How long do things take to decompose?


In a moment of idle curiosity, I wondered what would happen if for some reason, human beings suddenly became extinct. Not a cheery thought but I was curious about how long it would be before all traces of our existence disappeared so that a new future intelligent species or maybe extra-terrestrial visitors to Earth would see no trace that intelligent life had ever existed because all the artifacts we created had simply decomposed away. It seems hard to imagine since humans have covered the Earth with our creations but surely all things must pass at some time?

I came across this list put out by the US National Park Service of the times things take to decompose in nature. What struck me was that these times, long by our scale, were not so much when compared to geological times. Glass bottles were the clear winners, taking about a million years to disappear. I thought that plastic would take longer than 450 years and I was surprised that aluminum cans disappear after just 200 years.

This made me curious about how fossils of dinosaurs and even older objects survive for so much longer. It is because the bones have been transformed into rock.

But a buried bone isn’t the same thing as a fossil — to become a fossil, the bone has to become rock. The organic parts of the bone, like blood cells, collagen (a protein), and fat, eventually break down. But the inorganic parts of the bone, or the parts made from minerals like calcium, have more staying power. They remain after the organic materials have disappeared, creating a fragile, porous mineral in the shape of the original bone.

Other minerals reinforce this bone, burning into a fossil. Water gradually makes its way into the bone, carrying minerals like iron and calcium carbonate picked up from the surrounding sediment. As the water penetrates the dinosaur’s bones, some of these minerals precipitate into their microscopic pores. As this process continues, the bone becomes more and more rocklike. It’s like filling a sponge with glue — the sponge’s physical structure stays the same, and the pores and pockets within it fill up. The glue makes the sponge sturdier and more resistant to damage. Large, thick bones, which have more room for mineral glue, make better fossils than small, flat bones.

Over the course of millions of years, the sediment around these reinforced bones becomes sedimentary rock. Erosion, tides and other natural processes continue to deposit more sediment, and this sediment becomes rock, too. As long as they can withstand the pressure from the surrounding rock, the bones remain safely hidden and preserved. After millions of years, some natural process, like the gradual shifting of the planet’s surface, can reveal these layers of rock and the fossils they contain.

So it appears that the best (and perhaps the only) way to identify extremely ancient artifacts is if they have been transformed into rocks that have some shape that is not a random formation.

This aricle by Mayank Vahia of the Tata Institute of Fundamental Research suggests that ‘purified metals’ (which I assume are metals that have been reduced down to the basic elements) and alloys may last for hundreds of millions of years and in some cases re-homogenization may never occur. But how would we know that they were created by a species and not naturally occurring ores?

Of course the conditions matter. Objects that are exposed to the elements are likely to decompose faster than those that are buried deep, and the nature of the surrounding environment, whether wet or dry, likely matters too. But I was curious if there was anything that would survive for hundreds of millions or even billions of years. Will the fossils of human bones be the only things that survive hundreds of millions of years after we are gone or could human-made artifacts also become rocks and survive almost indefinitely?

Vahia thinks some things may survive.

However, the most long lasting impact of human existence, in the form of isolation of different materials and metals is likely to endure over millions of years until the normal geological activity absorbs these materials in the Earth’s crust and sends it down to the magma level where it can be melted and re-mixed. Some of these manmade differentiations may well be permanent, visible to alien visitors even a few hundreds of millions of years later.

The humble staple pin in the Tibetan Plateau may well become a clinching evidence of the one-time existence of humans on this planet several hundred million years after they disappear.

There is of course a great deal of uncertainty about estimates of how long things take to decompose so it may be impossible to get a definitive answer.

Comments

  1. otrame says

    We have stone tools, tools clearly made by an intelligent being, that are at least a million years old.

  2. Trebuchet says

    Purified metals? They corrode. Pure iron won’t last long. Pure aluminum, as in the can example, not much better. Gold artifacts are about the only ones that’ll stay that way.

  3. Randall Lee says

    “How long do things take to decompose?”

    Well,…. see… it depends. But it isn’t very long after we vote for them and send them into some position of government.
    .
    Now of course I’m talking about the brains of politicians. But then what would one expect considering that they are originally chosen from among and by partly decomposed brains in the beginning.

  4. Dan Gerhards says

    There are some things that will last longer than a couple 100 million years: spacecraft like the Voyagers and New Horizons.

  5. Pierce R. Butler says

    Assuming a reasonably sophisticated future alien archaeologist, equipped with comprehensive geological-survey tools, the absence of natural mineral/metal/fossil fuel deposits and veins, and their diffusion/concentration into waste sites, should provide a major indicator of artificial extraction that will last until approximately 100% crustal recycling.

    And the hardware left on the lunar surface, minus a few meteorite strikes, should last until the moon breaks up however-many-million years from now.

  6. says

    This made me curious about how fossils of dinosaurs and even older objects survive for so much longer. It is because the bones have been transformed into rock.

    Glass bottles probably disappear due to abrasion. A glass bottle buried in concrete (or cementified river mud) would probably last as long as a fossil.

  7. Reginald Selkirk says

    Thw World Without Us

    by Alan Weisman
    If human beings disappeared instantaneously from the earth, what would happen? How would the planet reclaim its surface? What creatures would emerge from the dark and swarm? How would our treasured structures–our tunnels, our bridges, our homes, our monuments–survive the unmitigated impact of a planet without our intervention? …

  8. StevoR says

    Plastic as I understand can last virtually forever if its one of themany varieties that doesn’t biodegrade. This is a huge problem especially when it ends up inthe ocean which I’ve read somewhere is actually more plastic than fish and which causes immense needless suffering, death and damage to seabirds and marine life.

    See for instance :

    WARNING :Confronting upsetting images & truths.

    Plastic also fragments into nano-plastics causing chemical pollution as well as the obvious physical hazard.

  9. lorn says

    One of the better films available:
    https://en.wikipedia.org/wiki/Life_After_People
    The whole thing is available on YouTube at a very reasonable price.

    Certainly some things will simply fail to decompose on anything less than geologic timescales. Gold, and some silver, artifacts will potentially last longer than the earth has left as an ecosystem. Glass and ceramics will mostly make it, but not necessarily in one piece. Primitive baked clay tablets easily outlast optical or magnetic media. Certain plastics, like Teflon and Kynar, and anything encapsulated in them, will hold up well, if not indefinitely.

    I’ve pulled out plastic and ceramic coated deck screws about, according to the homeowner, twenty years after they were used and they were like brand new after being exposed to wind, rain, sunlight, and corrosive salts in pressure-treated lumber.

    A whole lot of materials can last surprisingly long times if protected from the deleterious effects of water, or oxygen, or alkaline materials. Hoards of Roman iron nails have survived for well over a thousand years like new in favorable conditions. The iron on the outside of the tightly packed hoard were corroded but had, evidently, absorbed almost all of the available oxygen. Those in the center looked like they were made yesterday.

    Many materials submerged in clay or under water in areas with little free oxygen might remain in good condition. Similarly geosynchronous satellites and space probes can be expected to remain recognizable for a very long time. Robotic probes on Mars are going to be evidence of intelligent life for thousands of years. Climate run-away and traces of nuclear fallout might end up as clear evidence of our relative lack of intelligence.

    I suspect that some things will decompose but remain detectable signs of intelligent life and industry for a very long time.

  10. sonofrojblake says

    Some of the comments here are evidence of our inability to understand geological time.
    Post 1: “at least a million years old”
    Post 9: “Plastic as I understand can last virtually forever” (see original post’s figure of 450 years).
    Post 11: “Certain plastics, like Teflon and Kynar, and anything encapsulated in them, will hold up well”.
    Also “twenty years after they were used and they were like brand new”, “nails have survived for well over a thousand years “, “Robotic probes on Mars are going to be evidence of intelligent life for thousands of years”

    It’s not (I think) particularly interesting to wonder what would be observable post-sudden-human-extinction after ten, a hundred or a thousand, or even a million years. As has been accurately pointed out, we’ve certain knowledge of human toolmaking from twice that long ago.

    What constitutes an interesting gap is obviously arbitrary, but to me a good candidate for a minimum is the date since the dinosaur extinction – 65 million years. And when you’re in that ballpark, it’s no good looking at the moon to preserve our legacy – it’s expected that in between “just” 10 to 100 million years, meteorite impacts will have completely obliterated any trace of human activity. For comparison, one of my favourite “dinosaurs” when I was a kid was the sailbacked Dimetrodon, which died out 40 million years before dinosaurs ever turned up – over 270 million years ago. The stuff on the moon isn’t going to last half that long…

    And now I’m a bit dizzy and need a lie down.

Leave a Reply

Your email address will not be published. Required fields are marked *