My world has a huge crater (4400 miles in diameter) on a world that's roughly earth equivalent (so a circumference of 25000 miles). The native population doesn't need to know how it was formed but it would be nice if the world creator did! Around the edge is a ring of islands, inside and outside there's nothing but the sea.

I've been thinking about the crater from the meteoroid which wiped out the dinosaurs. I believe it's 180 km in diameter, obviously, this is far too small. I'm not sure a planet could survive an impact from something almost ten times the size (apocalyptic isn't an issue but the planet needs to survive and go on to create life afterward).

I've thought about volcanoes but I'm not sure that would create the shape I'm looking for.

I can resort to magical means as a last resort but I'd really rather keep this real world.

Ideally, I would like the crater to start in the polar regions and stretch down well towards the equator (almost like the death star) so there is a vast range of islands. It does not need to be particularly deep so doesn't need to go anywhere near the planet's core.

Could a crater this size occur without godlike intervention? If so how?

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    $\begingroup$ Thats a big hole you have there, its bigger than the Earth... How about an early event that made the Planet split and buildet a nearby moon out of the matter of the crater. An bigger world hitting on the one Side of the Planet and pops the crust on the other side out of the athmosphere. For Magical las resort "they dont like us! lets build our own planet out of this one" - Pop planet builded and crater on the old one is there. $\endgroup$
    – Fulli
    Commented Sep 22, 2014 at 7:39
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    $\begingroup$ How big is the planet itself? $\endgroup$
    – user
    Commented Sep 22, 2014 at 7:42
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    $\begingroup$ Also, if the planet is large enough to support the creation of a 16,000 miles (almost 26,000 km) diameter crater, it is probably large enough that gravity will present more than a bit of an obstacle to life, particularly early forms of life. $\endgroup$
    – user
    Commented Sep 22, 2014 at 7:45
  • $\begingroup$ I've done some maths and re-scalled a little $\endgroup$
    – Liath
    Commented Sep 23, 2014 at 10:26
  • $\begingroup$ just to clear that point - your world has a radius of about 4000 miles now and the Crater is 4400 miles. Where is the crater located? Poles? Aquator? backyard of the university? ;P $\endgroup$
    – Fulli
    Commented Sep 23, 2014 at 11:40

5 Answers 5


Actually while the title of your question asks for an enormous crater, the body is asking for something completely different - a ring of islands.

That could be explained by a couple of means that do not involve craters at all - both of these start in a planet entirely covered by water, but not too deep water.

Ring of Fire

A "hot spot" in the magma of the planet causes volcanoes to form above it, over time that hot spot moves around in a circle driven by Coriolis forces within the magma, as it does a line of islands appears above it as volcanoes punch up from the sea bed and into the surface.

When first formed you would get tall active volcanoes, these would then gradually erode down as the hot spot moved on until millions of years later the hot spot returned and built them up again, so you would have a ring of gradually eroding islands "behind" the active one.


A hypothetical corral species proves very successful and builds its way all the way up to the surface and then starts expanding outwards. As it moves out the old coral starts to erode and collapse back down. The growth of the coral absorbs certain nutrients from the sea shore so it always keeps expanding outwards in a ring.

Natural events, valleys in the sea floor, variation in the mineral, etc mean that breaks form in the ring as it expands leaving the situation after millions of years of a ring of islands with empty sea both inside and outside of the ring.


There have already been some great non-impact ideas, but I thought I'd add one more resource. consider Saturn's 3rd moon, Iapetus:

Scientists have several theories for Iapetus's characteristic equatorial ridge, but the most compelling to me is the collapse of an ancient ring system (this would have been from Iapetus's own ring system, not that of Saturn). See the Wikipedia page Equatorial ridge on Iapetus for the other, more boring theories.

Over time, all planetary ring systems collapse. If you wish to connect this cause to the present through stable rings that survived this collapse, they would require shepherd moons to keep them in place.

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    $\begingroup$ But if your planet has an atmosphere, you would not get the nice ridge. Ring particles are small (dust, mostly), and so their velocity would be reduced to zero before they hit the ground. So you'd have atmospheric dust distributed over the planet, much as micrometeoroids are on Earth today. $\endgroup$
    – jamesqf
    Commented Feb 4, 2019 at 17:33
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    $\begingroup$ Perhaps, but that's not conclusive. Rings are quite dynamic with so many variables, and their particles change size over time. The rings could fall before fully pulverizing, they could be massive enough to begin accreting, the atmosphere could've been originally frozen in the rings, they could have been a super fun cool idea, etc etc $\endgroup$
    – BoomChuck
    Commented Feb 5, 2019 at 14:02

A few ideas, I'm not sure how viable they all are:

Spin Changes

The planet formed and cooled spinning very rapidly as a result it had a flattened rather than spherical shape. It then captured a large moon nearly as large as itself, the resulting tidal forces slowed the spin down drastically (and led to some interesting times for anyone alive at the time).

The slowed spin meant that the poles were now significantly lower effective gravitational level and the water drained down into them, filling them with water.

This would give massive circular oceans at each pole.

Near miss

A large planetary body came very close to striking but in fact only skimmed the surface then headed back out into space. The glancing blow ripped out a huge chunk of land (unfortunately unlikely to be circular but less devastating than a direct impact).


A very dense body (for example black hole or neutron star) punched clean through the planet taking out a huge chunk of material as it did and then continued on across the solar system.

The rest of the planet then collapsed into the void and you had some pretty extreme geology for a while afterwards but ironically by just removing large amounts of material cleanly it might do less damage to the rest of the planet than a conventional meteorite impact would do.


Unfortunately, a single compact object hitting a planet with enough mass and velocity to create a crater this size would be an extinction-level-event that would most likely sterilize the planet and turn the impact site into a four-thousand-mile-wide pool of lava surrounded by the crater wall as it punched clean through the planetary crust and into the magma layer beneath (assuming that this planet is earthlike with a molten core). It is also likely that there would be massive fault-line ruptures radiating away from the site of such an impact.

On the other hand, such a large impact would leave its mark on the planet for millions of years, however it is likely that on an earthlike planet, such a great amount of elapsed time would result in the workings of plate tectonics disrupting the nice neat ring described, and it could take billions of years before advanced life appeared again unless there was a bit of panspermia going on.

Another option that may remotely work is a huge caldera, a volcanic system where a section of crust separates in a roughly circular area and subsides, leading to a ring of volcanic eruptions around its edge. However, calderas are not usually found on that sort of scale, they are typically tens of miles across, not thousands. Again, a caldera on this scale could also be an extinction-level-event.

The most likely non-magical event that I can imagine is also somewhat far-fetched. If a circular orbital habitat 4400 miles in diameter fell to ground on its side, it could cause the circular chain of volcanoes or crater wall mountains described that would be visible thousands of years later without necessarily causing a mass extinction event However, that would presuppose that a previous civilization was sufficiently advanced to create such a habitat, and then departed without leaving other traces - or that any other traces they may have left have long since decayed.

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    $\begingroup$ This may go down as quote of the day but I'm not actually worried about extinction-level-events, I can always populate the planet after it! $\endgroup$
    – Liath
    Commented Sep 23, 2014 at 13:12
  • $\begingroup$ Related watching: youtube.com/watch?v=pe3s2G8ulBc shows an animation of the scenario, complete with the aftermath. Crudely speaking: it would incinerate everything organic, burn away the atmosphere and boil away the oceans. There will be a crater large enough, but NOTHING can survive. $\endgroup$
    – Nzall
    Commented Oct 7, 2014 at 8:13

sorry i didn't read all the previous answer but a huge impact crater is terrible for live on your world. Have u consider to use a mechanism like the one thath create the blue holes?

Acidic rain erode and dissolve limestone creating lots of crack in it. So weaked the limestone can collapse in a second time under the action of an high pressure or maybe an earthquake.

sure it's not a crater, but is similar and surerly more life suitable than an asteroids


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