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What would happen to an earth-like planet where an event caused a large (think, the size of the Caribbean tectonic plate) section of the crust to get peeled away, exposing the upper mantle for a period of time?

Clarification from comments: By "Earthlike" I mean a planet like our own (iron core, surface gravity between 10 mps^2 and 12 mps^2 with an atmospheric pressure of approximately 760 Torr).

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    $\begingroup$ Depends on wether half of the planets oceans would flow into that hole and evaporate, or not. The question is what remotely realistic event could cause four million square kilometres of crust to silently vanish. ;-) Sounds like you'd need the combined evilness and power of all dark wizards in earth's literature. $\endgroup$ – Karl Jun 14 '19 at 20:30
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    $\begingroup$ The question came from a friend and I figured that this was the place to get the most cogent/coherent answers to the question. The fact that Luke suggests the portion of the crust has been "peeled away," means to me that it did not just disappear, but rather was removed intentionally (like Brainiac kidnapping Kandor). $\endgroup$ – Lee Carlson Jun 14 '19 at 20:59
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    $\begingroup$ (Who kidnaps whom? ;-)) $\endgroup$ – Karl Jun 14 '19 at 21:48
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    $\begingroup$ You'll get far more cogent answers if you specify what kind of world you're talking about. "Earthlike" can mean just about anything. What's it made of inside; what kind of universe is this? What sorts of physical laws are in effect? Etc.? $\endgroup$ – elemtilas Jun 15 '19 at 0:41
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    $\begingroup$ @elemtilas An earthlike planet has an iron core, an apparent gravity around $10m/s^2$, and light speed is 300000 km/s. Or so I would guess. ;-.) $\endgroup$ – Karl Jun 15 '19 at 8:06
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Chaos would happen. Here is a chronological order of events:

The size of the earth does not allow for such a hole to exist for very long. Gravity would compact the crust down and push the magma up, filling the hole. The compacting would cause massive earthquakes along every fault line as the tectonic plates compress together. Mountain ranges would suddenly form in areas such as the Mid-Atlantic Ridge, where the tectonic plates moving apart are suddenly mashed back together. The Mariana Trench would become suddenly much deeper as the Pacific Plate is shoved underneath the Mariana Plate. These massive earthquakes would cause massive tsunamis, obliterating everything along the shoreline that hadn't already been obliterated. A significant portion of living things would die (It is hard to speculate what exact percentage). After that, things would cool down for a while.

What happens if the chunk comes back to Earth? The size of the asteroid that is speculated to have caused the extinction of the dinosaurs was between 7 and 50 miles in diameter. This meteor is around 1,500 miles in diameter. When the chunk crashes back into the earth, the energy released would be at least 30,000 times as much as the dinosaur killing asteroid. The Wikipedia article on the Chicxulub meteor states the following:

The impact would have caused a megatsunami over 100 metres (330 ft) tall that would have reached all the way to what are now Texas and Florida. The height of the tsunami was limited by the relatively shallow sea in the area of the impact; in deep ocean it would have been 4.6 kilometres (2.9 mi) tall. A cloud of super-heated dust, ash and steam would have spread from the crater as the impactor burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, broiling the Earth's surface and possibly igniting wildfires; meanwhile, colossal shock waves would have triggered global earthquakes and volcanic eruptions.

Imagine that, but several orders of magnitude larger. I think it is reasonable to say that, "Everyone and everything would die."

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    $\begingroup$ I would think someone powerful enough to rip off a tectonic plate might also be able to put the debris into a stable orbit, or on an escape trajectory. ;-) $\endgroup$ – Karl Jun 15 '19 at 10:13
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    $\begingroup$ But then again, someone evil enough to rip it off in the first place would be evil enough to send it back at a high speed... $\endgroup$ – Bilbo Baggins Jun 15 '19 at 16:32
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    $\begingroup$ Bacteria would be fine. They would get their world back. Or maybe it has always been theirs. $\endgroup$ – Willk Jun 15 '19 at 16:48
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    $\begingroup$ Well, I find the first part of your answer much more interesting that the worn out impact story. +1 $\endgroup$ – Karl Jun 16 '19 at 12:27
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Two possibilities: If no ocean empties itself into that hole (it's in the middle of a continent), you just get a few hundred years or so of decreasingly interesting (=catastrophic) climate, because of the huge heat source. The hole gradually fills itself with magma over time, the surrounding continent sinks, and in the end you get a new ocean. Add a few spectacular new mointain ridges that appear around the hole, pointing towards it, and the crust on the opposite side getting ripped apart at every minor fault line.

If however large parts of your oceans immediately flow into that hole and evaporate within at most a day, your whole planet gets steam-boiled, and that's it. No survivors.

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Theoretically, the oceans of that world would diminish. Sea life would be interrupted drastically, potentially making many animal species go extinct. Due to this loss in sea level, the volume of sea water at the polar ends of the planet would recede, which could potentially cause the ice caps to melt. This would begin to raise the sea level again, but at the cost of yet another biome being altered. Life all around the planet would be affected by this change, and likely would cause most land species to begin to diminish. After a long period of time, the now water filled magma below the crust would begin to harden, forming a new layer of crust. Although most of the worlds life would have perished by now, the planet would begin a reboot. Mind you, this all would take course over thousands and millions of years. Hope this helps.

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