I'm considering a worldbuilding exercise focusing on a "second generation" civilization, which arises after a previous civilization from millions of years ago either died off or left the planet. One of the biggest questions, which I haven't found a good answer for yet, is what would happen to that previous civilization's plastics, specifically what transformation they might undergo after millions of years. If ancient organic material gets us oil, which gets us petroleum, which gets us plastic, what does plastic "get us"?

Would plastic still just be plastic, maybe more condensed or weakened, or would it undergo as dramatic a change as ancient plankton did when it became oil? Would plastic break down and turn back into petroleum?

Though I don't have an exact timeline locked down, for the sake of this question let's assume 20ma between the first civilization beginning plastic production, and the second civilization finding it (or its by-product) and, perhaps, beginning to mine it.

Obviously any answer here would be speculative, but I would like that speculation to be as closely-based on real geological and minerological processes as possible.

Thanks in advance!

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    $\begingroup$ Most plastic on land will get eaten by bacteria. Salt water is not a great place for those bacteria, but eventually the plastic will sink (possibly becoming fossilized) or wash up on a shore and eventually get eaten by bacteria. The bacteria that do this mostly need fresh water (in small amounts) and oxygen. Even the oxygen is optional. Excavated landfills from the sixties have disintegrating plastics next to still-readable discarded books and paper. So most of it will get consumed or converted to methane. $\endgroup$
    – DWKraus
    Commented Aug 26, 2022 at 21:39
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    $\begingroup$ Most kinds of plastic won't last all that long. In a few hundred years most plastic will be carbon dioxide and methane. A puff of gas. $\endgroup$
    – AlexP
    Commented Aug 26, 2022 at 22:02
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    $\begingroup$ @AlexP It should also be pointed out just how long 20 million years is. It's a really long time in terms of degradation of matter. All the most hazardous radioactive waste is gone after that long. $\endgroup$
    – DKNguyen
    Commented Aug 27, 2022 at 5:33
  • $\begingroup$ You didn't do a simple search first did you? How long does it take for plastic to biodegrade? - SaveMoneyCutCarbon $\endgroup$
    – Pelinore
    Commented Aug 27, 2022 at 9:18
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    $\begingroup$ Here's another How Long Does It Take Garbage to Decompose? . lots of sources out there that answer this for you . I've not read either of those . some are less trustworthy than others of course but judging that is normally relatively easy . a useful rule of thumb, the more hysterical or judgy they sound (rather than just providing dry facts) the less reliable they are. $\endgroup$
    – Pelinore
    Commented Aug 27, 2022 at 9:27

4 Answers 4


We are already seeing some fungi capable of consuming plastics, especially in landfills - https://www.colorado.edu/ecenter/2021/11/04/plastic-eating-mushrooms

If we imagine large reservoirs of plastics being left behind by an extinct civilization, it seems likely that fungi specializing in eating those plastics will develop, as well as fungi specializing in eating plants and animals that die from the ecological disruption plastics can cause, especially when eroded over time into microplastics. So, lots of energy for fungi, both in the forms of plastics, which nothing else can eat because it kills them, and in the form of dead stuff killed by the plastics. Fungi explode into a myriad of new shapes, sizes and types.

Especially considering what other pollutants the extinct civilization might have spread in order to produce those plastics, it seems like the far-future ecosystem would be very different. Fungi might come to hold an entirely different niche, while plants and animals might rely on different survival strategies depending on the damage done to water, carbon and nitrogen cycles. Whatever intelligent life comes to inhabit this new world, they probably have an important relationship with fungi; and there would probably be a lot of interesting fungi from which to choose. It could even wind up being an important turning point for them when they realize that a fungi important to their survival, or maybe even just to their culture, is dependant on plastic remnants, and those are in fact a limited resource that will inevitably run out.


Plastic is hydrocarbons so technically decomposes and degrades just like organic matter. The main difference is that there aren't that many microrganisms that can break it down...yet. Like cellulose/wood during the Carboniferous.

So I guess you have about the same chance as it turning into something coal, oil, or some other fossil fuel similar to like what happened with the pioneering woody plant . Except a lot more cellulose was ever produced than plastic.


some of it will breakdown some of it will work its ay into the mineral cycle.

We already see this, some plastic gets broken down by chemical or bacterial action, forming hydrocarbons or biomass. But some gets incorporated in to rock, natural rocks made of plastic have already begun forming. Plastiglomerate are just one example. As more and more plastic makes it way into the ocean you will likely see it becoming a measurable component of sea floor sediment. We even see plastic particles get frozen in Antarctic ice.

It is important to realize not all plastics are equal some are incredibly stable molecules other breakdown quite readily.



While breaking down some expanded polystyrene foam (i.e., Styrofoam) packing material to fit it into the trash bin, I watched how easily the little beads broke free and were carried off by the gentlest of breezes. I imagined a world of the future that's subject to dust storms, but where the dust is actually bits of plastic.

Just as ocean currents have created "islands" of plastic, wind patterns might cause plastic dust to accumulate in certain locations. Occasionally, a large windstorm would sweep through one of those spots and carry tons of plastic dust through the air. As blinding as a heavy blizzard and as bad to breath as the smoke from giant wild fires.

Now we have microplastics, not just from the chipping away of macro-sized plastic objects, but made specifically for abrading, sanding, exfoliating, and even for use as a laxative. Microplastics are now part of our food chain. In the past couple years, researchers have found plastic in human excrement (even from those who don't use a plastic laxative), their bloodstream, and even in placentas. Virtually all humans will now have plastic circulating in their bodies from the moment they are born.

When the humans die out and a new species forms a civilization, it seems likely they will have adapted to the fact that plastic is part of the ecosystem. Perhaps those adaptations will be the key to them rising up to become the dominant species on the planet.


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