If the Australian continental mass was hit at high speed by an Australian-continental-mass-sized meteorite, would the continental mass bob down in to the Earth’s mantle then bob back up again? Would it bob back up again with an almost equal amount of energy that forced it to bob down to begin with, resulting in the meteorite being relaunched back the way it came, “bouncing” off the Earth so to speak?

esselte said:

If the Australian continental mass was hit at high speed by an Australian-continental-mass-sized meteorite, would the continental mass bob down in to the Earth’s mantle then bob back up again? Would it bob back up again with an almost equal amount of energy that forced it to bob down to begin with, resulting in the meteorite being relaunched back the way it came, “bouncing” off the Earth so to speak?

No

How about if the two masses hit each other at a very low speed?

We’re talking about a crust 40 km or so thick in contact with an body 3000 km across, basically the size of Mars or Mercury.. The gravitation alone will shred the lithosphere regardless of how gentle the tap is.

dv said:

The gravitation alone will shred the lithosphere regardless of how gentle the tap is.

Yeah, I’m assuming so. I’m talking about the continental crust piercing the lithosphere down in to the mantle.

esselte said:

dv said:

The gravitation alone will shred the lithosphere regardless of how gentle the tap is.

Yeah, I’m assuming so. I’m talking about the continental crust piercing the lithosphere down in to the mantle.

Right. It will crumble. There will be no bounce.

dv said:

Right. It will crumble.

I’m trying to picture what that would look like (ignoring the apocalyptic destruction), a more familiar analogue…

Would it maybe look like launching a large piece of very rotten cork at another large piece of very rotten cork which is floating in water?

esselte said:

dv said:

Right. It will crumble.

I’m trying to picture what that would look like (ignoring the apocalyptic destruction), a more familiar analogue…

Would it maybe look like launching a large piece of very rotten cork at another large piece of very rotten cork which is floating in water?

You can’t really compare it to what you’ll see when you try it with small objects, because structural strength increases as the square of the scale, but mass and gravity increase as the cube of the scale.

OK, thanks for the answers dv.

dv said:

esselte said:

dv said:

Right. It will crumble.

I’m trying to picture what that would look like (ignoring the apocalyptic destruction), a more familiar analogue…

Would it maybe look like launching a large piece of very rotten cork at another large piece of very rotten cork which is floating in water?

You can’t really compare it to what you’ll see when you try it with small objects, because structural strength increases as the square of the scale, but mass and gravity increase as the cube of the scale.

I mean if you mock up an exact replica of the earth and the asteroid, make it of the same materials (somehow), but at like 10000000th the scale so it is about a metre across, then yeah, it will bounce.

Try it with the real thing and it will just crumble/melt the crust and spray the mantle.

OK. How about, if we levelled an area of the Australian land mass say 2,000km by 2,000km; and a similair area on the opposite side of the Earth. Then we get a very large adjustable clamp and fit the 2,000×2,000km sized tines (?? I dunno what the clampy parts of a clamp are called) to these levelled areas. Then we start to tighten the clamp. Ignoring other effects like gravitational if we may. Would Australia and it’s opposite land mass move towards each other, or would they crumble, splinter, crack and start to spread out over the lithosphere without moving appreciably towards each other at all?

The rebound thing comes down to elasticity. Big hunks of molten rock wouldn’t bounce much.

Would it be true to say that continental crusts do not have the structural integrity to be significantly submerged by force in to the mantle? The difference in the densities is too great, so it’s like trying to force a biscuit through a table top?

Too many questions from esselte to answer individually, so I’ll rotate the question about the clamps to re-frame it in a way that has already happened.

Just 18,000 years ago, the blink of an eye in geological terms, the Earth was put into just such a clamp as you describe. We call it the Ice Age. The weight of ice building up over the North and South Poles compressed the crust underneath. The crust near the North and South Poles sank into the mantle – slowly in human terms but very rapidly in geological terms. This ice was three to four kilometres thick, so weighed quite a lot. The oceanic crust is only 5 km thick.

If you want a visual image, think of a ping pong ball in honey. As pressure is applied to the top of the ping pong ball it sinks slowly into the honey.

When the ice disappeared 11,700 years ago, the crust rebounded. There was no shattering or crumbling. As far as I know, there were no significant changes to the shape of the tectonic plates.

As pressure is released from the ping pong ball it rises slowly back to the surface of the honey.

Most of the rebound has happened within that 11,700 years, but not all. A part of Canada is still rising at a rate of 18 mm per year.

esselte said:

Would it be true to say that continental crusts do not have the structural integrity to be significantly submerged by force in to the mantle? The difference in the densities is too great, so it’s like trying to force a biscuit through a table top?

Well parts of the continental crust are submerged into the mantle all the time. This is happening beneath the Himalayas.

dv said:

esselte said:

Would it be true to say that continental crusts do not have the structural integrity to be significantly submerged by force in to the mantle? The difference in the densities is too great, so it’s like trying to force a biscuit through a table top?

Well parts of the continental crust are submerged into the mantle all the time. This is happening beneath the Himalayas.

That’s a good point. Oceanic crust dives intact into the upper mantle to depths of up to 600 km. Occasionally and not very often, though, it does snap.

Continental crust is thicker than oceanic and proportionally stronger and less brittle. It could be submerged by a slow steady force into the mantle without breaking up. It could easily survive submersion at a rate of 4 cm per year. For higher speeds (eg. 1 metre per year, 10 metres per year, 100 metres per year) I’d have to do some calculations to see whether it would break up or not.

Density differences are relatively small, and the crust would become more dense due to the heat and pressure as it submerges. It’s an interesting idea. I don’t know of any continental crust that is being submerged right now.

mollwollfumble said:

dv said:

esselte said:

Would it be true to say that continental crusts do not have the structural integrity to be significantly submerged by force in to the mantle? The difference in the densities is too great, so it’s like trying to force a biscuit through a table top?

Well parts of the continental crust are submerged into the mantle all the time. This is happening beneath the Himalayas.

That’s a good point. Oceanic crust dives intact into the upper mantle to depths of up to 600 km. Occasionally and not very often, though, it does snap.

Continental crust is thicker than oceanic and proportionally stronger and less brittle. It could be submerged by a slow steady force into the mantle without breaking up. It could easily survive submersion at a rate of 4 cm per year. For higher speeds (eg. 1 metre per year, 10 metres per year, 100 metres per year) I’d have to do some calculations to see whether it would break up or not.

Density differences are relatively small, and the crust would become more dense due to the heat and pressure as it submerges. It’s an interesting idea. I don’t know of any continental crust that is being submerged right now.

Obduction: ophiolites, serpentinites, melanges, and blueschists are characteristics.

Oman Ophiolite is well documented and well exposed. (eg:

http://www.files.ethz.ch/structuralgeology/JPB/files/English/Omaneng.pdf

New Caledonia is a recent example of obduction. The serpentinite (and greenstone) belts of eastern Australia are more ancient examples.

Michael V said:

mollwollfumble said:

I don’t know of any continental crust that is being submerged right now.

Obduction: ophiolites, serpentinites, melanges, and blueschists are characteristics.

Oman Ophiolite is well documented and well exposed. (eg:

http://www.files.ethz.ch/structuralgeology/JPB/files/English/Omaneng.pdf

New Caledonia is a recent example of obduction. The serpentinite (and greenstone) belts of eastern Australia are more ancient examples.

Obduction? (Check web)

“Obduction is an adventure video game developed by Cyan Worlds. It is considered a spiritual successor to Myst and Riven. Obduction is a beautiful virtual world that shows the limits of VR”.

“Obduction. A geologic process in which the edge of a tectonic plate consisting of oceanic crust is thrust over the edge of an adjacent plate consisting of continental crust. Compare subduction.”

“Subsequently, this definition has been broadened to mean the emplacement of continental lithosphere by oceanic lithosphere at a convergent plate boundary, such as closing of an ocean or a mountain building episode.”

Is New Caledonia the wrong sort of obduction? ie. Is it a construction of continental lithosphere at a plate boundary rather than subduction of continental lithosphere under oceanic lithosphere?