First of all I've read the other posts and know what the effects of a black hole appearing on the earth's surface is. It goes through the crust, the tectonic plates break apart, volcanos appear everywhere and it moves around inside the earth. In my story a portal is opened up to an area close to a large black hole. Nobody can say the effects of gravity through a portal so I'm just going to treat it as having the strength of a 1mm black hole. It being a portal means the portal can be closed and end the black holes effect so it only lasts lets say 5 seconds. There are also powerful entities in that world that can control the elements enough to keep the tectonic plates from breaking apart and causing volcanoes everywhere. What I want to know is the visual effects on nearby landmasses on a map if a 1mm black hole existed for only 5 seconds, ignoring the effect on the tectonic plates on the rest of the world. Let's say, a distance of 1000-2000 km is what I'm wondering about. Would the land be caving downward because of the black hole? Would they be going in a spiral pattern toward where the black hole is? How much of the oceans water would be drained in that time? Would all the plant life at the distances I provided be destroyed? How far down would the black hole get in the 5 seconds or even in 10 seconds and if it gets to the mantle in that time? Let's say it landed in the Cayman islands, the effect it would have on Cuba and Mexico. Again, this information is so I can figure out how a map would look after the event happened. I don't know how to say this without sounding like I'm ignoring facts, but any effects of the black hole existing for 5 seconds that would destroy the entire earth is probably controlled so it doesn't in this situation. I only want to know how it changes the areas close to it. Lets say the land above is still affected and only the tectonic plates below are held intact.

This is a very important question to me, I'm not giving any of these specifications for no reason. This is not made for the same purpose as the other black hole question, so I do not believe it is a duplicate question and would like it to be reopened. Sorry If I wasn't supposed to change the question that much.

  • $\begingroup$ Hello Sociocat, welcome to Worldbuilding. When you have a moment, please take our tour and read through the first two bullet pages in our help center. It's worth searching our site before asking questions because a great many questions have been asked before. When you search, include "is:question" to narrow the search to just questions. I found that duplicate by searching "is:question black hole appear". I didn't look through the entire list. I wouldn't be surprised if it's been asked a couple of times. $\endgroup$
    – JBH
    Commented Apr 17, 2021 at 19:49
  • 1
    $\begingroup$ I recommend referring to the second answer in the proposed duplicate, it's most useful. $\endgroup$ Commented Apr 17, 2021 at 21:29
  • $\begingroup$ "Small, quiescent black holes can be considered as candidates for the missing dark matter of the universe, and as the core energy source of ball lightning." link.springer.com/article/10.1023/A:1001865715833 $\endgroup$ Commented Apr 18, 2021 at 14:59
  • $\begingroup$ See also newscientist.com/article/… $\endgroup$ Commented Apr 18, 2021 at 15:03
  • $\begingroup$ Seems like the answers can't agree on the mass of the black hole, even when using the same calculator. I'd be wary about upvoting answers which sound scientific, yet can't tell the difference between a radius and a diameter. $\endgroup$
    – Innovine
    Commented Apr 19, 2021 at 8:32

2 Answers 2


According to this Schwarzschild Radius Calcultor, a 0.5mm radius (1.0mm diameter) black hole would have 5.6% of the Earth's mass (so it would be about 4.5 moons).

This is far more mass than is needed to punch straight through the rock below, heading for the center of the Earth, eventually passing right through it and bouncing around inside the crust in a decaying hyperbolic orbit for ages until it settles in the center...and eating the Earth's mass the whole time.

However, the massive seismic waves generated by a point with 5-6% of the Earth's mass passing through the crust will shatter the tectonic plates (granite and basalt are great at compression, but lousy in tension)

Folks in low-lying areas of Cuba and Florida could expect a tremendous inrush of air and water (a quick death by surprise drowning). Folks unlucky enough to be on high ground would incinerate a few minutes later as the shattered tectonic plates no longer insulated them particularly well from the heat and pressure of the mantle.

All other life on Earth would die within the next few days, obviously. In a few years, you might be able to tell where the original continents had been...but then again, maybe not.

  • $\begingroup$ that is all very helpful, but I have a few questions. so firstly, I didnt even consider that the black hole would be affected by earths gravity and go down until now, so thats helpful, but Id like to know why the low lying areas would get an inrush of air and water, is that because the earth in that area will cave in toward that point and be lower? $\endgroup$
    – sociocat
    Commented Apr 17, 2021 at 18:15
  • $\begingroup$ @sociocat tides on Earth are caused by the moon's gravity. A much heavier and much, much closer "moon" would create huge tides, drawing in nearby rock but particularly the faster moving air and water. $\endgroup$ Commented Apr 17, 2021 at 20:26

Using the same calculator as the other answer, but different assumptions: a 1 mm hole with mass 11.3% that of the Earth opens up. I'll say that the portal remains intact until light (and gravity) travel 1 decimeter from the portal - then the mechanism is destroyed and the gravity immediately vanishes once more.

I don't have the general relativity chops to calculate gravitomagnetic effects, so I'll suppose the magic portal doesn't have any. Everything simply gets yanked on with force F=ma for 1E-1m/(3E8m/s) = 3E-10 seconds (0.3 nanoseconds). What could possibly go wrong in 0.3 nanoseconds?

The mass in kg is 6.7E+23 kg ... heh, just over a mole of kilograms - that would be a comical unit for working this type of computation, but, no. Let's just say 11% of Earth's mass has 0.11*9.8 m/s^2 = 1.1 m/s^2 of acceleration. At a distance of 6400 km, that is! Alas, some of the most scenic Florida beaches are a bit closer than this. The strongest acceleration we care about is experienced at about 1 dm (the rest of the material goes through the portal before it breaks). That is 6400E+3m/1E-1m = 6.4E+7 times stronger ... squared (gravity is inverse square law), or 4.5E+15 m/s^2. That means that in 3E-10 seconds it is moving at 1.4E+6 m/s. We don't need to make a huge relativistic correction, but we do need video for our favorite intergalactic PeerTube channel.

(I started the integral over r, but ran out of time before I ran out of issues. Maybe later. But in brief, consider that the pull decreases per r squared but the area of material affected increases per r squared, so the same catastrophe you see at a decimeter is going to be repeated moving outward, in slightly more dispersed form.)

  • $\begingroup$ I'm not very educated in that area so the only thing I dont really know is what E is specifically, but other than that thank you very much for giving me this information. having a black hole exist for a very small amount of time and the effects of the surrounding area from it is exactly what I wanted to find out about $\endgroup$
    – sociocat
    Commented Apr 17, 2021 at 18:44
  • $\begingroup$ Second time in my life I see somebody using a "decimeter" outside of the math textbook. $\endgroup$ Commented Apr 18, 2021 at 12:01
  • $\begingroup$ @DarthBiomech Reading that I thought you had to be American until I looked you up. I never thought there'd be so much variation among metric countries in what prefixes are used! I use it all the time here in Sweden, twice today I think :) $\endgroup$
    – EdvinW
    Commented Apr 18, 2021 at 20:29
  • $\begingroup$ @EdvinW It's just that decimeters always looked like such a useless unit of measure to me. It's not big enough, like a meter, that you can't just opt for saying "20 centimeters" instead of "2 decimeters" and "fifteen cm" sounds better and more readable than "one point five decimeters" $\endgroup$ Commented Apr 19, 2021 at 1:03

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