Can a cosmic impact event break the Earth's surface in such a way so it expose liquid magma over large areas with solid rocks floating in it as "icebergs"?

I wonder, will such magma ocean experience tides and storms so that onybody survived on the "icebergs" to be in danger due to their constant move.

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    $\begingroup$ Simple answer is that no one would survive an impact of that magnitude. $\endgroup$ – jamesqf May 18 '15 at 18:22
  • $\begingroup$ @jamesqf if u are in a well-enclosed steel room, why not? $\endgroup$ – Anixx May 19 '15 at 7:23
  • $\begingroup$ For starters, 1) Shock waves from the impact would splatter you against the walls; 2) The force of the impact would strip off most of the atmosphere, leaving you nothing to breathe; 3) Steel's a good conductor of heat, so you're going to get pretty warm - like 700 °C to 1300 °C, per Wikipedia. $\endgroup$ – jamesqf May 19 '15 at 19:07
  • $\begingroup$ You'll be dry-roasted and pureed in the well-enclosed steel room, you may end up a well-cooked meat pie $\endgroup$ – user2872 May 19 '15 at 19:28
  • $\begingroup$ In theory, a strong enough tidal force could generate a similar effect, like a neutron star flying past the earth, missing us by a few million miles or so - the tidal effects could tug a section of crust off the mantle like pulling the skin of a banana and you'd have your mantle/"magma ocean" - of-course, we'd probobly not survive that either and if we did, the earth's orbit would be so changed that we wouldn't survive long. $\endgroup$ – userLTK May 21 '15 at 6:09

Let me first correct a small misconception. Where you are talking about 'the magma ocean', you are implying that one exists. This is in fact false. There is no 'magma ocean' in the Earth at the moment (and it has been like that for several billions years). The lavas you see erupting in volcanoes are not coming from a magma ocean. They are coming from either the crust or the mantle, both of which are solid. Magma generation is a localised anomaly in what would otherwise be a solid mass.

That said, the upper mantle has a section called 'the asthenosphere' which is slightly more ductile than the rest. Think about chocolate in a hot day. Still solid - but it can flow given enough pressure. The modern lithospheric plates (of plate tectonics fame) sort of float on the asthenosphere already now, and that's why you have plate movement and earthquakes and everything.

The asthenosphere is quite deep - some 100 to 200 km deep below the surface. If a cosmic impact is going to create a crater 100 km deep in the Earth, no one will be left around to care about whether they will survive on the 'iceberg'-like lithospheric plates. Again - this is irrelevant. The asthenosphere is ductile in timescales of hunderds of years (at least). If you were to pick up a piece of it now, it will just be like any other rock (albeit quite warm).

A magma ocean event probably occurred very early in the Earth's history (which is not going to happen again since Earth is already here), and the moon forming event also likely liquified quite a lot of the Earth. But this is not going to happen since there is no other big enough body that seems like it's going to collide with us at the moment.

  • $\begingroup$ But wouldn't an impact on the scale posited by the OP create a magma ocean? $\endgroup$ – jamesqf May 19 '15 at 19:08
  • $\begingroup$ You would have to melt a significant part of the mantle for a magma ocean. So theoretically, yes. You can do it. It's just that there is no impactor that can do it around us, and no one would be left here to contemplate whether they could float on this magma ocean or not. $\endgroup$ – Gimelist May 19 '15 at 22:42

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