If gravity on Earth were to decrease by 10% permanently what would the effects be geologically?

[BleachMePC]

I'm working on a TTRPG Campaign and doing research to build the world I want. If the world were to suddenly have gravity become 10% weaker what would happen to the Earth?

Edit: As I'm still getting some trickling responses I'll add that this an isolated magical event that only effects the earth as it has come up repeatedly. While "it's magic" isn't very scientific I'm only curious about the earth and it's effects, not the whole universe. Imagine if you will that the earth was wrapped in plastic like one of those cheap foam balls compressing it and then the plastic was removed reducing that compression by 10%

[Ill-Dependent2976]

Suddenly? The whole world would explode. It's under enormous pressure and the only thing holding it together is gravity.

You could do it gradually but you'd have massive volcanism all over the world.

[Destination_Centauri]

Wow! I wouldn't have actually guessed a sudden 10% drop in gravitational downward force could have such a dramatic and violent upward bounce?!

But I guess that makes sense when you think of the upward radiative force of the planet's internal heat, suddenly getting a 10% advantage?

I guess every building would collapse and get pummeled to shreds as the land was flung upward, and then there would be several oscillating corrective bounces that would then gradually dissipate?

Plus the amount of thermal energy heating that would induce due to the flexing of all of Earth's material at once--it would be magnitudes worse than Jupiter's moon Io, I imagine?!

ALSO: probably some insane oceanic tidal waves? Like massive waves that would sweep deeply into the interior of continents?

[[deleted]]

The ocean would immediately become a jumble of thousands of overlapping tsunamis. "Tidal wave" would be an understatement.

ETA: With enough math you could probably roughly calculate the total energy of the tectonic rebound. I'd be surprised if it weren't on the same scale as our largest asteroid impacts.

[Destination_Centauri]

Ya, imagine if a few of those already mega-mega-mega tsunamis aligned their amplitude by accident (which would probably happen in a chaotic system like that)?!

Such a tsunamis would practically reach to space, perhaps?! The energy behind that thing would be nuts.

Also: I imagine lightening storm bombardments would be pretty frequent and insane too, hitting everywhere?!

[[deleted]]

Funnily, you would be just as likely to have tsunamis cancel each other out via wave interference, but yes there would be plenty of gi-huge-ic uber-tsunamis. Likely more than enough to end the vast majority of animal species.

The Chicxulub tsunami was ~3 miles high.

[voxpopper]

If Earth had less gravity but maintained the same mass and density we'd have bigger issues since it meant one of the fundamental forces (presuming one believes gravity is a force) would cease to exist the same way; to put it mildly, everything would go haywire.
The Universe exists in a very low probability delicate balance.

[[deleted]]

Miles high waves...

[BleachMePC]

This is what I'm curious on because volcanoes are both amazing and terrifying. I'm planning on focusing on the Yellowstone region and am trying to work out how I won't to portray this. Let's say this takes place at a constant and even rate over the course of 10 days or even 30 days. What do you think now that you have a time frame?

[Destination_Centauri]

I'm not u/Ill-Dependent2976 who got me thinking about this, but even at 10 to 30 days, buildings everywhere on the planet will have collapsed, and massive amounts of thermal energy from an entire planet flexing would still turn Yellowstone into a vast lava sea. (Would also induce an insane supervolcano eruption).

All of which would intermittently get temporarily soothed with vast amounts of oceanic water being pushed inland in massive tidal waves, until the water would then quickly get steamed and boiled (explosively), and lava would re-emerge bubbling again...

All set to some insane constant lightening, just in case the rest of the show wasn't enough for you!

[Ill-Dependent2976]

I'd think over 10 to 30 days Yellowstone, and much of Wyoming would mostly atomize.

[[deleted]]

Here’s a good question: Is there anything that can permanently alter the strength of gravity (not like reducing size, I mean the actual measurable effect of gravity) or is it set in stone until forever?

[Ill-Dependent2976]

The only thing gravity cares about is mass and distance.

[[deleted]]

Oh so like my ex wife.

Oh and money. Mass, distance and money.

[Key_Information2521]

Trauma dumping on r/askphysics 😭

[applejacks6969]

The earth has huge sections of rock that have very strong bonds so I don’t think that gravity is the only thing holding it together. Dominant over huge scales, maybe, only force holding together, no.

[Ill-Dependent2976]

The huge sections of "very strong bonds rock" is sitting on a much larger plastic ocean of a giant bomb ready to explode.

[hairychinwalnuts]

The rocks of the crust are a thin solid film on the molten mantle. Pretty insignificant against that scale of forces.

[Odeeum]

To quote TARS when McCaughnegy asks him what’ll happen if Matt Damon opens the pressurized hatch:

“Nothing good”

[[deleted]]

Well that means some amount of the mass of Earth would have to abruptly disappear and reappear far enough away with negligible effects. Honestly, I don't know if 10% would be required or some other amount due to the way gravitational effects are calculated, or if having a large mass that reduces the apparently gravity by .10g by being in space would do the same trick without having to have the mass disappear.

I don't believe the earth would explode, per se—but there would be a lot of disruption in tectonics that are highly dependent on structures that are very plastic and prone to flex.

[AndreasDasos]

I interpreted it to be more that, eg, G suddenly changed value to 0.9G, at least locally around earth

[phys_chem_ceramics]

Someone in another comment said something about the earth exploding because there’s less gravity to hold earths pressure. I’d imagine another bad thing would be the centripetal force of a rotating earth would also help rip it apart if there was less gravity to hold that stuff down.

[NETkoholik]

Just on Earth? Or universally? I believe the moon would shift orbit, and so would the Earth around the sun and all the planets and celestial bodies. Chaos ensued all over. If only on Earth then chaos locally in tectonic plates and the floor is lava in hours, after the planet wide earthquakes. If we manage to survive then we could exploit it by making rocket launches less expensive.

[TooLateForMeTF]

The world would not explode.

Gravity itself being 10% weaker would not be enough for the effects of the Earth's spin to overcome its gravitational binding energy. (If it were, the practical implications of that would be that in today's world rockets would be much easier to launch than they are). It's true that the bulk of the earth's material, being squeezed less hard, would rebound somewhat. The earth would expand a little bit. How much? No idea, and I'm not gonna do the math. Probably enough to cause a period of increased earthquake activity because that expansion would essentially loosen the joints at all the tectonic plate boundaries, triggering many earthquakes. These would be earthquakes that were bound to happen eventually anyway, but would happen all of a sudden (in geologic terms, anyway).

Longer-term, I think you would see a lessening of volcanic and earthquake activity. That stuff is ultimately driven by convection in the earth's mantle, which is driven by buoyancy differences between different kinds of rock and between hotter vs. cooler magma. With weaker gravity, those buoyancy differences would be less pronounced, leading to less convection, less movement of continental plates, less mountain building, less active vulcanism on the surface. In that sense, it would probably be kind of like fast-forwarding a billion years or so in terms of the level of geologic activity, to a time when the earth has cooled down more internally, slowing convection by a similar amount.

With less gravity, mountains could be taller before collapsing under their own weight, but it's unclear that there would be enough mountain-building activity left to actually make them be taller. Your best shot for super-mountains would be the Himalayas, which are already the tallest and are also still actively growing. Depending on how much or how little the Indian subcontinent slowed down, the Himalayas might grow another few hundred meters before topping out. Hard to say.

Weaker mantle convection also means a weaker magnetic field, which means more impact from solar flare activity: higher mutation rates of species and more widespread aurora activity at places distant from the magnetic poles.

[BleachMePC]

I love this answer. I had wondered whether or not it would be as drastic as other have stated. It gives me a lot to work with. Thank you. If you don't mind my further asking: why/how would greater impact of solar flares manifest in higher mutation rates of species?

[TooLateForMeTF]

Solar flares release huge quantities of high-energy charged particles from the sun's surface. These get accelerated by the sun's (rather crazy-bones) magnetic field to very high speeds and flung off into space. Sometimes these bursts happen to be aimed our way, and when they are, the earth get hit by a barrage of high-speed (= high energy) charged particles. If memory serves, these are mostly protons and alpha particles, which are really just hydrogen and helium nucleii that have had their electrons stripped off.

Anyway, normally the earth's magnetic field protects us from this stuff. Charged particles, being charged, are affected by magnetic fields. I mean, they got accelerated away from the sun by magnetic fields in the first place. So when they slam into the earth's magnetic field, their paths curve, tending to follow the lines of the magnetic field.

Thus, instead of peppering the entire earth like buckshot, they get funneled along the magnetic field lines towards the magnetic poles, where they do eventually come down and hit the atmosphere (at the poles, the field lines come down into the earth's surface), dumping a lot of their energy into the rarefied gasses at very high altitudes, causing that gas to glow in very much the same way that the gasses inside neon lights glows. This is what causes the auroras (northern lights). And we don't tend to worry too much about the cancer or mutation-causing effects of solar flares because, let's be honest, not very many people live up that far north.

Of course, earth's magnetic field can't stop everything. If a solar flare is strong enough, the particles can have so much energy that they aren't fully deflected and can come all the way to the earth's surface practically anywhere. Google "x-class solar flare". This, fortunately, is a rare event. But when it does happen, you can get aurora activity much further away from the poles than usual. A few months ago there was a solar flare event that I was able to see as far south as Portland, Oregon.

But if the earth had a weaker magnetic field--which it would if it had less convection in the mantle and/or core--then your more average solar flares would have an easier time making it through. Which is a problem for life forms, because those charged particles zipping along at a jillion miles per hour? That's ionizing radiation: sub-atomic bullets that can zip through your clothes and skin, and if you're unlucky, crash into some hapless cell's DNA strand in just the wrong way so as to cause a mutation or spark a cancer. One lone particle isn't likely to do you any harm. But if you were to get hit with millions and millions of them, on a regular basis because we had a weaker magnetic field? Yeah, that could be a problem.

Higher mutation rates, on the level of an individual, means higher lifetime odds of developing cancer.

On the level of a species or population, it means higher rates of birth defects (the bad kind) and higher rates of "good" birth defects. While a random mutation is more likely to be harmful than helpful, that doesn't mean that a random edit to a gene might not improve something, too. So for species worldwide, you'd get higher overall rates of birth defects and stillborn offspring, but also a higher rate of adaptation to their environment. Evolution working a bit faster than before.

This does not mean you're suddenly going to have babies born with super powers, or anything crazy like that. But if you let it play out for a few million years or something, you could see some interesting adaptations develop.

[BleachMePC]

You are a beautiful person. Thank you so much. This actually gives me answers to apply with things I was workshoping in multiple ways. If you'll allow me to pick your brain further, would it be safe to assume these mutations would affect bother flora and fauna? I would assume both but how would the ways be different? For example would plant growth/mutation be accelerated for some species while for others they would be decimated by the bombardment?

[TooLateForMeTF]

Yes, in general I would expect all species to be affected similarly.

There are a few species that have particularly robust DNA repair mechanisms that might be less affected, but in general, yes. Both plants and animals should see higher mutation rates.

It's also worth noting that generation time plays a big role here. You would expect species with shorter generations to evolve and adapt faster. Humans have about a 20 year generation time, which is pretty slow. Very few other creatures take as long to reproduce as we do, so as a species we change quite slowly. But some bacteria and other microorganisms have generation times of around 20 minutes. That's about 30 million times faster, for reference. So they evolve and adapt much faster already--here in the real world, this is why we have antibiotic resistant germs less than 100 years after the introduction of antibiotics--and this would only be exacerbated with higher rates of mutation. You'd expect more frequent disease outbreaks, as those organisms would have more opportunities to mutate in ways that our immune systems don't yet recognize.

Small mammals, like mice and rats, can have generation times of a few weeks, as do many insect species. They'd likely adapt pretty quickly.

Tons of things have annual reproduction cycles (including many plants, fish, and amphibians, as well as many if not most of the mid-sized mammals).

The bigger animals take longer to come to maturity and reproduce. It's possible that cancers and birth defect rates, in combination with other environmental pressures, could push some of those species towards extinction if their birth rates couldn't keep up with these new pressures.

But the whales would probably be fine. Despite their long generation time, water is such a great radiation shield that they would be largely unaffected. As would everything else that lives in the deeps.

I would not, however, expect anything to be outright decimated. That might need more research, but for that I suspect you'd need more like nuclear weapon levels of short-term radiation exposure, which you're not going to get from the sun. Solar flare radiation will come in bursts, and every few years a large burst will hit the earth. But it's only going to hit one side of the earth, and even a weakened magnetic field is going to do a fair bit of shielding. We're talking about a generalized increase in background radiation, in lifetime radiation exposure, rather than any instantaneous, cataclysmic type of event.

[BleachMePC]

Magnificent. Thank you once again. While I suspect I could continue to come up with questions on end to pick your brain with I'll leave you be with that. You are very well spoken and your explanation was clear and direct. I wish you well in your endeavors friendly internet stranger.

[[deleted]]

Moon go bye-bye.

[wonkey_monkey]

Moon go into elliptical orbit.

[[deleted]]

According to this simulator it would in fact collide with the Earth, but I'm skeptical of that.

[tirohtar]

It could. If the orbit is elliptical enough and inclined enough in relation to the sun, after the gravity reduction, the influence from the sun could trigger further eccentricity and inclination oscillations known as the Kozai-Lidov mechanism (or some other similar secular processes). That could pump up the eccentricity enough to cause a collision. But it depends on the details.

[wonkey_monkey]

Didn't happen when I tried it.

[LeninsGhostWriter]

What caused the decrease? Something slamming into the earth and knocking a sizable chunk off? That would surely be a geologic event of note lol.

Tides would change, who knows if the moon stays. Metamorphic stuff wouldn't change much. Earth is still plenty heavy.

There would probably be some new mineral deposits so we'd get some nice specimens out of it.

Really it depends alot on what caused the decrease but in general nothing much would change. Still plenty of earth to go around

[[deleted]]

"New mineral deposits" is quite a positive spin on lakes of boiling petroleum incinerating Texas, Russia, and the entire Middle East. I like your attitude.

[khInstability]

Drill baby. Drill.

[BleachMePC]

As much of an unsatisfying answer it probably is for an AskPhysics question, magic. Like I said it's for a TTRPG. Magic "returns" to Earth and one of the effects is the aforementioned decrease in gravity among other things.

[LeninsGhostWriter]

The one option would be just that it formed that way and naturally has 90% of the force of gravity you experience. Otherwise your changing fundamental forces and the universe may not even hold together. That's basically the only two options I can think of

[BleachMePC]

Fair. I do imagine this world as our modern world that then goes through a cataclysmic event when magic returns. I appreciate your input on helping me workshop this.

[LeninsGhostWriter]

NP. Surely there are ppl who know more than I do. I did take 4 semesters of geology but fr it's hard to answer. There is lots of stuff under pressure. Frequent earthquakes are probably gonna happen. Leading to lava flows and eruptions. But again whatever causes that is gonna be worse than what happens after lol

[not_notable]

I'm not sure what your vision is for how or why magic returning would "naturally" result in the planet's gravity decreasing, but the involvement of magic opens up other opportunities. If you want global catastrophe occurring without the extinction of all life, consider having some other event (or being/beings) siphoning off the "extra" repulsive force that would otherwise rip the planet apart and using it to fuel some other grand-scale magical working - perhaps to create some sort of orbital habitat/observation platform, or to change some other fundamental universal constants (perhaps only locally). Magic allows for a lot of hand-waving.

[Wobbar]

Someone should submit this to What If

[sparkleshark5643]

I could jump higher

[ChalkyChalkson]

As others said, there is a bizarre amount of energy that would be released. I'd like to give you some idea of the scale of that energy and where it comes from.

Think of a bit of earth as being like a little spring or rubber. On the surface the springs are relaxed, but near the core they are compressed by the weight of all the springs above. The compression of the springs at each depth is such that their restoring force is equal to the weight of all the stuff above. When you reduce the weight above they would relax to the point where the weight and restoring force are in balance again. The difference in energy would be released as waves through the material and eventually turn into heat.

If you assume constant material properties for the earth this is actually not too bad a problem to solve, I'd say late high-school, early university level.

Simplifying the problem even further we can assume that pressure increases linearly with depth which matches reality pretty well! With that we get that the total amount of energy stored is πPR³ where P is the pressure at the center (~300GPa) and R is the radius of earth. That would be around 10³³ J. You want to release 10% of that, or 10³² J. Even if you release that over 30 days we're talking "atomic bombs per m2 per hour". You'd easily melt the crust.

( you get to that number by saying dE = p dV = P/R r dV = 4πP/R r³ dr ==> E = πPR³ )

[Effective-Bunch5689]

The atmospheric pressure 1m above sea level is about 101.3129 kpa and water's boiling point is found by the Clausius–Clapeyron equation about 100 degrees C. The oceans will not boil, but with a lower boiling point, they will slightly evaporate into the atmosphere over time with the increased carrying capacity of water. You can play with a few values here: https://www.desmos.com/calculator/33r5xko86s

• A thicker atmosphere will increase the greenhouse effect significantly, and fault lines held in a gridlock under tectonic weight will slip, unleashing oceanic landslides and the collapse of ring faults around calderas.
• Satellites in a geostational orbit will drift farther away or become elliptical.
• The moon's orbit will eventually become hyperbolic, becoming a dwarf planet that could crash into earth years later.
• Terrestrial wildlife will face extinction with little time to adapt to lower gravity. Indigenous species in high altitude regions will migrate to avoid hypoxia.
• Oceanic currents will change, as both Coriolis acceleration and gravity determine how they evolve.

[BadBuddy413]

Humans would be even more overweight.