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I've seen questions here about what cities would be like after being abandoned for a thousand years, but what would be left after timescales of tens of thousands of years? Ancient settlements like Catalhoyuk or Gobekli Tepe can provide some answers, but these were made from stone; buildings made from materials like concrete (reinforced or unreinforced), steel and glass are a much greater mystery to me.

In this scenario, let's assume that a monumental portion of humanity was killed by a mysterious disease, and the humans that were left decided to go nomadic rather than live with the diseased corpses. The cities were therefore left mostly untouched for the next many thousands of years, where humans are still living nomadic lifestyles. Let's also assume that over the course of these thousands of years, there have been no major catastrophes except for those that already occur on a cycle.

I am asking this question because I am unfamiliar with the effects of time on a city over such immense timescales, which many of the questions here don't cover.

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I'm no expert, but to the best of my knowledge, depending on the construction materials and the environmental conditions...

  • building paint can wear enough to significantly expose the materials underneath in as little as 10 years.
  • The best roofing I'm aware of comes with a 100 year warranty, and is made of metal. Assuming every structure has the best roofing (they don't), we can expect a small amount of rust to develop within 300 years. Rust starts off very slow, but then speeds up exponentially as it spreads to affect more surface area. I'd give the metal structuring on the best roofs less than 1000 years before the rust has developed significant breaches in the roofing. As it decays it will expose whatever was under it.
  • Once the roof has holes in it, water that drips into the building will survive longer than water that dripped onto the roof, and so rust buildup on interior metal fixtures will go quicker than that on exterior metals after it starts.
  • Wood left untreated can deteriorate in under a hundred years if termites get into it, and especially if it is bearing significant weight.
  • In spite of whatever your school taught you about the time it takes plastic to degrade, the plastic siding on your house will only have about a 15 year warranty on it, because plastic becomes brittle and starts to significantly decay into dust in as little as 50 years with regular exposure to the sun.
  • Foams and insulation will also degrade quickly when the sun touches them. I'd also give these about 50 years after start of exposure.
  • Drywall doesn't stand up well to water and wind either; probably less than 50 years after start of exposure.
  • Stone fixtures will be the last to go, because they don't rust and they aren't hurt by sunlight. Likely, any nice & sharp 90° edges around the structures will wear away within the first 1000 years or so after exposure, especially if it rains a lot, or if you live in an area with violent wind/dust storms. The rest of the rock will wear away over a much longer time. Given that, as far as I know, modern cities don't use stone in their walls, the elevated stone will probably break up quickly as its support structure collapses and bends underneath it. The foundations will be buried under a thick layer of dust, and will be likely the last things to break up.
  • Glass will break up as the structure around it bends.

So, keeping in mind that the outer layers will have to decay before the interior ones do, I give our skyscrapers, unmaintained, 5 to 10 thousand years before they are difficult to distinguish from the surrounding landscape. In 50000 years, an archaeologist might think herself lucky to find a slab foundation buried deep in the ground with a few mostly-decayed pipes encased in it.

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    $\begingroup$ Glaciers will smash human edifices. Changing sea levels will drown coastal cities...or bury them in sediment miles from the new coast. Tsunamis and cyclones will also, over time eventually wash away many seacoast cities. Rivers will change courses, carving new paths, making and breaking lakes, and eroding away high-elevation cities. Wildfires will take a toll. Fun! $\endgroup$ – user535733 Jun 14 at 0:13
  • $\begingroup$ Sounds about right. Nothing left except the hardest underground stuff. $\endgroup$ – Bilbo Baggins Jun 14 at 4:19
  • $\begingroup$ That was a really generous estimate, though. It could be as quick as 2000 years or less... $\endgroup$ – boxcartenant Jun 14 at 23:06
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Assuming an extinction level event takes place today, the majority of our cities will vanish withing 25 years. The vegetation will cover/bury everything except the tallest of buildings.

Quick timeline of "the day after":
A few hours later: Most of the lights around the world would shut off because power plants would run out of fuel and solar panels would get covered in dust. The only power plants still running would be hydroelectric stations.

Two or three days later: Most underground train systems would flood because the pumps keeping water out would stop working.

Ten days later: Pets and farm animals would die off while packs of big dogs would form to hunt down other animals.

A month later: The cooling water in nuclear power stations would have evaporated. This would lead to a series of disasters across the world, stronger than Fukushima and Chernobyl.

One year later: Satellites around Earth will start to fall from their orbit, creating strange ‘stars’ in the sky.

Twenty five years later: Vegetation will cover the world with some cities being buried in sand.

Three hundred years later: Metal buildings, bridges, and towers will start to break apart because of corrosion.

10,000 years later: The only evidence of our existence will be the things we’ve made with stone like the pyramids in Egypt, the Great Wall of China and Mount Rushmore.

Personal Opinion:
I don't think the great stone structures will survive either, after 10.000 years. Lets check on the existing mega-structures (pyramids, great wall). Although not as old as 10.000 years, both structures get periodic maintenance from the humans (specially in the last century). Left untenanted, i guess they will collapse at some point, due to weather/moisture/vegetation.
So for a far-future archaeologist (50.000+ years), best case scenario is to find some ruins/rumbles of such a structure. But that's all.

Video source: https://www.youtube.com/
Article source: https://www.unilad.co.uk/

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