I’ve thought about this question a lot. As an archaeologist who has done quite a bit of Roman-period archaeology (including at Pompeii) and artifact conservation, I can definitely say that, unless unusual depositional or environmental factors are present, time is VERY effective at erasing traces of humanity. Without being suddenly or catastrophically buried (by a volcanic eruption, for example), most materials (including architecture) degrade very quickly.
In most circumstances, a modern concrete-and-steel city in a temperate climate would in my opinion become unrecognizable as such by about 1000 years after abandonment: by a few thousand years after the funpocalypse, most structures would either be buried or eroded, and the corrosion of the steel and crumbling of the concrete would render most buildings into shapeless masses of rust and rubble.
Most metals degrade very rapidly except under very particular conditions (such as highly arid or anoxic conditions); silver and gold are exceptions. Glass and stone objects are very tough and often survive indefinitely. Landfills and other major garbage dumps will probably be recognizable as such a million years from now largely from the glass; most everything else will likely have become unidentifiable sludge.
One work of humanity that would be easily recognizable for a surprisingly long time is the road system: in many forested areas, these would be visible for some millennia due to the shallower soil buildup, which would lead to relatively stunted foliage; this would be visible from the air as unnaturally straight lines of less developed woodland.
A lot of artificial materials would disappear. Plastics, for example, would probably be consumed by new strains of bacteria. Metal would last an exceptional amount of time, though.
What would last the longest of the great human endeavours? I didn’t think of satellites, but space junk would be up there for a long time. Would fallout shelters and other secure bunkers (dare I say Vaults ) last?
Uranium 236 has a half-life of some 20 million years, and is a by-product of nuclear reactors. So aliens looking for signs of previous intelligent habitation might find higher then expected traces of it and its daughter elements.
And of course we dig up fossils from hundreds of millions of years ago (billions, according to wiki) from creatures that were far less numerous then humans are. So I suspect our alien archaeologist may do the same with human remains.
This questions seems to be asked about once a year and the speculation is always interesting. One thing that is usually brought up and seems the most plausible is that evidence of mining would be the most obvious evidence of humanity millenia after we disappear.
All the earth’s crust eventually gets subducted down into the mantle again. That would be a pretty thorough eraser of anything and everythng on the surface. But how long it’ll take for some city at the center of a plate to migrate to the edge & subduct is not a question I can answer.
Thanks for a great reply. The bit about the roads is particularly interesting.
Will glass outlast things such as high-end drill bits on fancy metals? Medical prosthetics? I know they are still metals, but they seem to be alloys specifically designed to resist corrosion.
I’m not certain about modern high-falutin’ alloys, but stainless steel will definitely corrode given enough time and regular steel often turns into a lumpy mass of pure rust after less than a century of burial. Possibly esoteric alloys, etc. would survive, but given enough time very few materials are that stable. Glass survives because it’s just silicon and rocks, of course, hang around for billions of years.
More than mere millennia. Weisman wrote that some tools would last so long that new species would have time to evolve to sapience and dig them up and use them.
Another incredibly long-lasting artifact would be Mount Rushmore. It was carved out of solid granite in an area with minimal geological activity. Weisman said it will still be a recognizably artificial structure millions of years from now.
There’s lots of crap strewn about down there (albeit not very densely); however the likelihood of recovery is IMO pretty poor.
Everything we leave behind, on land or in the sea, will be in either erosional or depositional environments, both of which pose recovery issues some millennia down the line: in depositional environments (like the bottom of a hole), our junk would be buried in sediment, often very deeply, and therefore less likely to be recovered; very deep excavations are hazardous and difficult (Oak Island, anyone?). Erosional conditions (like a mountaintop) are hazardous to artifacts both in terms of preservation due to the exposure to the elements, and also to in-situ recovery, since in time they’ll tend to effectively roll down into a depositional environment.
Deep-sea environments pose the same problem, particularly in the case of depositional environments; ocean basins tend to form veeeerrry deep deposits, particularly of silts, sometimes in quite short periods of time; there a few supposedly treasure-laden 18th century shipwrecks off the California coast that, even though their general locations are known, are thought to already be buried in many meters of sediment.
So you might end up with well-preserved stuff down there (though many materials actually preserve less well in oxygenated salt water than on land) but it might be very sparsely distributed in one thin layer of a vast amount of mud or, given enough time, of rock.
I have ordered Weisman’s book. In the reviews it mentions that things made of bronze might last in recognizable form for millenia. I find this at odds with steel corroding in centuries. What is it about bronze that would make it outlast steel?
HC pissed me off when they started airing that “Men Who Killed Kennedy” nonsense. I started watching it out of curiousity and it was 10 times worse than Oliver Stone ever thought of being. They had people stealing Kennedy’s body and altering the wounds to fit with the Warren Commission theory.
Then last night I watched “End of Conspiracy” or some such thing which made every conspiracy argument about the Kennedy Assassination seem silly. They debunked all of the arguments one by one in a scholarly and thorough way.
They have had a lot of good programs in recent years. They just need to get rid of all of the UFO crap.
Iron is a great material for tools, machinery and construction when pampered, but once left on it’s own will degrade faster than most. When rust sets in, any temperate-region air will contain enough moisture to keep the corrosion advancing at a rapid pace. In the archeological record, things like (unglazed) pottery, burnt wood, horn and bone, any stone and any other metal will outlast iron implements. Just leaving a carbon-steel blade inside a mildly moist sheath will visibly corrode the blade overnight. Leave a wet bronze blade lying around and it will be almost pristine decades from now.
The thing that makes corrosion so bad for iron is that rust begets more rust. When an iron object rusts, bits flake off or bubble, exposing more surface area, and allowing for more rust to form. With many other metals, though, the oxide that forms produces a thin layer which protects the underlying metal from further oxidation, so corrosion is limited until some mechanical process exposes more metal.
When Gutzon Borglum sculpted Mount Rushmore, he was informed the granite erodes at a rate of about one inch per 100,000 years. Borglum is reported to have said he added another foot to Washington’s nose so as to “give him another million years.”
As mentioned, the key issue is how interested and informed the observer is and what resources they have. If the question is “How long before an average North American wouldn’t notice anything while casually walking across the site?” I’d wildly guess between 150 and 600 years for a suburban development in the temperate U.S. (somewhere with reasonable rainfall; a freeze/thaw cycle helps too), and 1,000 for a real urban area. All bets are off in real desert areas.
If the question is “How long before a reasonably funded archeological expedition with current technology could find conclusive evidence?” the answer probably is “until an ice age pushes glaciers across the site or a fault drops the area into the sea.” For intermediate levels of resources and expertise, the answers will be in between.
For instance, in New England there are lots of farmsteads abandoned between 150 and 100 years ago. Casual walkers might not notice them, but even without digging, a careful observer can discern the not-quite-natural hole that was a cellar, take note of the surviving apple trees, trace the ragged line of rocks that was a wall, and possibly make out the path of an old road. Tree growth patterns can be made out after 150 years, too. It would take at least 300 or 400 years to create a northeastern forest indistinguishable by an expert from true old-growth (and maybe not then).
Longer in the west of North America. (“Hmm… why is there one isolated stand of 1,000 year old redwoods, but all the rest are only 500 years old max?”)
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