How would Earth turn into debris drifting through space without everything at its surface being destroyed in the process?

Question

Consider planet Earth in a distant future. Human technology has advanced, although it still respects the currently known laws of physics. I am trying to build a story where:

• An Event makes it so the whole planet Earth, or at least a significant portion of its surface, is shattered into debris that go drifting into space. Earth surface is NOT destroyed in the process, so that human constructions - buildings, small stuff, etc - are more or less intact. The debris can be of various sizes, from dust to country-sized debris. It is okay that all living beings on Earth died in the process.
• My protagonists managed, by chance, to escape Earth right before the Event using a technologically advanced spaceship. They are now also drifting into space in the middle of the debris, and they can explore the debris and find various remnants of human civilization that can be used for my plot: that's what I'm getting at, and that's why the surface must not be destroyed during the Event. Their spaceship is smart and manages not to crash into the debris.

The situation I'm describing - my protagonists drifting alongside the debris and exploring them - would need to last at least a few years. What happens after that doesn't matter.

Currently, I've considered the possibility of a "big explosion". An industrial accident of human origin makes Earth's inner core explode, and the whole planet is shattered into debris that go drifting through space in all directions. But it doesn't work because such an explosion would definitely destroy everything at the surface.

Ideally:

• The Event would be of human origin, no alien intervention, no random asteroid crashing into Earth.
• The Event would be rather sudden, happening in a matter of minutes.

What could the Event be?

Answer 1

Not possible within physics as we know it:

I cannot think of a way this can be done within the parameters requested, as any event of sufficient energy to disassemble a whole planet will:

1. Necessarily be of such an energetic nature as to pulverize the surface many,many,many times over. The energy needed to just break the gravitational bonding energy of the Earth, for example, is about 2.49×10^32 Joule. This is enough energy to heat the entire planet by some 94000 Kelvin.

2. Require that every bit on the surface be accelerated away from the main mass of the planet at a speed of something more than 11km/s (some 40000 kilometers per hour), otherwise gravity will simply cause everything to fall back into a new, very hot, planet.

3. Even if you miracle a non-energetic way to break open the planet
   For example by having a rogue black hole do a close flyby of the Earth, well within Earth's Roche limit. But what such a rogue black hole or sufficiently massive planet will do to the solar system as a whole and possibly the Sun is....nasty. We're looking at ejected planets at the minimum, almost certainly gravitationally-induced solar instability leading to a Nova, which destroys the very same surface artifacts you want to retain!
   But then you are still spilling the contents. Earth's crust masses about 2.7x10^22 kg. The whole earth masses 6x10^24 kg. So only about 1/2 of a percent of the Earth is not currently molten.

So by whichever means you are disassembling the Earth, you will get 200 parts molten rock or iron, for every 1 part of solid material.

And of this solid material, less than one part per thousand used to be Surface, or within 20m of the surface. (assuming average crust thickness of 20 km)

But outside of physics as we know it:

To get viable surface installations and artifacts drifting in space, you would need to do something really strange

• First successful test of interstellar Hyperdrive. Footnote: Do NOT activate while on planet surface! Calculate range of effect! Do not use without programming destination!!
• Force field experiment, but the experiment cut the region off from all gravity, causing Earth's rotation and Orbit to cause it to fly off into space?
• Time travel that did the same, neglecting to remember that the Earth rotates, orbits the sun, and even the Sun orbits the Galaxy, all rushing off to somewhere else than where the timetravel zone was when activated?

p.p.s If you want alien visitors to discover Human artifacts... We have scattered satellites and probes and rovers and landers all over the place. We have even sent 2 golden gramophone records in the direction of not-so-nearby stars.

And there is a fully functional Tesla Roadster (red, with spacesuit, low .. ahem. very very high mileage) floating out near Mars orbit.

Answer 2

You're Looking for the Roche Limit

The Roche Limit is a term for how close one celestial body can pass to another one, and still stay together under the force of gravity. If the Earth was a mass of chargeless dust or non-viscous liquid, at this closeness to another larger gravitating body Earth would begin to fall apart.

But Earth isn't a chargeless dust or inviscid fluid. We have a solid crust that lends some mechanical strength and oceans of water that loves to stick to already-wetted surfaces. Earth won't just fall apart. '

But let's see how close it has to be to be in danger. The equation for the Roche limit is :

$RocheLimit = Radius_{HeavierPlanet} ( 2 { density_{HeavierPlanet} \over density_{LighterPlanet} } )^{1 \over 3}$

As you can see, the only time the Roche Limit is outside the larger planet's radius is when the larger planet is made of denser stuff.

That's not hard. The mean density of Earth is 5 g/cm^3. Many things are denser than that: an Earth-sized rogue planet (we assume these are floating around the universe) made of lead (11 g/cm^3) would have a Roch Limit a little further out than the orbit of the moon. An Earth-sized rogue planet made of gold (19 g/cm^3) would have a Roche limit out to almost 19,000 kilometers.

How would it work?

Imagine an Earth-sized ball of lead passes close to the Earth, but not a direct hit. The internal pressures of Earth, usually balanced out by gravity, are now loading the crust. Oceans will try to jump ship, as will the atmosphere. Would be reasonable to say that some poorly anchored structures, or structures on poor foundations (swamp, mud) might just fly up and fly off like so much dust.

Let's see if Earth splits open.

All but the outermost 50 kilometers of Earth is liquid. It's made of basalt (SiO2), which is basically glass (33 MPa ultimate strength). Imagine it as a 50 km thick glass bowl. The rupture strength is (33 MPa x 50 km = 1.65 TPa) (please correct me someone if I've mis-calculated this). This is holding back a peak fluid column 12,000 km high of material 5.5 g/cm^3 in density.

The pressure depends on how close the other planet passes. Let's says it's close enough so there's a net 1g acceleration on Earth during the passage. That would be a column pressure of 0.6 TPa.

Increase the density of Rogue Planet to gold (19 g/cm^3), and have the approach be almost touching during the pass, and the gravity goes up to 4g.

The glass bowl cracks, and a decent portion of Earths fluid spills out like so much quickly-cooling formerly liquid rock into a new ring orbiting Rogue Planet.

What's left, assuming there's enough velocity difference that Rogue Planet sails on it's way without permanently capturing Earth (v > 15,797 m/s) would probably consist of a lot of things as described in the original post.

Answer 3

Earth has developed matter displacement technology (aka beaming in Star Trek). It was designed to move resources from the asteroid belt or moons into earth orbit or directly onto the surface in a matter of seconds.
So they built a giant space station, collecting huge amounts of energy from the sun and what-not and aiming at what was to be displaced.

However, either through a malfunction or by criminal intent, the station displaced the entire molten core of Earth into the solar system, essentially a fatal reversal of the stations functionality.

As it happened within minutes, Earth is now only a shell, fragmenting quickly with the core gone. The Earth shell would probably still be in orbit around the sun and - with safety systems in place - humans could still have survived on the surface, but life would be impossible.

Answer 4

Condition: The event must happen in minutes

Impossible. Really, it's impossible. Consider an earthquake, for which minutes is a very, very long time. It flattens buildings. An earthquake lasting minutes would flatten cities. The destruction of the world in just minutes would flatten literally everything. Worse, it would churn everything into dirt soup. Considering how much mass is involved, your explorer would be lucky to find a brick, much less any other kind of proof of civilization.

But, maybe... if we change the timeline to weeks or months...

An impact won't work, that means no surface explosion of any kind. Too fast, too violent, probably won't leave the Earth in its current orbit, and the energy sufficient to split it up would spread it all over the place. So, no impact.

But... what if the core started to expand? Slowly...

There will be destruction. After all, the mantle is sitting atop a molten soup of fluid and even the most even expansion will cause uneven results. Everything from volcanoes to calderas would erupt. But if it happens slowly enough, it might not be terrestrially devastating.

So, let's consider something along the lines of the movie The Core. In that movie, the theoretical threat of an enemy state weaponizing earthquakes led the U.S. to develop Project Destini (Deep Earth Seismic Trigger INItiative), a tool that would (theoretically) produce focused earthquakes anywhere on the globe.

Except that when they tried it, it stopped the core from spinning. No magnetic field. Birds can no longer navigate. Pacemakers stop. Dogs and cats... living together! Mass hysteria! (Whoops, mixing my movies... sorry.)

What if the nations of your Earth were doing the same thing? Maybe they were simply trying to probe the core and the "probe" exploded, causing the core to expand. The planet's honking big! So such an effect would take time.

And time is what you want (sorry). It takes time to gently rip the Earth apart — but only so much! Because you want those chunks (similar to what was seen in season 5 of Agents of S.H.E.I.L.D..) to gently stay put, both in orbit and generally in locale, such that your explorer can swoop around in between chunks.

Remember, there's a LOT of planet to deal with

A huge part of your problem is that 99.99% of the Earth is everything except the surface. What you have in mind seems like it would be huge — but compared to the rest of the planet it's insignificant. It would actually be really, really hard for your adventurer to find chunks with buildings on them. But that's great! Because you want the process to take years.

"Realistically" the localized asteroid belt once known as the Earth would be a bunch of chunks floating around, hitting one another, and causing a ton of grief. Most of it would be metal, not rock (I think, the outer core might be mostly rock, so maybe it will be mostly rock...). An no matter how gently it was expanded into chunks, some of those chunks would eventually come into contact with the Moon. Now we have the proverbial toilet brush whisking chunks of the Earth around like a whirlpool.

Actually, now that I think about it, the effect would be pretty cool. The Moon would act somewhat like the guardian satellites that help shepherd the rings of Saturn. The math might not work out... but the concept of a moon-shepherded whirlpool in space is cool beyond reason.

On the other hand, what you could also end up with is the surface (mantle) of the Earth broken up and orbiting the swollen-and-now-cooling core. A new, mostly metal (maybe) planet having almost exactly the original mass but now surrounded by a ring system that happens to be made up of fairly large chunks. Astronomically, they'd eventually plunge back to the planet's surface. But that could take a while. A long while.

But, in the end, you'd have the effect you want... so long as you don't want atmosphere

So, we slowly expand the core like a swelling appendicitis until the either the surface breaks up and floats away or the entire planet breaks up. It's slow, because that's the only hope any building has of surviving.

The result is either a new planet with a ring system containing the chunks you're looking for or a localized asteroid field made up of the mass of the planet that eventually becomes this whomping cool whirlpool shepherded by Luna, the Guardian Moon.

What you do NOT have is atmosphere. Anywhere. There's no way to keep it around the chunks in any circumstance. The buildings and desiccated human remains are in a complete vacuum and might not be protected any longer even by the Van Allen Radiation Belt. (A curious thought, that, but I'll leave it to another to analyze.)

Answer 5

It is imposible due to how planets (and astroids) get their's form at fist place.

If we somehow magicaly remove half of the Earth, the remaining half would immediately start to turn into a sphere (80% of former diameter), greatly heating in the process. This would certainly destroy all the surface.

Same goes for all large parts (> 1km) - they would collapse in sort-of-spheroids (the larger - the more spherical it would be, the more heat would be produced in process). Parts having mass of about 2/3 of a Moon (say continents) would always become glowing spheres of magma.

Smaller parts that would not be turned into piles of stones (from several hunderd to hundred meters across) would just have not enough gravity to keep themselfs intact. Tidal forces of Sun's gravity and gravity from larger parts would just rip (or better say - put) them apart into individual stones/bricks/cars/etc.

So it doesn't matter what Event created this debris - they just woudn't keep "surface" intact while drifitng through space.

P.S. You may have a barn or a two-three-four floor peice of a reinforced concrete building or section of a bridge floating in space (i.e one single object) , but no surface - no lawns, roads or whole ranch (nothing made of bulk materials)

P.P.S. And it would be much better if you are not telling anyting about The Event. Let it be mistery even for your hero.

Answer 6

Known physics: Impossible.

Unknown physics:

First, you can rule out any possible explosion or accident. Even a tiny percent of the energy involved will destroy the surface, nothing indiscriminate could possibly be so selective.

Second, some answers have suggested somehow turning off gravity. That's going to fling off pieces from equatorial regions but they aren't going to snap cleanly. Your surface gets destroyed by earthquakes.

We need some way of cleanly removing chunks and that's only going to happen by deliberate action.

Enter Acme Planet Mining Company. They take useless worlds apart for raw materials. Shattered bits are hard to handle so they've learned to do it cleanly: The planet is surrounded by projectors that generate a very intense gravitational field. They're all turned on at once and the planet gets pulled apart into 3D pieces of pie. Once the pieces have been pulled far enough a second set of projectors slices up the pieces of pie.

The fields are powerful enough that the rock simply shears that the boundaries and the boundaries are straight so there's no rubbing. As the acceleration is gravitational the material being lifted isn't subjected to high forces, it comes up basically intact. (The projectors are highly focused and thus the normal inverse square doesn't meaningfully apply.)

The end result is squares (or perhaps hexagons) of crust of whatever size they choose to work with, the pieces of core cut off are molten and turn into blobs.

You imbeciles, that was the wrong planet you chopped up!

Answer 7

Sending parts of Earth drifting into space, as you require, will need something capable of imparting those parts escape velocity, around 11 km/s.

The only thing I can imagine producing such effect would be a massive impact which results in a spallation of the planet, kicking out large chunks of the planet surface on the opposite side with respect to the impact site.

The planet would be turned into a molten ball of rock, but some of those ejected chunks would keep some remains of the original features.

The acceleration would be such that I doubt any building would remain untouched, but under the remains one could still scavenge for something useful.

Answer 8

Von Neumann probes

A Von Neumann probe is "a spacecraft capable of replicating itself." They necessarily need to extract material flom planets to make new copies.

If they are planet or dwarf planet sized and come to Earth, they might cut city or small country sized chunks of the planet's surface and take these to space far from Earth. With Kardashev level 2 or 3 technology, which is jargon for technological magic and handwavium, these chunks may reach space relatively intact.

The probe then takes the materials from the bottom of the chunk to make a new probe, discarding the surface side. Now you have two probes, and the process repeats. The probes will multiply increasingly faster, so the planet may be disassembled in short time.

In the Steven Universe cartoon, an alien species disassembled their own home planet for raw materials like that. It has been discussed online that the aliens in the show are also Von Neumann probes (but mostly human sized, unlike what I propose here), and indeed a lot pf the lost planetary mass was used for reproduction. Their planet is hollow and looks like the image below (black patches are voids) - and i remember it having chunks orbiting it in unnatural patterns:

Answer 9

'Respecting the current laws of physics'.

Ahh, yes, therein is the rub. Respecting the laws in reality, or respecting the laws in a fictional setting?

There is the 'possible', the 'plausible', and the 'believable' criteria level.

'Possible' is completely adherent to what an engineer, physicist, or scientist works within. As many posters have stated, there are lots of physical reality reasons why your scenario is not possible, among them gravity, the earth's molten core, and just the tremendous amount of energy necessary to accomplish the task. But, really, I don't think there is any disaster movie or fictional novel that strictly adheres to 'possible'.

'Plausible' agrees substantially to what the general public, with non-specific knowledge of engineering, physics, or science knows or understands about reality. Fortunately for science fiction, the general public does not know that much. Things like escape velocity, the molten core, and energy requirements are vaguely understood by most people, so disaster movies generally take liberties with the laws of physics and define them within the generally accepted public perception of them, not he mathematical models of pure science and engineering.

'Believable' generally acknowledges that the general public can be convinced of almost anything, if the writing is convincing enough. There are an abundance of disaster movies that fit this level.

TL:DR

Do you want the event to be valid within every law of physics, or within the realm of the public's generally believed and generally accepted laws of physics, but not completely confined by ALL of the laws being applied at once? That is, obey a few that are necessary to create the event, but ignore others that are inconsequential (inconvenient) to the plot?

Most people understand plate techtonics. They understand earthquakes, the movement of these plates, and the fact that they 'float' on a solid mantle. Okay, that layer is more 'plastic' than 'solid', but whatever. There is a 'wrap' that keeps the insides of the earth inside, that continents float on. People also understand that earthquakes are a result of a tremendous amount of energy. How much energy, they really are not concerned with, but just the notion that this energy can move an entire plate and cause mountains to form is overwhelming to them. They have also been inundated with factoids about asteroids, and the asteroid belt, being remnants of a planet that somehow self-destructed, without being re-constituted by the gravitational forces. Add to this all of the scare stories about mankind having enough nukes to blow the world apart ten times over? Everyone knows about unrestrained chain reactions. How about anti-matter? The general public perceives this to be able to produce phenomenal energies. Nuclear fusion plants and containment vessles gone bananas, releasing huge amounts of energy? And the creation of artificial black holes in large colliders? The fact that all of these things are created in lightning storms aside, the public is really gullible when it comes to armageddon disaster theories. What if they all happened at once, in some 'perfect storm'? One event leading to another?

So an event that is so powerful, it breaks up and knocks one or more of these techtonic plates off of the earth? It is not 'possible' escape velocity wise, when one does the actual calculations, but can it be made 'plausible' or 'believable'? An earthquake that is so powerful, it breaks these plates up into pieces, like a broken saucer, and flings them into space? Peeling them away from earth, leaving only the molten core, wrapped up in its 'mantle', behind? Notwithstanding the fact that most human structures and buildings depend on the existence of a gravitational force to 'hold them down' in compression, and could probably not retain their structural integrity in low gravity, especially during an earthquake, could the general public be convinced structures could survive on these plates?

Could a very convincing author make the general reading/viewing public believe that an all-out nuclear armageddon, where all nations fire off their nukes at the same time, even perhaps having progressed to anti-matter or controlled black hole weaponry, cause an earthquake of such magnitude that the earth shakes off its plates?

Answer 10

Impossible.

Let's take earth and just for the thought experiment split it in 8 with 3 slices and then magically pull them apart without destroying everything. Let's ignore the huge energy that is needed to get this done and the destruction that alone would bring.

These chunks are all 1/8th of the weight. Their desired center of mass is just R/2 from their own crust, but it currently is at 3/8th of the former earth Radius - considerably furth on the outside. The resulting forces are tremendous and first pull the pyramidal inner section of the chunks back up, which flattens the body. As a result of inertia, the center of the bent triangle former surface breaks open in the center and floods over with molten rock, the surface melting as it sinks to the center. Lacking the support of the (now missing) center, the corners of the former surface get pulled center wise harder, break down and melt. After a (geologically) short time the quarters have returned into their natural shape of mostly molten spheres - ruining their former surface - and start to cool again, rebuilding a surface.

Answer 11

Impossible...

"Human technology has advanced, although it still respects the currently known laws of physics"

That's your problem right there. In a matter of minutes, you need to displace a mass of the order of magnitude of the Earth's (everything but the crust). Apart from the energy requirements (the gravitational binding energy of the Earth in a matter of minutes could only be supplied by a Kardashev level III civilization), this implies accelerations whole orders of magnitude beyond what most human artifacts could withstand.

Removing the core and leaving the surface in place would almost work, albeit requiring matter displacement (which violates the laws of physics), but that would still imply the cessation of the Earth's gravitational field, thereby liberating megatons of surface tensions (the Earth is not a sphere but a geoid, and after being hollowed out, any sizeable mountain would start rebounding) which, together with the atmosphere exploding outwards, would make short work of, again, most human artifacts.

...but could we cheat?

"Magic" can usually be made compatible with hard science fiction in one of two ways: nano- or pico-technology, or simulation. Move your "universe" inside a computer and literally everything becomes possible, often including self-contradictions in the laws of physics.

Nanoengineering the disappearance of the core is still impossible (plus the energy requirements explode, both literally and metaphorically).

What about simulation? For some reason, humanity decide to Ascend into a virtual world, and to do so, they use nanotechnology to map and possibly gradually replace the Earth's surface (nobody cares about the deep underground). When they have done so, they upload the Earth into the VirtuaEarth engine...and a bug hits.

Possibly even due to the nature of the process. Maybe they were using alien technology they did not fully understand; maybe they saw that the aliens had already made use of the same technology but had mapped their whole solid planet, which took millennia; the canny humans believed they could do this in mere months by skipping everything deeper than 15 kilometers, and did not realize that the virtualization engine would not keep the planet together under those conditions.

You are left with a world where an impossible gravity keeps people on the leftover chunks of surface, air is still there and still breathable, but the Earth's surface has become a flock of floating islands. The process can probably be fixed, if the engine can be accessed and properly reprogrammed.

Answer 12

Highly advanced force-field bubbles would need to exist.

Let's imagine in the future it's possible to create an impenetrable bubble that would protect you from a nuclear explosion. These bubbles can be small enough to protect a house, or large enough to protect a city. They're spherical, so they contain air above and ground below. The force-fields can be sustained almost indefinitely by fusion power plants.

Presumably there is a way for a spaceship to pass through these bubbles, using similar force-field technology.

(We might also need some kind of artificial gravity to keep the air in the bubble from filling up with floating dirt. Luckily everyone already has gravity pads, because they like to turn down the gravity when they want to relax.)

Having established this technology, it no longer matters much what destroyed the earth. As soon as the earth's core got turned into anti-matter (or whatever) the seismic detectors were triggered, and the bubbles activated in time for the people in them to survive the immediate aftermath of the event.

Answer 13

This suggestion isn't within the known laws of physics - I agree with others that that's probably impossible - but I quite like it.

If you were somehow able to make the force of gravity almost zero but very slightly negative in the region around the Earth, that would do it. It could be the result an experiment gone wrong, or maybe a doomsday weapon.

On the surface of Earth it would feel like there was no gravity. The atmosphere would expand out into space with nothing to hold it down, which would kill everyone pretty quickly, but buildings and chunks of landscape and so on would pretty much hold together, because there'd be no forces pulling them apart.

However, because of the slight negative gravity, each part of the Earth will be feeling a slight force pushing it away from the rest of the Earth, and on large enough scales this should be enough to rip it into big chunks and send them slowly accelerating away from each other. The effect can wear off once they've eventually reached each others' escape velocities, because then they won't just fall back down again.

It should be noted that this violates the conservation of energy fairly drastically, so if you're going for hard science it's not a good choice. But if you're willing to wave your hands about that sort of thing, it's a reasonable way to achieve the effect you're looking for.

Answer 14

I think other contributors have amply demonstrated that the desired effect is not possible with any technology (or even any natural process) within our current understanding of possibility.

If you consider a speculative technology such as teleportation to be one that "respects the currently known laws of physics", however, here is a possible way to do it:

At some future date humans develop teleportation technology capable of moving large masses over interplanetary distances. Like the Star Trek teleporters, this technology requires a teleportation station on only one end of the journey, but unlike the Star Trek teleporters it is capable of teleporting something the size of the entire USS Enterprise over the distance from Earth to Mars.

This technology is implemented in numerous places around the Earth as a method of interplanetary travel that is rapid and relatively safe (compared to sitting in a spaceship bathed in radiation for several months).

In order to safely teleport an object over such a large distance, the station must be extremely stable, and so it uses teleportation technology to "stabilize" the crust of the Earth within 20 or 30 kilometers radius of the station (in three dimensions), essentially by continually teleporting that entire mass through tiny distances (measured in nanometers) in order to compensate for any disturbance.

Unfortunately there is a bug in the software that controls the "stabilization". One day an earthquake very near one of the teleportation stations exceeds the normal bounds of operation and triggers this bug. Instead of teleporting a few nanometers, the teleportation station (under the control of the software) teleports itself and a nearly-hemispherical chunk of the Earth's surface 20 or 30 kilometers in radius to somewhere around the distance of the Moon, and leaves this chunk with enough velocity to remain in orbit far above the Earth.

The resulting shock waves caused by the removal of this chunk (and the collapse of the hole left behind) trigger the same software bug in stations around the globe, and they also teleport chunks of similar sizes to similar orbits, turning the tectonic plates of every continent into something resembling Swiss cheese.

The effect on the remaining surface of the Earth is cataclysmic, destroying every structure and human artifact and wiping out all life. The effect on the teleported chunks is also disastrous -- whatever part of the atmosphere was teleported is quickly lost -- but each chunk has such weak gravity that there is no deformation or fracturing of that piece of crust. Buildings are severely damaged by explosive decompression but many artifacts remain.

---

This is still quite fantastic and I'm not sure even I would be able to comfortably suspend disbelief this far, but I think you could do a lot worse.

Answer 15

You could look into something similar to the Spindizzy from James Blish's Cities in Flight. In the novels, the spindizzy is used to separate entire cities from the gravitational field of Earth, establish artifical gravity, and allow them to travel faster-than-light, eventually leading to these cities acting as roaming contract workers for various outposts of human civilization.

If you want this to be a sudden event, you could postulate some mad scientist inventing this technology, and using it to accelerate large chunks of Earth's surface out into space. This would still leave most of the Earth behind (since less than 1% of Earth's surface is the Crust we live on), but from humanity's perspective, only the chunks that were broken off would be habitable.

Answer 16

A localised effect "disabling" gravity

The "problem" preventing our planet from flying apart is the gigantic amount of energy required to move matter outside of its own gravity well. But what if some magical technology disabled gravity for a time?

The earth's crust would be ripped apart by the pressure of escaping magma from the earth's mantle and its pieces would be accalerated away on pillars of slowly cooling flame. Normally this acceleration would probably be relatively slow, and people might even survive if they don't drift of into space when trying to walk outside, asphyxiate in the very quickly escaping atmosphere, get hit by splatters of crustal magma or commit sudoku when realizing they might never reach their loved ones again. But whatever process disabled gravity might cancel the gravitational energy debt imperfectly, also resulting in a superheating of the earths core, instantly sublimating/evaporating the metal and blowing away the fragments on a shock front made from high-pressure metallic gas...

In-depth ideas

Energy Source Depending on the hardness of your scifi story, it might be necessary to postulate some source for the energy necessary to cancel the gravitational energy debt - after all, spontaneously violating conservation of energy would seem pretty implausible to most readers. Perhaps the researchers were utilizing some zero-point energy source to power an experimental flying car antigravity engine, but underestimated the actual energy output of the energy source, leading to a much less localized effect (spatially as well as temporally). Or perhaps the energy was pulled from the future, through some kind of time rift... In any case, the combination of some gigantic source of energy and antigravity experiments could very plausibly cause big parts of the earths crust to fly around relatively undamaged.

temporal energy source

If a spatial rift was the energy source used, there might be a mysterious anomaly flying through the wreckage, sucking up all energy from its vicinity to send back in time.

Is the effect permanent?

If (active (1)) gravity was permanently disabled for all matter that once was part of old earth, it would spread out rapidly. Distances would quickly grow huge, and after a few years a very loose asteroid belt would form (its looseness increasing at a speed depending on the initial speed of the "explosion" - there would be no internal gravitational interaction to stabilize the belt). If, on the other hand, gravity returned slowly, the outcome would depend on the speed of that return - ranging from stable asteroid belts (like the one we have) over clumps of wreckage barely bound together (and sometimes escaping), to an unstoppable cataclysm in the nearer future (few years to few millenia) where all pieces would crash back together (and obliterating all traces of humanity in the process).

Other side effects

The gravity-altering technology might have created other disturbances - some pieces having an earth-like gravity at their surface, others actively repelling nearby objects. This might (depending on your desired hardness) require additional handwaving to explain.

Conclusion

Whether a mindbending experiment gone wrong or a society playing with technology they didn't understand until reaching a catastrophic tipping point, the question of circumventing gravity is probably at the heart of every plausible solution to your problem, since any source of energy capable of quickly removing a large piece of our planet out of an intact gravity well would vaporize it in the process. More "mundane" sources of anomalous gravity (like the Roche limit idea mentioned by James McLellan, or perhaps an alcubiere warp drive experiment) might work equally well and might be more plausible, but would probably destroy most of the surface structure of our crust as well. If gravity was turned of "peacefully", very large chunks of crust could stay intact, and there would only be minimal to medium (depending on acceleration speed and "tectonic" collision rate) damage to buildings - even precarious skyscrapers could very well survive intact.

Notes

(1) by active gravity I mean the effect of matter distorting surrounding spacetime. "passive" gravity (as in still being attracted by the sun) would still work, as it is an intrinsic effect of the geometry of spacetime. But the spacetime-distorting property of matter would be much easier to counteract, at least in the (my) current understanding of the nature of gravity. Just imagine exotic (negative) dark matter swirling around, canceling the (active) mass of our normal matter... (and there are probably many more good explanations of decent scifi hardness).

Answer 17

Humans aren't currently at any sort of technological state where we could consider disrupting the structural integrity of the Earth. If you want the disaster to be man made, you are already so far beyond what we currently have access to or know that you can basically say whatever you like. It's sufficiently advanced technology.

I like trying to compress Jupiter into a red dwarf using mass from an artificial white hole so that we can wrap it in 9 / 10ths of a Dyson sphere. Use magnets and the open part of the dyson sphere to turn it into a generation ship. Lots of things that can go wrong there.

It seems that you could tune a black hole or neutron star's speed and mass so that forces the earth into a really good spin (kind of a reverse gravity assist), so that it tears itself apart. Kind of like an Olympic discus thrower trying to set a distance record with an uncooked pizza. If an npc proposes it as a hypothesis, and another lampshades the improbability of such an event, you're basically there. From there, you can sidestep the further and much greater improbability of human artifacts surviving. Your characters already know it's improbable. If your players need a better explanation of the event, they can attempt to figure it out themselves.

Answer 18

So I know this is rather old, but I feel like most of the other answers neglected a possibility, so I wanted to add one.

Strictly speaking, this is not currently acheivable based on current knowledge of physics, and depends on the exact composition of the earth's core, which is not currently known. However if there is a "real" answer to your question, it's this:

You do not want to slow things down, but rather to speed them up. Instead of happening over minutes, it needs to be near-instantaneous. (You can still potentially have some lead time where you know it's going to happen, but when it does, it's instant.)

Other answers have rightly noted the massive energy release required, but they have generally assumed the energy release would be omnidirectional. Instead, we want it to be linear (vorticity), or at least planar (implosion).

They've also assumed you need to account for the entirety of Earth's mass-energy, or alternatively, to gravitationally unbind the crust from the entirety of the core/mante. This is unnecessary, and actually counter-productive. The debris would scatter outward in all directions. This includes perpendicular to the solar plane ("up" or "down") and to the orbit itself as viewed in-plane (either sunward or "Jupiterward"). Seriously bad news if you've imparted any acceleration and don't have gravity to pull things back together. Within very short order (hours, days, weeks at most) everything will be too far apart to be worth the fuel cost to travel between them.

Instead, you want most of Earth's mass to remain exactly where it is, for the fragments to remain loosely gravitationally bound, and for them to continue to coorbit at Earth's general distance from the sun.

The most plausible scenario I can come up with involves assuming the "inner inner inner core" is in fact composed of fissionable elements, currently held by the heat and pressure of the planet above in a very specific homogenous crystalline structure/state such that the rate of fission is tightly controlled AND significantly more stable against collapse than alternative arrangements. I don't know of any such structure, but there's nothing I know of that says it can't exist.

Assuming you have all this, then the core (at least this deep inner portion) is basically a giant nuclear bomb waiting to be detonated. So detonate it. Provided your process shapes the blast right, you should be able to direct most of the energy along a line (ideally passing through the poles), or a plane (ideally along the equator). Equitorial probably makes the most sense, and has the "benefit" of channeling excess explosive force into uniform "thrust" separating the halves.

The remaining core and mantle will tend to "glob" and "stretch" back towards the center of gravity, leaving the crust unsupported against collapse itself, with an "outward" momentum, and the shifting gravitational attraction of the various globs pulling it differentially.

Since we don't know the exact composition of the core, you can play around a bit with some of the plausible options without violating the currently known laws of physics, and you can even "dial-a-yield" by picking the option best suited to your result.

The important thing is that this happens incredibly fast. It must be a highly asymmetric and chaotic process. As others have noted, anywhere close to this level of energy release is going to tend to level every conceivable structure. However, if it happens fast enough, theres not actually enough time for most of that energy to be transferred to the remnants before it passes through to space. There will still be global shockwaves - and earthquakes of such a magnitude that we've never even conceived. But they won't have time to coalesce before the earth itself breaks apart, and the intensity will vary immensely. Most of the surface will be unrecognizable. But sizable portions could easily go relatively unscathed. At least in terms of vibrations.

The real problem is atmosphere. Not just what we have now, but what would be released as the crust disintegrated. I'm not sure how you manage any version this without essentially lighting the air on fire. So you'd have to explain why that fire didn't destroy everything on the surface. There's probably a way to do that, but it's beyond me.