhile, debate, rather than sampling and study surrounds most oceanic craters, such as Silverpit, Shiva and a few others. After all, drilling for oil is far more important than silly endeavors like science, right? :/

By: Lugosi

Lugosi — Thu, 12 Jan 2012 16:52:43 +0000

Lost in this discussion is the fact that the Earth’s surface is 3/4 water. There are no doubt many more impact craters under our oceans’ surfaces (though a crater probably wouldn’t survive very long under water).

By: Peter B

Peter B — Thu, 12 Jan 2012 15:43:26 +0000

Alex Hall @ #30 said: “I simply can’t imagine a 6 mile wide meteor being completely vaporised at impact… I know scale is a massive thing, but if part of it is still poking out of the atmosphere surely there’s not enough energy to completely destroy the whole thing?”

What do you mean by saying a 6 mile rock would be still poking out of the atmosphere at impact? The Earth’s atmosphere is much deeper than that. Six miles is only a little higher than the top of Mount Everest – at that altitude there’s still just enough oxygen for people to stay alive.

“Or is my imagination just not big enough yet?”

This might be the problem.

The important thing to keep in mind is speed. These rocks hit the Earth at speeds something like ten times faster than bullets, and possibly more. That’s the difference between you walking down a path and a car driving down a suburban street. And remember energy is proportional to the square of the speed. Something that big travelling that fast is going to release a lot of energy when it hits.

By: Peter B

Peter B — Thu, 12 Jan 2012 15:36:53 +0000

Scott P @ #28 asked: “If you end up with a bunch of liquid rock at high temperature, why doesn’t it spread out until you get a big flat puddle of molten rock?”

It doesn’t spread out to form a puddle because the strength of the shock wave caused by the impact is sufficient to blast material out for tens, hundreds or thousands of kilometres, or even into orbit. It’s the difference between pouring water out of a plastic bottle onto the ground, and dropping the full bottle onto the ground from the top of a building.

“Why do you get a ring range?”

The raised rim is caused by compression of the rock surrounding the impact site. The rock is close enough to the impact site to be slightly softened by the heat of the impact and compressed by the shock wave generated by the impact. The rock is shoved outwards, but before it can settle back it cools sufficiently to solidify again.

“I would think that much liquid rock must take years, if not decades, to cool enough to solidify.”

Why should liquid rock take years to solidify? Lava is liquid rock and it generally cools solid in a few days at most.

Can I recommend you read the Wikipedia article on Impact Craters? It answers your questions better than I can. But in summary, I think you’re missing the sheer amount of energy released by these sorts of impacts. They’re incredibly fast and incredibly violent.

By: Nigel Depledge

Nigel Depledge — Thu, 12 Jan 2012 12:39:58 +0000

@ Alex Hall (29) -
Supersonic shock waves cause a lot of heating.

By: Nigel Depledge

Nigel Depledge — Thu, 12 Jan 2012 12:31:00 +0000

@ Scott P (28) -
Well, I don’t know for sure, but I can take a guess. Or guesses:

1) It probably does not take much time for most of the liquified rock to cool down enough to become very viscous. Some types of lava eruptions are pretty viscous and gloopy even at their hottest.
2) You’re sure to have a gradient of temperature from the point of impact outwards, so at some point it’ll be hot enough to be liquid but not hot enough ever to get runny.
3) Rock is a fluid anyway – at least, it behaves thusly when considered over long time periods (the only way to model plate tectonics is to treat the Earth’s crust as a viscous liquid). It may not ever need to melt to transmit a large transverse wave.