What magnitude would a quake have to be in order to make the crust sink into the mantle?

Question

The crust is just a thin skin of cooled magma floating atop a roiling, churning sea of molten rock and metal. As such, while it would be highly improbable, I don't see anything that would necessarily make crust sinkage entirely impossible. How bad would the Earth have to quake at a fault line in order for at least some of the crust to sink all the way into the mantle, opening up a gaping chasm of molten "planet guts"? I know the crust does sink via subduction (right word?) but I'm talking about a larger event than merely business as usual.

Answer 1

No tectonic force could ever cause the crust to have some kind of massive earthquake that opens it up to the mantle. First of all, even in the thinnest places along the ocean bottom, it's still 7km thick.



But most importantly, the crust is less dense than the mantle. It floats. Put a piece of Styrofoam in a bucket and figure out a way to make it sink. You can't. It will float. There is nothing that will ever rip it apart that way.



Even the massive Yucatan impact did *NOT* penetrate the crust. The mantle is never exposed, unless you count natural volcanic eruptions (which you shouldn't).

Answer 2

It couldn't. The first problem in the scenario you've set up is that the mantle is not "a roiling, churning sea of molten rock and metal", it is several layers of rock which, while they are under large temperatures, are solid. Plastic deformation is not the same as being a liquid. Secondly the crust as a whole is less dense than the mantle, It can't sink in the way you describe. Subduction (yes it was the right word) is caused by the thickening of a plate to the point at which the net density is greater than the mantle, causing it to sink. It may drag less dense crust with it (with difficulty) but without subduction, "sinking" into the mantle can't really occur. The closest you're going to get to seeing "planet guts" is at a divergent plate boundary, or preferably a divergent triple junction (there's one in Afar in Ethiopia) but even then you're only seeing partial melt from the mantle, not the mantle itself.

Answer 3

The mantle is not a liquid, and is under tremendous pressure as well. The crust is not going to sink into the mantle, that doesn't make sense.

Answer 4

The asteroid that caused the extinction of the non-avian dinosaurs and tore through the earth's crust caused an earthquake that has been estimated as a 13.5.