On a planet with Earth-like attributes in every way, including physics and geography, what repercussions would there be for a relatively deep 'dent' in one quadrant of the spherical body? This is only slightly related to this question, as there are many key differences.


It's about 2 thirds deep into the mantle, at its deepest point, of an Earth-sized planet, ie. there is one third of the mantle remaining at the deepest point of a rather steep hole.

In the image below, the red area represents the hole (deeper than I wanted but never mind).

Assume the worst - this isn't a hole in the middle of an ocean, you can already see that it takes out some of a polar ice cap, and a lot out of the equator, so there's going to be a lot of land that disappears.

I'd prefer answers that are structured like a relative-to-hole-appearing timeline; with a few years after, a few hundred years after, a few thousand and a few million (or whatever you like) that gives local updates on how everything is coping with this dent.

I'd like to see the following things covered in this post:

  • Gravity and overall physics of both the planet and its Moon-sized moon.
  • Geography; what happens to the land around this dent? The oceans?

Please note than in this question, the reason as to why this dent exists is, for the sake of this question, magic

As a side note: I will be splitting this question into at least two parts. The following will be posted in another question, of which the link will be placed here.

  • Ecology; how do living things cope and interact due to this dent?
  • Meteorology; does this dent get rained on? Does it generate unusual weather somehow?
  • And anything else you think is essential that I missed
  • $\begingroup$ How deep is this hole? For that matter, how big is the planet? $\endgroup$
    – March Ho
    Jan 27, 2015 at 13:03
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    $\begingroup$ Have you considered posting that question to What If? $\endgroup$
    – eimyr
    Jan 27, 2015 at 13:51
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    $\begingroup$ Is 'magic' keeping this dent in existence? Otherwise there would just be total devastation of everything on the planet as it collapses back into a spherical shape under its own gravity. $\endgroup$ Jan 27, 2015 at 14:00
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    $\begingroup$ This question is also relevant. $\endgroup$ Jan 27, 2015 at 17:57
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    $\begingroup$ @eimyr What-if #46 Bowling Ball actually already covers something similar. $\endgroup$ Jan 27, 2015 at 22:50

6 Answers 6


The Earth is round because it is in hydrostatic equilibrium: its gravity is strong enough, and the material comprising most of its bulk fluid enough, to make it flow into the shape that minimizes its gravitational potential energy: a sphere.

As a fairly reasonable analogy, you can think of the Earth as behaving like a floating drop of water; while the forces that hold the Earth and the drop together are somewhat different (gravity for the Earth, surface tension for the water drop), the overall behavior is similar. The important thing to realize is that, when it comes to the overall shape of the planet, what matters is the gravity and the hydrostatic properties of the core and the mantle; the relatively thin solid crust on the surface is neither thick enough nor solid enough to make any significant difference.

Once you appreciate that, it should be obvious what would happen if a large piece of the Earth were to magically disappear somehow: the remaining bulk of the planet would just flow into the hole, filling it. Once things settle down, you'd just end up with a slightly smaller, but still spherical, Earth.

Now, that's the long-term outcome. The slightly more interesting question is, what would happen in the short term? For that, I'll have to agree with the other answers: utter chaos and destruction. Basically, as bowlturner notes, cutting a large chunk out of the Earth would be very similar to, and only slightly less destructive, than hitting it with a large planetoid — like, say, the one that produced the Moon.

Basically, here's a number of things that would happen:

  1. The atmosphere will flow down into the hole. This will actually take several hours, since the wind speeds will only(!) rise to around Mach 1. Indeed, the initial rarefaction wave will only travel at Mach 1, so on the opposite side of the Earth, it will take a while for the air to start moving at all. Still, in less than a day (if nothing else interferes; see below), pretty much all of the atmosphere should be down in the hole, leaving the rest of the planet (temporarily) airless.

  2. The oceans will also flow down into the hole. The speed at which this happens will depend somewhat on the location of the hole (but probably not as much as you'd think; see below). Sound travels about five times faster in water than in air, so the initial disturbance will circle the globe pretty fast, but the actual quasi-equilibrium flow rate may (initially) depend on undersea geography. In any case, you'll likely end up, briefly, with some of the biggest waterfalls ever seen on Earth (and there have been some pretty big ones).

    As the water flows down into the hole, it will hit exposed magma and vaporize, joining the air that is also flowing into the hole. This will also cool the surface of the magma very efficiently, forming a thin solid crust, but since the magma will also be moving (see below), that crust is unlikely to be very stable.

    Depending on geography, and the extent of crustal damage (see below), some standing bodies of water might be left in lakes and oceanic trenches. If so, as the atmosphere above them disappears down the hole, the pressure will drop until liquid water becomes unstable. At that point, the water will boil and freeze at the same time; the part that boils will spread out as vapor (and, if it doesn't condense as frost, eventually end up in the hole with the rest of the atmosphere), while the rest will remain behind as solid ice. (If the ice gets thick enough, some liquid water might remain, trapped underneath it.)

  3. The lithosphere will also flow down into the hole. Remember, I said that the Earth's mantle will flow under gravity until it becomes roughly spherical again. As the mantle flows, it will drag the crust along with it — but since the crust is solid (on short timescales, anyway), it's going to break up as it flows, much like ice on moving water. The size of the chunks is hard to estimate, but I'd expect them to range from dozens of kilometers (comparable to the thickness of the crust) up to maybe a few thousand (comparable to small tectonic plates today) in diameter.

    Besides causing the mother of all earthquakes, the breakup of the Earth's crust will open deep fissures all over the planet. That's why I said that undersea geography may not matter so much — many of these fissures will open up under oceans (where the crust is thinner anyway), some likely along existing mid-ocean ridges, and the water will just flow into them and hit the exposed magma there.

    The details of what happens next will depend on many things, like the depth of the water and the spreading rate of the fissure. Initially, at least in the deep ocean, the water can probably absorb the heat from the emerging magma, as it does in ordinary undersea volcanic eruptions, but as the fissures keep spreading and the ocean keeps draining, eventually the remaining water may get hot enough to boil away.

So, over the first few minutes to hours after the hole forms, what we'll see (hopefully from a safe and comfortable vantage point, say, on Mars), is basically a massive avalanche of magma, water and air all rushing in to fill the hole. Without running an actual lithodynamic simulation, it's hard to say exactly how long it will take for this avalanche from hell to reach the middle of the hole (or, rather, to fill it up — of course, the magma will also be flowing up from the bottom of the hole), but when it does, it's likely to do something interesting.

It will splash.

Basically, all that magma, water and air will accumulate a considerable amount of momentum as it rushes into the hole. Once it reaches the center, and meets the flow coming in from the other side, it has nowhere to go but up. And up it goes. Basically the same effect happens when you drop a rock in water: at first, it just basically makes a hole in the water, just like your magic hole in the Earth, but most of the splashing actually happens only when the water falls back into the hole.

Again, it's a bit hard to estimate the magnitude of the splash off the top of my head (though an experienced planetologist perhaps might), but I'd expect it to scatter at least some magma into the sky. Some might even escape the Earth's gravity entirely, but most will likely just fall back somewhere else on the planet. So, just after you thought it couldn't possibly get any worse, any surviving bits of the Earth's surface will likely experience a rain of burning rocks from orbit.

Finally, as the dust settles (literally), the remaining atmosphere of the planet (probably somewhat thinned by the splash, but augmented by a whole lot of water vapor and volcanic gases) will gradually settle down again. Most of the water vapor (which used to be oceans) will rain down again, at least once the surface cools down below the boiling point of water, and eventually the clouds will break up and the sun will shine on the surface once more.

(At least, that's if you're lucky, and all the extra heat, water vapor and volcanic outgassing isn't enough to kick off a runaway greenhouse effect like on Venus. We don't really know enough about climate dynamics at such extreme conditions to say if such an equilibrium shift would even be possible on Earth, or if so, what would be needed to trigger it.)

OK, at that point, you may wonder what all the is going to do to any life there might be on the planet. Well, obviously, the simple answer is "nothing good". Indeed, I would expect all this destruction to kill off well over 99.99% of all life forms on the planet — what doesn't get crushed or burned or boiled might end up vacuum-frozen or just plain poisoned by toxic gases.

Still, life on Earth is pretty tenacious, and absolutely everywhere. There are microbes that live inside solid rock, deep within the Earth's crust, and others that can tolerate anything from freezing to boiling. Short of actually melting the entire crust, I doubt it would be possible to exterminate all life on Earth (although a runaway greenhouse state as mentioned above, if possible, might just about do it).

Depending on the extent of the disruption, it's even possible that some multicellular organisms might survive — perhaps even some macroscopic land animals like insects, some of which might survive as eggs or pupae. I'm not seeing much hope for vertebrates, though, but if you really wanted to stretch plausibility, I just might buy a few fish surviving in some lucky refuge somewhere, perhaps in a deep frozen lake. That would be a pretty long shot, though.

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    $\begingroup$ Rather than flowing down, the mantle and core will explode upwards at the release of pressure. $\endgroup$
    – Oldcat
    Jan 27, 2015 at 23:48
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    $\begingroup$ "what doesn't get crushed or burned or boiled might end up vacuum-frozen": calculate where life needs to be to get grilled medium $\endgroup$
    – smci
    Jan 28, 2015 at 1:07
  • $\begingroup$ in the long term, the greenhouse effect probably won't matter much, because disruption of the planet's magnetic field will mean the solar wind will start stripping the atmosphere off the planet (as it has mostly done on Mars). $\endgroup$
    – Michael
    Jan 28, 2015 at 4:19
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    $\begingroup$ @Michael: You seem to be assuming that the core dynamo would stop, and not start up again. I can surely believe that it would be temporarily disrupted, but I don't think we know enough about the Earth's core dynamics to say what the long-term consequences for the magnetic field would be. $\endgroup$ Jan 28, 2015 at 11:39
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    $\begingroup$ Per my answer, I don't think there's anything that could keep the planet from readjusting itself into a sphere pretty much immediately, which I think will cause the crust to dissolve everywhere as seismic shockwaves circle the globe, so although your oceanic ideas are really fun, I don't think they'd have any time to happen, at least not with a hole of the size shown in the question. Now, if the hole were MUCH more shallow, so the crust didn't need to re-adjust, then what you say about the oceans would hold. $\endgroup$
    – Dronz
    Jan 28, 2015 at 21:08

Let's assume that The Dent just suddenly appeared because Magic, poof! and it's gone as if hit by a huge D-cannon of WH40k fame. It's replaced with nothing (vacuum).

Minutes to years: Complete and utter destruction of everything on the planet.

Appearance of The Dent caused everything on the planet to jerk violently as the eccentricity and rotational momentum of the planet changes. Most living creatures die, depending on the size of the dent. Athmosphere became sucked into the void, creating winds of unprecedented force and hurricanes that last for years.

Eccentricity of the planet changes, and a new center of rotation is found. Depending on the relative position of Star-Planet-Moon, this might change orbit of the planet and/or orbit of the moon. It's not very likely that the planet's orbit changes significantly, but it might increase its eccentricity, meaning that difference in energy being delivered to the planet changes more than for real life earth (less than 10% of radiation change, not enough to dominate seasons). Changes in planet's rotational momentum change the cycle of night and day.

There is a reason why planets are more-or-less spherical. By being misshapen, depending on the depth of The Dent, planet-moon system will experience increased tidal forces. Potentially it can lead to destruction of either (that's how Saturn got it's rings) in the future.

If The Dent exposed deeper crust, mantle or, fingers crossed, the planet's core, expect extreme amount of hot debris and gases being released into the athmosphere. Also, since rocks underneath all that stuff surely contain some gaseous fraction, even if tiny, it is under extreme pressure, and with that removed, will explode. Plate tectonics stop working, as the molten inside of the planet rearranges itself. Total chaos. Fumes and debris cloud the athmosphere, expect a period of extremely hot greenhouse state for a time, followed by glaciation that lasts until all the fumes clean up (and provided the orbit is still okay to support life).

Also, if anything survives that, remember that magnetosphere is gone. Without magnetosphere directing a lot of solar wind towards the poles, expect that to simply hit earth.

Also, if the dent happen to have some connection to any ocean, expect that water will fill The Dent as well, evaporating while it does. Athmosphere follows.

Hundreds to thousands of years: complete rearrangement of the planet

Provided the planet is not ripped apart, the surface is now toxic, blasted by cosmic radiation, unlike anything you'd expect from the planet before The Dent.

Millions of years: A new planet that is

You might as well create something from scratch at this point. Anything is possible, but statistics show that your planet would be just another rocky barren world.

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    $\begingroup$ Would your answer be affected if the dent was filled with standard atmosphere (according to the planet's atmosphere) instead of a vacuum? A simple yes or no would be fine, as I intend on asking a different question in which the dent is replaced by air, instead of nothing. Accepted, upvoted. Very good answer. $\endgroup$
    – blaizor
    Jan 27, 2015 at 21:23
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    $\begingroup$ If it would be replaced by air, skip the part when I mention athmosphere being sucked in and resulting hurricanes. $\endgroup$
    – eimyr
    Jan 28, 2015 at 10:39

The whole planet surface will crumble apart right away, and the atmosphere will be filled with volcanic dust. The planet will re-form into a sphere.

All masses greater than a certain size form themselves into nearly-spherical objects because all mass attracts all other mass via gravitational force, and for something the size of a moon or greater, there is so much mass that it exceeds the ability of any arrangement of matter to retain any other large-scale shape than a sphere.

So, what Ilmari Karonen said, only much faster and more fluid than he describes for the "solid" parts of the planet. (There will be no kilometer-plus-sized chunks.) The crust with nothing to hold it up with fall apart immediately, and become an avalanche falling into the gap at great speed.

For example, see the speed with which the single volcanic eruption of Mount Saint Helens fell apart during its 1980 eruption:

enter image description here

The sideways velocity of the mountainside is estimated at over 330 miles per hour, and that's just one eruption.

Your scenario would lead to an immediate avalanche the size of a continent and the height of a moon. This would destroy the planet even if it weren't magma underneath. If it is magma under the crust, the shockwaves going around the crust will not only cause the whole crust to crumble apart, but will also cause the magma to be released everywhere. At the speed of propagation of shockwaves through rock. Because of this, I don't think what Ilmari Karonen wrote about oceans falling into the hole would have time to occur, because the shockwaves from the collapse would dissolve the crust under the oceans, and the large-scale planet deformity would readjust itself before oceans would have occasion to "flow into the hole."


The simple answer? It likely kills most life on the planet. If some magic just makes it disappear, poof there one minute, gone the next, it would be the least violent way to make it happen but, the next few months years would still be violent enough to be devastating.

There is a huge hole that need to be filled in. Lots of things are going to rush to fill it in as too. The mantle and magma will slide from the surrounding area and level out from the bottom up. the oceans will be pouring into the hole at an alarming rate, most of earths oceans could fit in a hole that size. Where the magma moves from under the crust, the crust will collapse causing terrible earthquakes (I'm guessing) beyond the 10 point Richter scale. The earth near the hole will begin to look like a cracked egg.

Back to the Oceans pouring in. Cold Ocean water pouring into hot magma will cause lots of steam, likely enough to blot out the sun for years. This would not only kill large parts of the ocean life but the plants would die from lack of light, and the animals slowly after as food continued to become scarce.

It would almost be as bad as being hit by a huge asteroid...


Total armageddon. In fact, virtually no answer posted to this already seems to have a real sense for the scales involved here, which are phenomenal.

At planetary scales, all matter effectively acts as a liquid. A planet is really best thought of, dynamically, as though it were a droplet of water, than it were a spherical chunk of solid rock. This is because at the speeds and scales involved, the resistance to deformation provided by intermolecular forces is negligible compared to larger-scale gravitational and kinetic (impact) forces. This is why that planets are spherical in the first place: it is essentially exactly the same reason as to why a droplet of water or other liquid will naturally assume a spherical shape without any other perturbing forces acting upon it.

Thus, the answer to this question is, in effect, the same as asking what would happen to a spherical droplet of water were that a hole of proportionate size and geometry made therein. The answer is that it would quickly reform into a slightly-smaller spherical droplet of water.

So the planet will do the same - it will "ooze" its way toward a somewhat-smaller sphere. In particular, the exposed material on the sides will tend to balloon out to fill the hole, while the rest of the planet contracts.

And that's why I say "total armageddon", because the crust on the rest of the planet will essentially break up like the thin crust of detritus floating on top of, say, a pot of food being cooked, or the oxide scale on a droplet of hot metal. And not only that, this will happen very fast - while it's actually "quite" incorrect to use the ordinary surface gravitational acceleration as the gravitational field has now become much more complicated, it still suffices as an order of magnitude - in particular with an order of magnitude of $10\ \mathrm{m/s^2}$, the magic of the metric system lets us rewrite with no calculation that this is $10\ \mathrm{(km/s)/ks}$ and so we get that after as little as a kilosecond (1000 s, about a short break; for comparison of this timescale to that of planetary phenomena keep in mind the orbital period of the ISS around the Earth is about 5.6 ks), things will already be happening at speeds on the order of 10 km/s - vaporizy-strong speed and you can expect likewise several thousand km of displacement, i.e. the hole is already well on its way to healing at this point. Think about your liquid drop, again: it will seem to respond, in the scale of your perception, "instantly" to perturbations - well here, a kilosecond is the scale of "instant", just as for your everyday droplet a millisecond or less may be, and "non-instant" is thousands of kiloseconds (e.g. an orbit of Earth about the Sun, i.e. a year, takes 31 558 ks).

Hence, you can expect that the crust will be jumbled and broken apart and likely rapidly covered and melted by the exposed mantle material oozing out from underneath from the enormous, visible-from-space cracks (again, think about your pot with crust on top, breaking up) of widths measured in hundreds of km or more, propelled by the pressure underneath. The effect will be essentially like that of the impactor which formed the Moon against Earth: the most likely end state will be the complete conversion of the surface to a "magma ocean", meaning it is 100% liquid rock at temps exceeding 1000 °C. For anything living on the planet, it is a guaranteed, 100% fatality rate. The only way to survive would be to not be on the planet in the first place while it's happening.

Over the very long term, this magma ocean will eventually resolidify and form a new crust, but in this case, the planet will be left with completely different terrain from when it started - effectively, it has been "reset to zero" geologically.

Regarding atmosphere, again, the answers here don't really do it justice: it will be a complete atmospheric replacement, as the exposed mantle material rapidly outgasses as the entire planet effectively becomes one single gigantic volcano (as in the actual vent itself, more or less). Once the planet has cooled and a new crust has formed and caked hard, the new atmosphere will have composition quite unlike the old one.


Look at the planning and findings of the Rosetta probe and its lander. The worldlet is shaped like a toy duck, with a huge eroded cleft almost chopping it in two. It's not fiction, but a real navigation and mapping nightmare.


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